Mulching Trees is Good and Bad

Mulching Trees is Good and Bad

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

It seems to be human nature... if something is good, a lot is even better.   Unfortunately, that’s not always the case.  For example, one of the best things you can do for the trees in your yard is to mulch them with an organic mulch.  Excessive amounts of mulch applied improperly by enthusiastic gardeners can cause more harm than good.  I want to talk about the problems caused by excessive and improperly applied mulch.... but first let’s review the benefits of mulching along with the proper way to apply mulch.

Benefits of Mulching

Competition with Turf  —  Trees and turf are competitors that don’t get along particularly well.  They compete for the same water and nutrients that are available in the top foot of soil. Because grass roots colonize faster and take up more of the soil space, they often “win” the fight for nitrogen in the soil.  Both struggle against each other for the available water. They also compete for light, with trees having the uncontested advantage because of their height.  As they get taller and wider, trees intercept so much light that the grass growing in the tree’s shade can’t get enough light.  As a result the grass thins out.

Grass has a secret weapon in its fight against trees. Grasses apparently release antagonistic chemicals into the soil that slow the growth of tree roots. This phenomenon is referred to as allelopathy. Researchers have found that tree growth is reduced when trees are grown in turfed areas compared to trees with mulched root systems. They attribute the improved growth of mulched trees to the absence of these antagonistic chemicals along with the other benefits provided by mulches.

A layer of mulch over the entire root zone of trees reduces the contentious competition between trees and turf... allowing both to grow better in peace.

Conserving Moisture  —  Mulching conserves moisture by reducing the amount of water that’s lost through evaporation from the soil surface.  A layer of mulch reduces the amount of water that must be applied by irrigation.

Adds Organic Matter to Soil  —  As organic mulches decompose they improve soil fertility and structure.  Decomposing organic mulches also provide food and a favorable environment to beneficial soil organisms, such as earthworms and “good” fungi.

Provided Insulation  —  A layer of mulch acts as insulation for tree roots, protecting them from extreme summer and winter temperatures.

Discourages Weeds  —  Mulches discourage weeds, which also compete with trees for water and moisture.

Reduces Soil Erosion  —  Mulches reduce soil erosion and soil compaction, as well as improving water entry into the soil.

Protects Trunk  —  A circle of mulch around a tree decreases the chances of damage to the tree trunk from string weed trimmers or mowers.

Looks Nice  —  An attractive layer of mulch around trees and shrubs provides a more uniform look to a landscape.

The Proper Way to Mulch

So just what is the “proper” way to mulch trees?  Proper mulching involves applying a two to four inch layer over the area around the trunk, extending out to the dripline or beyond.  The larger the area, the more beneficial to the tree. The ideal way of mulching is to apply mulch over the entire root system... which can be an area as much as two to three times the spread of the branches and extending well beyond the dripline.  The practical way of mulching is to apply mulch to an area at least four to five feet in diameter around the trunk.  However, it’s very important to keep the mulch six inches away from the trunks of young trees and one foot away from the trunks of older, mature trees.

A well-aerated, composted organic mulching material is best for properly mulching a tree.  Some of the preferred materials for mulching trees include bark, grass clippings, shredded leaves, and pine needles. If grass clippings are used they should be mixed with some coarser materials to discourage matting.  Wood chips make especially good mulch if they’re composted first and then mixed with leaves and bark.  Fresh wood chips and sawdust should never be used because their decomposition ties up available nitrogen in the soil, depriving the growing tree of nitrogen needed for growth.

Improper Mulching Can Kill Trees

Improper mulching or mulching mistakes are easy to make. One of the most common mulching mistakes is over-mulching.  Too much of a good thing isn’t better... in this case it can kill trees by suffocation.  When too much mulch is applied, it limits the amount of oxygen that’s getting to the roots of the tree. The excessive mulch also slows evaporation of soil moisture and the soil stays wet for long periods.... exacerbating the lack of oxygen in the soil.

Another common mulching mistake is placing the mulch against the trunk of the tree.  The base of the tree where the trunk flares out must be able to “breathe” .  It’s not root tissue and can’t tolerate a continually moist environment or a lack of oxygen.  Frequent irrigation that keeps a tree trunk saturated, a change of grade that buryies the flare, or mulch applied directly to the base of a tree can cause the death of the inner bark.  When the inner bark dies, the tree is no longer able to send food to its roots and the roots eventually starve.  Without roots that can take up water and nutrients, the tree dies. Excess moisture at the base of the tree can also favor bacterial and fungal diseases that attack and kill inner bark tissues. 

A very thick a layer of materials that are not fully composted, such as a thick layer of green grass clippings, can also lead to trouble.  This thick layer may actually heat up and go through the composting process, much like a compost pile.  The heat of early decomposition may lead to temperatures as high as 120 to 140 degrees in the mulch layer.  If this “composting” mulch is directly in contact with roots or trunk tissues, these temperatures can kill them.

Excessive insulation from a very thick layer of mulch can delay the hardening process in the fall, making a tree more susceptible to winter injury from cold temperatures, especially those occurring early in the winter.

When mulching was first advocated, some enthusiastic gardeners applied thick layers of mulch to their trees.  They later found that as a result of this thick layer the tree roots grew close to the soil surface... probably because they needed air.  These shallow roots didn’t have problems until severely cold temperatures arrived, killing them... and the trees.

Finally, a more recent and interesting way to improperly mulch trees are with mulching “volcanoes.” Mulching volcanoes occur when gardeners apply a tall pile or mountain of mulch (or sometimes soil) around the base of the tree.  These do nothing to help a tree... and are an easy way to kill it through suffocation and collar rot. 

So remember... mulch is very good for trees, but a lot of much mulch or mulch that’s applied improperly is bad for

Cold Temperatures Can Damage Plants

Cold Temperatures Can Damage Plants

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

How cold can it get before plants suffer cold temperature injury? It’s important to keep in mind that a plant’s potential hardiness is genetically determined.  Plants that are native to a geographic region have evolved in response to that area’s climate and weather patterns.  It can be thought of as “survival of the fittest”... the hardy plants that can survive the winter weather of a region produce offspring that are also hardy.  Through this natural selection process, native plants are usually able to survive the winter cold of their particular native region.

However, a hardy plant in the middle of summer can’t withstand the same amount of cold that it can in mid-winter.  This is because hardiness to cold develops as a series of physiological changes within the plants.  The first change to occur is a response to the shortening days of late summer and early fall.  Plant hormones are produced in response to the shorter days.  These tell the plant that winter is coming and plant tissues develop greater resistance to damage from freezing temperatures. 

As fall proceeds and temperatures become increasingly cooler, the plants develop even greater hardiness.  The rate at which plants develop hardiness varies from species to species.  The rate also is dependent on the degree of cooling that occurs in the fall.  If fall temperatures remain mild, plants may fail to acclimate fully to cold temperatures.  Because our late fall and early winter has been so mild, it’s very possible that some plants sustained some cold temperature damage from the recent deep freeze. 

At some time in the middle of winter, plants develop their ultimate mid-winter hardiness.  This point is genetically determined.  It’s interesting to note the part that genetics and geographics play in plant hardiness.  The ultimate mid-winter hardiness can be quite variable even within the same species.  For example, Douglas firs that evolved in the Rocky Mountains are hardier than Douglas firs that evolved in the Cascades.  Similarly, a flowering dogwood that is native to the New York state region is hardier than one from the Florida or Georgia regions.  In both cases the plants are exactly the same genus and species, they just evolved in a different climatic region.

After the plants receive their ultimate mid-winter hardiness they begin to deacclimate or lose some of their hardiness.  Like the acclimation process, deacclimation is usually gradual.  However, deacclimation can occur quite rapidly during an extended warm spell.  Plants have the potential to acclimate to colder temperatures again and again when temperatures drop, but they lose this potential as spring approaches and growth begins.

Many gardeners ask how they can protect their plants from cold temperatures.  Plants aren’t “warm blooded” creatures.  Putting blankets around a tree trunk won’t keep them warm.  Blankets can’t help, but other things can be done to provide a measure of protection:

1. Select plants that are hardy for the local climate.  Nurserymen will indicate a plant’s hardiness by noting the USDA hardiness zones for which it’s suitable.  The zone for the Tri-Cities area is Zone 6.  Gardeners close to the river or in the Walla Walla area might be able to push it to Zone 7.  You may see “borderline hardy” plants in area landscapes that do well here for several years when the winters are relatively mild winters, but then they succumb to a particularly cold winter.  That’s because they’re not truly hardy for this area.
2. When planting conifers (evergreens) in the landscape, situate them to limit their exposure to sun and wind.  If you live in a particularly exposed location where there is no “protected location,” plant a windbreak to protect them in winter months.
3. Keep plants in a healthy condition.  Healthy plants are better able to withstand the rigors of winter.  Stressed plants are more susceptible to winter injury.
4. Plants that are actively growing late in the season fail to acclimate as winter approaches.  Avoid fertilizing, pruning, or irrigating excessively towards the end of the summer and in early fall.  Keep in mind that the fall fertilizer that you put on the lawn may well be reaching tree roots and encouraging late season growth.  Try to avoid fertilizing trees and shrubs in the fall, especially those that are not extremely winter hardy or ones prone to winter injury.
5. While you don’t want to water plants excessively and encourage late season growth, it’s also important to note that water stressed plants are less hardy.  Don’t let your plants go into winter with dry soil.
6. Mulch the roots and crowns of tender plants, plants prone to winter injury, and recently planted trees, shrubs, and perennials.  Use loose mulch, such as finely shredded bark, that won’t mat down or exclude air from the roots.  Roses will usually benefit from an application of bark to the crown to protect them from cold and dessicating winds.
7. Protect the trunks from sunscald.  Sunscald damage happens when bark surfaces on the south and west sides of a tree are warmed by bright winter sun and then the temperature drops rapidly when the sun goes down.  This abrupt temperature change can damage the bark and cambium underneath.  Newly planted trees; trees with dark bark, such as cherries; or young trees with thin bark, such as maples, ash, crab apples, and tulip trees are particularly susceptible to this type of injury.  Wrap the tree trunk with a commercial trunk wrap or paint it with an inexpensive interior white latex paint to reflect the sun and prevent damage.  Bark wraps should be removed in the spring.

Early Fall Cold Snap Can Lead to Damage on Landscape Plants

Early Fall Cold Snap Can Lead to Damage on Landscape Plants

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

In our area we typically have relatively long periods of mild fall before we finally get a hard freeze.  In most years, the leaves senesce and normally fall off the tree as leaves do in autumn.  However, occasionally an early cold snap will take us and our trees and shrubs by surprise.  One very obvious sign of the sudden end to the growing season is all the leaves left hanging on the trees.  Just like us, they weren’t ready for the severe cold.  They didn’t have time to form the “abscission layer” that develops at the base of the leaf stem and causes the leaf to fall. The leaves stay attached to the trees.  This is a common trait of trees such as oaks, but unusual for many other trees and shrubs.  Some of these leaves will hang on for most of the winter, but others will fall every time we have a strong windy day. All will drop off by spring when the new leaves start to develop.

How Plants Get Ready for Cold Winter Temperatures

While these persistent leaves are not harmful to the plants, they are a sign that many trees and shrubs were not ready for record breaking severe cold temperatures.  Plants go through a physiological process to be able to avoid damage from cold temperatures.  This begins in late summer and early fall as the days grow shorter and the temperatures begin to decline.  Plant hormones formed in response to these changes trigger the cessation of plant growth, stimulate the formation of the abscission layer, and bring about physiological changes that allow the plant to withstand colder and colder temperatures.  In response to gradually cooling temperatures, the plant attains its winter hardiness through this physiological process called “hardening” or “acclimatization”.  By midwinter it has attained its ultimate hardiness, which is genetically determined.

The potential problem caused by our recent record-breaking cold spell is that many plants were not fully acclimated and hadn’t achieved their ultimate winter hardiness.  While they might be able to withstand 15 degrees, 10 degrees, or even 5 degrees in mid-winter, they may have sustained damage with the cold temperatures coming so early.  It won’t be until the next spring and summer that the amount of potential damage can be assessed.

Symptoms of Damage

For gardeners, the next spring and summer are the “moment of truth” for appraising the extent of possible damage.  The first clue that damage actually occurred will be the failure of a plant to leaf out and grow.  In some cases, leaves may form while flowers fail to develop because of injury.  There may also be plants that die abruptly after appearing unscathed with leaves and flowers growing normally.  On these plants, the buds were not injured but stem and branch tissues were severely damaged or killed.

When cold is sudden and sustained over a period of several days, roots of plants also may be damaged.  This usually takes longer, perhaps several months or more, to become evident on established plants.  Symptoms include outright plant death or gradual thinning and dieback of the crown.

Help for Injured Plants

Once spring arrives, there are some things gardeners can do to help an injured plants. First, be patient. Wait until late spring, after there has been sufficient time for the plants to fully leaf out, and then prune out dead wood.  Be sure to prune properly.  Don’t make stub cuts.  Prune back to a bud, stem or trunk with live green healthy wood.  Only remove dead and severely damaged wood.  Follow that with tender-loving care, proper watering, and mulching to retain soil moisture.  Injured plants are already stressed; so don’t add to their stress. 

Experts disagree about fertilizing winter-injured plants.  Some advise against it, others recommend it. One thing you don’t want to do is encourage excessive growth with heavy fertilization because injured roots and transport tissues may not be able to support this growth.  I recommend watering properly and fertilizing only lightly in the spring.

Minimizing Cold Temperature Damage

There’s not much we can do to protect our plants from record-breaking low temperatures in the fall or winter.  But we can minimize losses to our landscape and gardens by selecting plants hardy to our region.  Many landscape plants that we commonly use are “exotics” and are better adapted to climates where they’re considered natives.  Exotic plants respond differently to local climatic clues than do native plants.  Native plants have a greater chance of surviving winter cold temperatures.  However, gardeners like to test the limits and try growing many different plants... so even if we are planting “exotic” trees, shrubs, and perennials, we should make sure they’re hardy for our zone.

Several late-season gardening practices can help plants avoid winter injury, whether they’re natives or exotics.   It’s important not to promote late-season growth. Avoid fertilizing trees, shrubs, and perennials late in the growing season.  Don’t prune plants late in the growing season.  Wait for trees and shrubs to become fully dormant.  Late winter is the best time to prune.  Don’t drought stress your plants, but do cut back on the watering as the weather cools.  They certainly don’t need as much water in late summer and early fall as they did during 100-degree heat.  (It’s still important though not to drought stress your plants, especially needled and broad-leaved evergreens, as they go into winter.)

Fall and Winter Watering

Fall and Winter Watering

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

When irrigation water is turned off and systems blown out in the fall, your landscape plants still need water.  Foggy mornings, heavy frost, and cloudy skies often give a false impression that plants have adequate soil moisture.  Relatively dry air and low or no measurable precipitation lead to dry soils.  This means that even in the fall and winter... trees, shrubs, and lawn grasses need water to avoid drought stress.  This is particularly true during the fall and early winter when there is little or no snow cover... or when there is an extended warm fall.

Fall and winter watering can be crucial to having healthy plants in the landscape.  During the summer we are clued in to water stress by wilting and dropping leaves.  Plants that experience fall and winter drought can’t tell us something is wrong until the next year when they fail to thrive the next year.  Fall and winter drought can lead to root injury or death.  These drought-injured plants may not show symptoms of the problem until the next season or even the next year.  In fact they may leaf out and flower just fine in the spring, relying on stored food reserves.  Once that energy supply runs out plants weaken and start dying back.  Even if a plant isn’t killed outright, it is made more susceptible to insect and disease attack.

This type of “winter” injury seems to be more common than we realize in this area.  For the past several years, we have been losing many of our area birches.  This has been attributed to past winter droughts and cold temperatures without the benefit of insulating snow cover.  Birches are particularly sensitive to fall and winter drought.  Weakened by this, the birch trees that weren’t killed outright have become increasingly susceptible to attack by the bronze birch borer, which attacks and eventually kills weakened birch trees. 

Other shade trees are also susceptible to winter drought damage, especially those with shallow root systems.  This includes Norway maple, silver maple, linden, Colorado blue spruce, Norway spruce, and many other evergreens.  Shrubs are also vulnerable to winter drought damage, especially those growing up close to the house or in a warmer location.  This includes junipers, Oregon grape-holly, and euonymus.

So what’s the answer?  It’s simple.... water the landscape in the plants and the fall.  Yes, it will be work and you’ll have to use your domestic water but, the effort will be worth it when you see that your plants survive and others around you don’t do as well as yours.  I’m not talking about watering plants every day.  .... you’ll probably only have to water a couple of times in the fall to prevent damage. 

The most critical time to water is in the fall just before cold weather hits or during extended warm weather.  It’s interesting to note that not so many years ago, horticulturists thought it was a good practice to insure dormancy by drought stressing plants in the fall and this decreased the chance of winter injury.  Research since then has indicated that the reverse is true... so now we try to insure plants aren’t drought stressed in the fall.  The soil should be kept slightly moist down to a depth of 18 inches for most shrugs and a depth of 18 to 24 inches for trees.  Water only when the air temperature is above freezing and the soil isn’t frozen... which isn’t usually much of a problem during the fall but can be during winter dry spells.  Water early in the day to allow water time to drain away from the bases of plants.  (Frozen water next to the bark can physically damage trees and shrubs.)  Soaker hoses work well for applying the water slowly and where needed.

Since fall and winter watering will be more labor intensive for you, apply the water where it counts the most... in the root zone.  Consider that established trees have roots that go out at least as far as the tree is tall and usually further.  It is in the “dripline” and just beyond where most of the water should be applied.  The “dripline” is an imaginary vertical line that is perpendicular to the longest side branches of the tree and perpendicular to the ground.  Water applied at the tree trunk base is wasted because there are no water absorbing roots there.

Watering recently planted trees and shrubs is a different story.  Their roots don’t go out that far yet.  In this case you will want to water the root ball zone and just beyond.  The aim is to water where the roots are.  This makes sense doesn’t it?

Keep in mind that even if we do get lots of rain during the fall, shrubs and trees close to the house foundation or located under eaves may still need watering.  Located in these areas, they receive little precipitation and they lose more moisture than other plants because of their proximity to the structure and reflected heat from the walls.

Trunk Cracks and Wood Rot

Trunk Cracks and Wood Rot

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

Have you ever seen cracks in tree trunks?  Radial separations of bark and wood are usually referred to as frost cracks.  Almost every reference you’ll look at on tree care blame frost cracks on extremes fluctuations of temperature in the winter.  In fact, most sources blame frost cracks on the phenomenon of “southwest winter injury.”  This type of injury is attributed to the bark and wood of trees warming up on the southwest side on a sunny but cold winter day.  The tissue deacclimates, coming out of its complete winter dormancy.  When the sun goes down and the temperature drops rapidly, the tissues can’t reacclimate quickly enough and the water in the tissue freezes, causing cell damage.  The theory has been that sometimes this tissue damage can result in wood and bark cracks.

For many years bark cracks and trunk splits were attributed only to frost.  It wasn’t until the last twenty years, under the guidance of former U.S. Forestry Service scientist and author, Dr. Alex Shigo, that researchers have realized the real cause of split trunks and bark cracks.  Apparently “frost” cracks start from a wound that may have happened much earlier in the tree’s life.  These wounds are believed to be formed at the death of a root or branch.  This wound can create an internal “crack” that develops as a result of stresses from drying, wind, or temperature extremes.  When other pressures occur, such as the thawing and freezing that occurs in the “southwest winter injury” scenario, the internal cracks develop outward.

While the frost leads to a visible crack and open wound, it’s not the real cause of “frost cracks.”  The real cause of the cracks are the death of major roots at planting time; physical injury to roots from construction or soil compaction; wounds created by flush cut pruning; dead limbs resulting from topping cuts, physical injury to the tree trunk, and poor graft unions.

What is the problem with cracks in the tree wood?  Even though callus forms at the edge of the crack and may appear to close it, the wood will never re-knit together.  It’s not like broken bones on humans.  Once those wood fibers are split, they are split forever.  This weakens the mechanical support of the trunk or limbs involved. 

Which came first the chicken or the egg?  It was thought that trunk cracks developed first and then the decay developed in the center due to the opening created by the cracks.  Shigo discovered that the decay is there before the crack, coming from the dead roots, branches, or wounds which instigated the crack.  Additional fungi and insects may attack the tree as a result of the outward crack, but decay organisms are already present before the outward crack develops.

What can be done about split trunks?  Not much really.  One must realize that most of the wood cells of a tree are dead cells.  As I mentioned, they will not “knit” or grow back together.  In the case of most trunk cracks, the internal wood of the tree is already subject to wood decay from fungi.  The tree may function quite well with little effect from the crack or internal decay, since    the vital functions are carried out in the outer few inches of the tree's circumference.  The real concern is for the tree’s structural integrity.  If wood decay becomes substantial, the tree will become a hazard.

Don’t try to paint or seal the split with any type of compound.  They don’t help and they can aggravate the wound.  The best you can do is clean or smooth the edges of the wound with a sharp knife.  Start at one end of the split, smooth around one side of the wound, going no more than one‑half to one inch back from the split bark.  Stop at the other end and do the same procedure on the opposite side of the split.  This aids in callus development.  Sterilize the knife between cuts by dipping for several minutes in a 1:10, bleach:water solution or a 70 percent alcohol solution.

Occasionally “bleeding” or slimy seepage occurs from cracks and wounds.  This is called wetwood or slimeflux.  It is caused by an infection of the wood by a bacterium.  This bacterium feeds on the sugar in the wood and produces a foul-smelling gas and liquid basically through a fermentation process.  The infection can kill some of the bark cambium and can stunt growth but, usually doesn’t kill the tree.

Not all “cracks” are serious cracks that form in the wood.  Some cracks are simply bark splits and are not likely to be fatal to trees, although they will, in some cases, allow entry of disease organisms which can lead to wood decay.  Most of these bark splits are fairly superficial, forming mainly in the outer bark.  Splits occur vertically along the trunk or main branches.  In many cases, bark splits will often close or callus over completely leaving only a slight ridge in the trunk.

Causes of these splits include various environmental factors, such as rapid growth spurts, drought, fluctuating conditions of excessive and deficient, temperature extremes, southwest winter injury, late fall growth, and sun scald.  Trees that are most susceptible to this type of injury are those with thin bark, such as Kwanzan cherry, maple, and certain fruit trees.  Young trees also seem more prone to bark‑splitting than older, established trees.

So the next time you see a bark split or a crack in the trunk of a tree be aware that the real cause of the problem is not frost.  They are reminders that tree care from the time of planting to pruning all contributes to a tree’s health, now and in the future.

Lawnmower Blight

Lawnmower Blight

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

Watch out for "mower and line trimmer blight" damage to your trees.  This dastardly disease is the result of tissue damage to the trunks of trees from careless mowing techniques and overenthusiastic weed whacking.  The bark of young trees is especially vulnerable to attack because its thin and tender, but older trees with thin, smooth bark are also susceptible.

Lawnmower blight damages the cambium tissue layer that lies right underneath the bark.  It's only a few cells thick.  The cambium is important because it produces the cells, which develop into the conductive tissue in the trunk.  Damage to the cambium results in decreased flow of water and nutrients upward and sugars downward in a tree.  A tree is "girdled" if damage occurs to the tree all the way around the trunk.  Eventual death is the result of girdling.

Too often we carefully plant and tend our trees, but fail to protect them from "lawnmower blight."  Once a tree experiences this type of damage to its trunk, there isn't much you can do.  Cambium tissue cannot be replaced... it doesn’t regenerate and the tree can't heal itself.  The best control is prevention!

The very best way to prevent damage to the trees is to eliminate grass and weeds growing close to the trunk.  Create at least a 12-inch “weed-free-zone” around the trunk of the tree.  This eliminates the need to mow and potentially damage the tree.  (If you use glyphosate or Roundup to eliminate weeds around the trunk of your tree, be sure to keep the chemical off the bark.  Young bark can absorb the chemical and result in damage.)  Edge the “weed-free-zone” with vinyl edging, bricks, or cedar bender board.  Mulch the area with bark or compost.

Keeping the “zone” weed and grass free isn't just aesthetically pleasing; it's also good for your tree. Research has shown that allowing grass to grow over a young tree's root zone will stunt the tree's growth.  This happens because the grass is a good competitor for water and nutrients.  Another reason is that grass exudes chemicals that retard root growth. 

If maintaining a “zone” around the trunk of your trees just isn't going to work, you can cage your trees with wire fencing or heavy plastic netting that is about 12 inches high.  However, you should have at least a three-inch, weed-free-zone right at the trunk base and keep the caging material from direct contact with the trunk.  This three-inch zone is important because otherwise the grass and weeds will grow up inside the cage resulting in an overgrown, unkempt mess.

Some folks use 6-to12-inch long pieces of large, plastic irrigation pipe to protect the trunk of their trees.  They slit the pipe on one side and then they pry it open to slip it over the trunk.  While this might not be the most attractive way to protect the tree, it does the trick of preventing mower blight.

Hot Weather and Leaf Scorch on Trees

Hot Weather and Leaf Scorch on Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

It takes longer for trees and shrubs to show signs of distress from lack of water or improper watering practices than other smaller plants in vegetable and flower gardens..  Leaf scorch is a possible sign of water stress.  Mild leaf scorch appears as the yellowing and then browning and drying of the tips and margins of leaves.  More severe scorch appears as the yellowing and then browning of tissues between the main veins of the leaf or large angular blotches of brown tissues between the veins. (In many cases no yellowing occurs at all, tissues just turn brown.) Extremely severe scorch involves the browning and death of entire leaves.

Leaf scorch is a physiological problem related to watering.  The leaves are telling us that they’re losing water at a faster rate than can be replaced by the plant.  Leaf tissues that are furthest from the veins (the margins and areas between the veins) are the first ones affected by the lack of water. Environmental conditions that increase the water demands on a plant tend to increase the severity of leaf scorch.  Hot temperatures, low humidity, high light intensity, and wind all increase the rate of water loss from the leaves.

Sometimes leaf scorch is simply due to a lack of adequate irrigation during the demanding summer weather.  However, anything that interferes with the uptake and transport of water to the leaves can lead to leaf scorch.  Compacted soils; girdling, choking, or underdeveloped root systems; root damage from excavation; and trunk injury from mowers and weed trimmers are often involved. Certain disease and insect problems may also be part of the problem.   Recently transplanted trees with unestablished root systems are frequent victims of leaf scorch.  

Some types of trees and shrubs, such as Japanese maple and flowering dogwood, are not well adapted to hot summers in an arid climate like ours.  To survive and thrive in our region, they do best where they’re protected from excessive heat, afternoon sun, and drying winds.  In more exposed locations or surrounded by reflected heat from buildings and pavement, they will often develop leaf scorch.  They also benefit by having their roots kept cool with a layer of bark mulch. 

Strangely enough, leaf scorch can also be caused by too much water. Saturated soils don’t allow roots to get the air that they need.  Suffocated roots die and the plant can’t take up water... creating the same symptoms that appear if there isn’t enough water.  Saturated soils can lead to fungal and bacterial root rots and eventual death.

If leaf scorch develops on a tree or shrub of yours, try to determine the cause.  Don’t simply start dumping more water on it, assuming that too little water is the problem.  Check the roots and trunks for problems.  Consider all the possible factors that might be involved, but don’t forget to check soil moisture too.

It’s easy at this time of year to be derelict in your watering duties. Contrary to the belief of many, most trees don’t have tap roots and aren’t able to make use of the water in the water table. Most tree and shrub roots that absorb water are located in the top 18 to 24 inches of soil.   Trees growing in our region depend upon us and irrigation water to supply their moisture needs.

During the hot part of summer, trees and large shrubs should be watered with “deep” watering about once a week.  More shallow lawn watering is inadequate to supply their needs. Use a soaker hose to provide this deep watering.  Soaker hoses are made of porous canvas, plastic, or rubber. They allow water to seep out slowly and are useful in watering trees, flowers, vegetables, and shrub beds. Don’tprovide the water at the base of tree trunks where it’s wasted.  Apply water to the area known as the “drip line”, the outer edge of the branch spread.

Special attention should be given to trees and shrubs planted within the last two years.  Make sure they’re getting enough moisture by checking their root balls.  They can’t “tap” into surrounding soil moisture until their roots grow out of the original root ball. It’s important to keep their root zone moist... but not wet.

How much water does a tree need to keep it “happy”. Research has shown that a mature silver maple can lose over 000 gallons of water a day through its leaves, and a mature oak tree can lose over 400 gallons a day.  However, rather than worrying about how many gallons of water to give your tree, you should be checking the soil moisture in the root zone.  Use a spade, shovel, or probe to see if the soil is moist in the top two feet of soil. 

Prevent hot weather stress in your yard and garden by watering the right way!

Watering Trees

Watering Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

Proper watering is always important, but it becomes crucial when the temperature hits triple digits and stays there.  If plants are only provided with shallow waterings every day, they’re probably only getting a fraction of what they need.  Let’s look at the water needs of trees.  Large amounts of water evaporate through tree leaves.  This “pulls” water through the plant.  Water is the “vehicle” that nutrients use to enter the plant.  These nutrients are needed for plant functions and growth.  Water is also needed for vital physiological plant processes.  A lack of water means stress and disruption of plant functions.

While humans are supposed to drink at least eighty ounces of water a day and more in hot weather, trees need gallons of water.  A mature silver maple can lose up to 58 gallons of water per hour on a hot summer day!  This translates to over1000 gallons in a twenty four hour period.  Wow! Watering your tree for 10, 15 or 20 minutes a day just isn’t going to give that big old maple the water that it needs.  It’s like getting only a little glass of water after standing outside all day.

To water trees adequately, water should be applied over a longer period of time that will thoroughly moisten the soil to a depth of 18 inches or more.  Water should be applied slowly enough so that it soaks into the soil without running off down the driveway, sidewalk or street.  An excellent way to apply water slowly is with a soaker hose.  Soaker hoses are made of porous canvas, plastic, or rubber.  They allow water to seep out slowly and are useful in watering trees, flowers, vegetables, and shrub beds.

If you have a solid set irrigation system and numerous trees, soaker hoses might not be practical for you.  You’ll need to work with your system’s timer so that you can apply water for a longer period of time in the same location.  You may want to “pulse” the water, applying it several times with “resting” periods between applications to allow the water to soak in.  Check the soil several hours after you water to make sure that it’s moist to a depth of 18 to 24 inches.

It’s important to point out that the tree roots that absorb water are not right next to the trunk of established trees.  Water applied in the tree trunk area is wasted water.  The fine feeder roots that absorb most of the water for the tree are located at and beyond the “dripline.” The “dripline” is the outer edge of the branch spread.   Newly planted trees are an exception to this rule.  The water absorbing roots of recent transplants will generally be in the area of the root ball.  Care should be taken to keep that root ball moist and water may have to be applied close to the trunk to accomplish this task.

What’s the best time of day to water during the hot weather? It’s probably best to water in the very early morning, but irrigation water isn’t always available when you need it.  If you have a choice, water during the cooler part of the day, morning or evening.  If you water during the hottest times of the day, you lose considerable moisture from evaporation before the plants even get a chance to use it.

It would be a good idea to check out all your sprinkler heads and make sure they’re working properly.  During hot weather, a broken or clogged head might mean a stressed tree, a dead garden plant, or a brown patch of lawn.  If you rely on drip systems to water trees and landscape beds, make sure all the emitters are functioning correctly.

Caution: Even though you want to keep your plants supplied with the water they need, you shouldn’t drown them.  Saturated soils can lead to root rot and the eventual death of trees.

Leaf Scorch

Leaf Scorch

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

Hot weather or drought conditions sometimes lead to a physiological problem called leaf scorch. Leaf scorch shows up as a browning and drying of the leaf edges. In severe cases the brown areas will extend into the leaf tissue between the main veins. In extreme cases, the entire leaf turns brown.

Leaf scorch is often more severe on the side of the plant where the heat stress was greater, usually the south or southwest sides. Reflected light from light-colored surfaces and heat radiated from cement or asphalt put additional heat stress on plants and can increase the severity of leaf scorch. Rock mulches also absorb and radiate heat causing plant stress.

Leaf scorch is associated with inadequate amounts of water being available for the plant to use, especially during hot weather when plants have higher water demands. However, the cause is not always a matter of heat stress and too little water in the soil. A number of other factors may contribute to the development of leaf scorch. These factors include:

Inadequate or poorly developed root systems due to recent transplanting or poor planting practices.

Girdling roots choking the tree.

Injury to the roots or trunk, especially mower and weed trimmer injury.

Vascular disease.

Root disease, especially root rot.

Interference with water uptake and utilization due to soil compaction, poor penetration of water into the soil, water run off on slopes; and high salts in the soil.

Excess water and saturated soils exclude oxygen from the roots and also interfere with root function and water uptake. The result of too much water can be the same as too little!

Leaf scorch is a sign that a tree is under some sort of stress. When scorch develops on tree leaves, it's important to analyze the cause and then take remedial action to alleviate the stress and keep the tree as healthy as possible.

Trees With Yellow Leaves That Should be Green

Trees With Yellow Leaves That Should be Green

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

The leaves of most trees should be green, but sometimes in the Mid-Columbia region, tree leaves turn from green to yellow.  It’s a problem called chlorosis.  Chlorosis is an abiotic (not caused by a living organism such as a fungus or virus) disease.  It’s characterized by the greenish-yellow to yellow leaves.

Much of the chlorosis seen in our area on trees is actually iron chlorosis or chlorosis caused by a lack of iron in the plant tissues.  Iron is needed for the formation of chlorophyll, the green pigment in leaves.  Without the green pigment that allows a plant to utilize sunlight for production of food and energy, the plant will die. 

Plants with iron chlorosis first turn yellow-green to yellow between the veins, with the veins remaining a darker green.  With more severe chorosis the leaves become pale yellow and develop brown spots between the main veins.  Leaf margins may also turn brown with the leaves later drying up and falling off.  Tree growth slows to a stop and dieback of branches can occur when iron chlorosis is extremely severe.

Iron chlorosis is quite common in our area because we tend to have alkaline soil, those are soils with a high pH, often over 8.0.  While our soils actually contain adequate amounts of mineral iron, its in a chemical form unavailable to the plants due to the high pH of the soil.  The yellowing or chlorosis can involve the entire tree, or may be restricted to one side or even just one branch.  Within the same yard, there may also be perfectly healthy green trees growing right next to ones with iron chlorosis.

Certain types of trees and shrubs are more prone to iron chlorosis than others because they are more sensitive to high pH soils.  Those trees most likely to show symptoms of iron chlorosis include Pin Oak, Flowering Dogwood, Sweet Gum, Silver Maple, Tulip Tree, Magnolia, Catalpa, White Oak, Holly, and White Pine.  Acid-loving shrubs, like azalea, blueberry and rhododendron, are also prone to iron chlorosis.  These types of trees and shrubs should be avoided when planting in soils where pH is extremely high.

While it’s common to encounter highly alkaline, calcareous soils in our region, a high pH is not the only cause of iron chlorosis.  First of all, it may not even be iron chlorosis.  Chlorosis can be confused with similar symptoms expressed by mineral deficiencies such as magnesium, manganese or boron.

Cultural factors can also lead to symptoms of chlorosis.  Overwatering is probably the most common cause of chlorosis, in fact iron chlorosis can be induced if soils are kept excessively wet as a result of overwatering, compacted soils, or poor drainage.  I frequently see tree and shrubs that have developed “lime-induced chlorosis” as a result of overwatering. 

Chlorosis can also be the result of root damage, girdling roots, or trunk damage from mowers and weed eaters.  This is because root restriction, root injury and trunk injury all impair the plant’s ability to take up and transport soil nutrients.  Chlorosis can also develop in extremely dry soil situations because mineral nutrients must be in solution for a tree to be able to absorb them from the soil.  In dry soils they can’t absorb the nutrients.  I commonly see chlorosis on many silver maple trees in situations where half of the root zone or more is located in an area that isn’t irrigated regularly, such as a dry lot, a gravel driveway, or a ditch bank area.

What can you do about chlorosis?  If the cause is excessively wet soils, adjust the watering so the soil doesn’t remain saturated for any length of time.  You will still need to water the tree adequately so it doesn’t undergo drought stress.  Watering should be done slowly enough to moisten the soil to a depth of at least 18 to 24 inches without saturating the soil.  If the soil is compacted, aeration may help the water penetrate the soil more quickly.  If the soil is too dry, the remedy is simple... water regularly to maintain moist soil to a depth of 18 to 24 inches in the entire root zone of the tree.

If watering or compacted soils doesn’t seem to be the problem, check for trunk or root problems.  Look for trunk injury from physical wounds to the base of the tree from mowers or weedeaters.  See if the bark on the southwest side of the tree has been damaged from winter injury.  Check for girdling roots, constricted roots, or damage to the root system.  When possible, these problems should be corrected.  In some cases the tissue damage may not be able to be corrected and the tree may eventually succumb to its injuries.

Finally, correcting iron chlorosis in alkaline soils isn’t an easy task.  Before you do anything, it’s a good idea to have a soil test performed to find out the alkalinity of your soil.  Once you have determined that your soil truly is alkaline, there are several approaches you can take to attempt to correct iron chlorosis.

One of the simplest approaches is to acidify the soil.  This is most easily achieved by adding sulfur to the soil prior to planting, but acidification is a slow process and pH change will be slow.  To acidify the soil you should add sulfur to the soil, prior to planting.  The easiest sulfur to use is prilled sulfur.  This should be applied at the rate of 25 pounds per 1000 square feet of landscape bed and mixed well with the soil to a depth of 18 to 24 inches.  If trees and shrubs are already established you will need to apply acidifying fertilizers, such as ammonium sulfate.

Another method of correcting iron chlorosis is the application of iron chelates to the soil.  Chelated iron is less affected by soil pH and more readily available to plant.  Iron chelates must be placed in the root zone by drilling holes in the soil or working it into the soil. 

Iron chelates in soluble form or iron sulfate can also be applied to plants through a spray to the leaves.  These foliar sprays often result in a quick “greening” of the leaves, but these effects are generally temporary.  New growth that develops after application will still be chlorotic.

There are also methods available for injecting iron right into trunk tissues with implants or injections, but these cause wounds to the tree trunk it’s not advisable to use them in most situations.

So if your trees are turning yellow, determine the cause and take action.  Keep you thumb and your trees green. 

Common Root Problems

Common Root Problems

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

Root problems are the number one cause of tree and shrub death in the home landscape. When the general care of a tree or shrub is adequate, root problems are suspected when a plant is growing poorly or showing signs of dieback.  Other symptoms of root problems include chlorosis (yellowing of the leaves) and leaf scorch (brown tips and edges of the leaf). Here are some of the most common root problems that lead to plant death in our local landscapes.   

PLANTING TOO DEEP:  Tomatoes don't mind a deep planting, but most trees and shrubs suffer from being planted too deep.  Plant roots suffocate and die when planted too deep because the roots can’t get the oxygen they need. Trees and shrubs should be planted at the same level as they were in the nursery or just slightly higher to allow for some settling of the soil.  This means that the top of the root ball should be at or just below the soil surface.

If you have trees or shrubs planted in the last five years that haven't grown very well and have exhibited chlorosis, leaf scorch, or die‑back of the twigs, check to see if the roots are too deep.  It's easy to check.  Just take a garden trowel and start digging down at the base of the trunk.  Just under the top of the soil you should be able to find the main roots radiating out from the trunk.  If you must dig down more than two to three inches to find the main roots, you and your plant have a problem.  While troublesome, replanting at the right level is probably the only real solution.

IMPROPER PLANTING: Improper planting should be suspected if the plant looks OK but doesn't grow much after several years from the time of planting or when the plant begins to decline after several years, often with symptoms of leaf scorch and chlorosis. 

When planting a tree or shrub that’s been grown in a container, encircling roots should be gently spread before planting.  Dense masses of fine fibrous roots need to be cut and teased out of the root mass.  Paper pots and burlap should be torn away or removed from the root ball once the plant is situated at the right level in the hole and positioned correctly.  This is especially important if the burlap is green or has a green tint to it.  This burlap has been treated with a copper solution to keep it from rotting in the nursery.  Copper is toxic to roots, so roots will not grow through this copper "barrier" even after the burlap begins to rot.

You can check for root growth by digging down at the edge of the original root ball.  Look for any root growth that has occurred since planting.  If plant roots weren't loosened or spread, it's likely that roots have not grown out from the original root mass. 

This problem occurs most frequently with plants grown in plastic pots, especially when the plants are simply "popped" out of the pots and plunked into a hole without any loosening of the root ball.  If someone else planted your landscape for you, you may want to check the situation of the roots before problems become apparent.  Replanting is the only recourse in this situation too.

WATERING: Because many container grown plants are grown in a "soilless potting mix" which is a coarse mixture high in peat moss or other organic matter, water does not move easily from the surrounding soil into the root ball.  You may have to water the individual root ball directly and not rely on sprinkler irrigation to provide adequate water.

It's especially important to keep the root ball of recently transplanted trees and shrubs moist for the first several months after planting.  Their roots are confined to a limited space, so they're unable to “reach” for moisture further outward or downward in the soil.  With our typical local summer climate, it's essential to pay close attention to watering the first summer after planting.  However, it's also important not to drown plants with too much water.  Don't allow water to puddle around the trunk or keep the roots saturated for any length of time. 

How can you tell if the soil is moist, too dry, or too wet?  Use a trowel, shovel, or soil tube to check for moisture in the root ball.  A dry soil will crumble when you try to make a ball of it in your hands; a moist soil will form a ball; and a wet soil will be muddy with excess water that can be squeezed out.

ROOT DAMAGE: Root damage can occur from any number of situations.  The most common cause of physical damage is from construction, such as trenching for utilities, driveway installation, or digging of septic systems.  Roots are often cut to accommodate the construction without the consideration of the damage caused to the tree.  Cutting off major roots, severs the main "pipelines" for the tree.  The tree can no longer access water and nutrients through the severed roots.

Root damage can also occur from a change of grade.  Most of the water and nutrient absorbing fine roots are in the top one-foot of soil.  Changes of grade in excess of four inches can severely reduce the amount of oxygen and water reaching these roots, eventually killing them.  In some cases, soil compaction and not extra soil from a change in grade can lead to the decline and eventual death of a tree.  Soil compaction also restricts oxygen and water uptake by the roots, leading to root decline and death.  Severe soil compaction is associated with roads, driveways, and the use of heavy machinery.  Moderate compaction can develop over time simply from human and animal foot traffic or from impact sprinkler irrigation.  This type of compaction can be partially alleviated with the use of a mechanical aerator.

Careful Use of Weed Control Chemicals in the Lawn Protects Trees

Careful Use of Weed Control Chemicals in the Lawn Protects Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

"Weed and Feed" or plain "weed" fighting chemicals can cause serious problems for trees and shrubs.  These materials are generally considered safe to humans, pets, and wildlife, if label directions are closely followed.  However, many weed control chemicals are not as safe to the desirable trees, shrubs, and ornamentals growing in the lawn or in surrounding planting beds.  The problem is not really with the weed killing products, but with us... those applying the products.  We frequently fail to read and fully understand the precautions given on the label.

Many products, especially those containing the active ingredient dicamba, advise against use in the root zone of desirable trees, shrubs, and other ornamental plants.  Damage often occurs when gardeners don't realize just where the root zones of these plants are located. 

Trees and shrubs are not tap-rooted (with a few exceptions). Roots expand laterally out from the trunk where there are larger "pipeline" roots that divide and subdivide, becoming smaller with each division until the very ends where the fine-feeder roots are located. The fine-feeder roots are responsible for absorbing water and nutrients from the soil.  The roots of most trees are found in the top 18 inches of soil, with over 50 per cent of their roots found in the top six inches of soil.  They're situated in this top layer of soil because that's where they can get the air, nutrients, and water they need for growth and survival.

Roots go out much further from the tree trunk than once believed.  It was once thought that most of the fine-feeder roots were located primarily in the drip-line zone. This is the area beneath the reach of a tree's side branches... the place where rain "drips" off a tree.  However, research has revealed that the root systems, especially the fine feeder roots, extend out laterally much further from the trunk.  In fact, tree roots often extend out from the tree trunk as far as the tree is tall... or even further.

Before we apply "weed and feed" or other weed controlling products to our lawns, we should step back and assess where tree, shrub, and ornamental plant roots may extend.  In most cases, it would be best to apply the chemicals only to the patches of lawn with weeds, not to the entire lawn where roots of desirable plants may be located.  If you have a large lawn with lots of trees and shrubs, these spot applications may be annoying, but they're important in protecting your plants from damage.

Of course if you don't have many weeds, it's much healthier for your trees and shrubs to either dig out the weeds by hand or spot treat individual weeds.  It's overkill to repeatedly apply herbicides to your entire lawn when you don't have many weeds.  The very best form of weed control is a healthy, dense lawn.  Proper watering, fertilizing, and mowing go a long way in helping you avoid the use of weed control chemicals... and protecting your trees, shrubs and gardens. 

If you chose to use weed control chemicals on your lawn. here are more precautions you should follow to protect your trees and shrubs:

Calibrate—  If you're applying a "weed and feed" or other weed controlling products, be sure to only apply the right amount. If it's a granular product and you're applying it with a drop spreader, be sure to calibrate your spreader correctly and only apply the amount needed for the square footage of lawn.  If you're applying the material as a liquid spray, you should also check the calibration of your sprayer or applicator.  Damage to desirable plants in the lawn or nearby plants is often due to over-application.

Do Your Math—  Do you know the square area of lawn that you're planning to treat?  Don't guess. Actually measure your lawn area and determine the square footage being treated. If your lawn is not an easy rectangle or square, determine the footage by breaking the area into rectangular, square, triangular, and circular blocks.

When Using a Granular—  Many of the granular materials will direct you to apply them when there is dew on the grass or when the grass is wet from a recent rain or irrigation. This helps the material stick to the grass and work more effectively.  If applied to dry grass, they will be much less effective.  There will also be directions on how long to refrain from watering and mowing, whether the material is applied as a spray or a granular.

Sensitive Plants—  Some plants are particularly sensitive to the application of dicamba and phenoxy (2,4 D, MCPP, and MCPA) herbicides in their roots zone.  Damage may occur even when you closely observe all the label precautions.  Trees and shrubs that are particularly sensitive to damage from either dicamba or phenoxy herbicides or both include apple, birch, box elder, catalpa, dogwood, forsythia, grape, honey locust, Norway maple, redbud, Siberian elm, sycamore, and walnut.  Extreme caution should be employed if you have any of these plants in your yard and you use these weed killing chemicals in your lawns. 

Avoid Drift—  Many broadleaf weed controls are applied to lawns as liquid sprays.  Extreme caution must be utilized to prevent drift to nearby areas.  Don't spray when it's windy.  The lower to the ground the application and the larger the spray droplets, the less likely drift will be a problem.  It's worth repeating...don't spray when it's windy or breezy.

What Weed Is It?  —  Get your weed identified first, BEFORE applying any chemicals for control.  If grassy weeds are the problem in you lawn, the typical "weed and feed" products for controlling broadleaf weed chemicals will not touch them.  If they're perennial grassy weeds, they will have to be spot- treated with a non-selective herbicide, such as Roundup or other glyphosate containing product.

If they're annual grasses, you will need to apply a preemergent herbicide, such as we discussed in a recent column. Effective herbicide applications for both perennial and annual weeds in the lawn depend on the type of grass and the appropriate timing of the application.  Get your weeds positively identified before attempting control.

Picking An Herbicide—  Not every weed is controlled by every herbicide chemical.  Most broadleaf weed control products for home gardeners contain more than one active ingredient.  2,4 D has been a popular ingredient for many years and provides good control of a number of common broadleaf weeds, such as dandelions and plantain.  However, there are other weeds for which it really doesn't do a good job of control.  Mixes of 2,4 D andMCPP or MCPA, are typically utilized.  These broaden the number of common broadleaf weeds the material will control, including chickweed, ground ivy, black medic, and clover. 

Many mixes now also contain dicamba to get at some of the more difficult-to-control weeds, such as bindweed, prostrate spurge, mallow, and oxalis.   Mixes that contain dicamba, 2,4 D, and MCPP are often referred to as "Trimec" when listed on the active ingredients of the label.

There are even tougher weeds, that the Trimec combination sometimes won't control.. A newer material, triclopyr, is now available to home gardeners to use on their lawns for control of some of the toughest lawn weeds, such as oxalis, violets, and spurge.  The only home garden product for use on lawns in Washington that contains triclopyr is Monterey Spurge Power that also contains dicamba and MCPA

Do Black Walnuts Poison Other Plants?

Do Black Walnuts Poison Other Plants?

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

Black walnut leaves, husks, and bark have been rumored to be toxic and not good for use in gardens and compost piles.  The start of this rumor comes from the phenomenon of certain types of plants wilting and dying when growing in close proximity to black walnut trees.  This is called “black walnut toxicity” or “walnut wilt.’ This phenomenon is not new.  In fact, it was noted by Pliny in Roman times.

Scientists have determined that there is a plant chemical called juglone that causes black walnut toxicity.  The largest concentration of juglone can be found in the buds, hulls, and roots of black walnut trees, but it’s also found in the leaves, bark and stem tissue.

Juglone is toxic to many other plants.  Black walnut toxicity primarily occurs when the tree’s roots exude juglone.  Juglone sensitive plants with roots in close proximity to the black walnut roots are affected.  Scientifically, juglone inhibits respiration, denying plants the energy they need for plant growth and metabolism.  Plant responses range from sudden wilt and death to stunted growth.  There are other members of the walnut family, such as Persian walnut, butternut, pecan and hickory, which also contain juglone, but don’t seem to produce it in sufficient quantities to produce toxicity symptoms.

There are a good number of black walnuts grown in this region.  Some area gardeners have been concerned about using the leaves and hulls in their compost because of the rumored toxicity.  According to an Ohio State University Extension factsheet “walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria.  The toxic effect can be degraded in two to four weeks.  In soil, breakdown may take up to two months.  Black walnut leaves may be composted separately, and the finished compost tested for toxicity by planting tomato seedlings in it.” 

However, fresh sawdust, shredded leaves, or wood and bark chips from black walnut should not be used for mulching plants sensitive to juglone.  According to the Ohio factsheet, composting the bark for a minimum of six months provides safe mulch even for plants sensitive to juglone.

While rain may leach juglone from leaves and hulls, the juglone is highly reactive and quickly inactivated in the soil.  Juglone is also poorly soluble in water and doesn’t move very far in the soil.

Juglone sensitive plants include apple, asparagus, azalea, birch, blueberry, cabbage, ornamental cherry, chrysanthemum, crabapple, eggplant, lilac, linden, saucer magnolia, narcissus (some,) pear, peony, pepper, petunia, pine, potentilla, rhododendron, tomato, and more.

Ohio reports that problems with black walnuts are not limited to plants.  Horses may be affected by black walnut chips or sawdust when they’re used for bedding material.  Allergic symptoms in both horses and humans may also be produced by close association with walnut trees while their pollen is being shed in the spring.

When Staking is Left On Too Long

When Staking is Left On Too Long

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

Staking wire cans girdle a tree and lead to its death.  As a tree grows in girth, encircling wires “girdle” or choke the plant so that water and nutrients can’t move up and down the trunk.  Tree staking should be removed from trees as soon as possible after the tree becomes established.  It should never be left on the tree for more than a year. 

There are some cases where the tree grows over the wire and “grafts” to itself.  In those cases the tree survives but there is a permanently weak place in the trunk that is prone to breakage.  In cases where the wire isn’t completely enclosed in tissue and you can remove the wire without significantly harming the surrounding tissue, give it a try.

Keep this in mind too when there are plant tags, twine, or wire located on branches or trunks.  These materials don’t look harmful when the plant is young, but as the plant grows they can cause damage.  Even biodegradable strings and twines may not deteriorate before they girdle the plant.  Remove all “tying” materials when the tree or shrub is planted.  Don’t forget to remove staking as soon as the tree is established.

Take a look at recently installed commercial landscapes, those planted within the last two to three years.  In some locations, you're likely to see a number of trees where the staking has been left on much longer than needed and the wire is already cutting into tree trunks.

Planting Trees Correctly to Ensure Their Success

Planting Trees Correctly to Ensure Their Success

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist

Planting a tree isn’t as simple as digging a hole and sticking it in.  Many trees fail to grow and thrive because they aren’t planted correctly.  To determine just what is the “right” way to plant a tree, let’s look at some of the most common mistakes made when planting a tree or a shrub.

Too Deep

Probably one of the most common reasons for failure in planting trees and shrubs is that the root system is planted too deeply.  Roots need air.  They should always be planted at the same level or slightly above the level that they were growing in the nursery or in their container.  Planting them deeper than this deprives the roots of air and will eventually kill most plants.  Death is not usually a sudden thing.  Trees planted too deep will fail to grow well.  Leaves and new growth will be undersized.  Leaves may develop leaf scorch along the edges or become yellow and drop off the tree.  Keep in mind that trees aren’t tomatoes, most won’t tolerate deep planting.

Planting With Material Around the Root Systems

Any material around the roots... plastic twine, burlap, boxes, paper pots, and plastic pots should be removed at planting time.  Most gardeners don’t have to be told to do this, but you’d be surprised at how many people belatedly find trees and shrubs planted with the roots still in a plastic pot.

Whether it’s a non-degradable material like plastic or a material such as paper which will eventually rot, it should be removed.  Materials like cardboard boxes and pressed paper pots do rot, but they don’t rot quickly enough in our garden soils.  They impede water movement and restrict root growth.

The same holds true for burlap.  Even though burlap seems to rot readily it doesn’t rot as quickly as we would anticipate.  Some burlap is even treated with a copper material to retard decay.  Since copper is toxic to root tissues, the roots won’t grow through the burlap layer even if the burlap has decayed.  In some instances, plastic burlap is used.  That definitely won’t decay any time in the near future.

Dense Root Masses and Encircling Roots

When left growing in containers too long, many trees and shrubs develop cramped root systems, which make it difficult to grow them with success.  For plants to survive and grow, you will need to loosen the roots of those with dense, fibrous root systems.  When roots are dense and matted, cut them with a shovel, spade, or knife.  Make six to eight shallow vertical cuts into the exterior root mass.  Use your fingers or a hand fork to loosen the cut roots and help spread them out.

Another method of remedying problems with dense root masses of container grown plants is to “butterfly” the bottom of the root mass.  Use a shovel or spade to divide the bottom half of the root mass, creating two flaps or “wings.” The “wings” are kept apart by with soil, a stone, or a stick before filling the planting hole with soil.  Make six to eight vertical cuts to the exterior roots in the uncut, top portion of the root mass.

Plants with thicker, woodier roots, often develop encircling roots when grown in a pot too long.  These roots will keep growing in circles, if they aren’t disturbed at planting time.  The roots should be cut and spread as just described above.  With encircling roots the plant eventually chokes itself to death. Circling roots can’t reach out in the soil for water and nutrients needed for healthy growth and the poorly established root system doesn’t perform its anchoring function very well.

Clay Root Balls

Many quality nursery plants sold in this area are dug from nursery fields in the Williamette Valley in Oregon.  Many of these fine plants have one major drawback... the soil in the root ball is a heavy clay.  This clay is very different from local yard and garden soils.  It’s dense and holds onto water very tightly.  When the landscape is watered to accommodate our lighter, more droughty soils, the plant ends up with roots that are constantly too wet because of the clay soil around the roots -- this often leads to root rots.

One can avoid the problem by avoiding the purchase of plants with clay soil or one can try to remedy the situation by gently forking soil away from the root system and exposing the roots.

Creating a Bathtub Effect

Adding organic matter such as peat moss, compost, or mulch to the backfill soil of a planting hole is not a good.  It generally is not helpful and can create a “bathtub” effect.  Water easily enters the coarser soil in the planting hole but drains our slowly because the surrounding soil is more dense.  Again, the roots stay wet for long periods of time and root rots are very likely to develop.

However, when planting an entire landscape bed or border amending the soil with organic matter is a good idea.  Loosening the soil and adding the organic matter fosters good root growth.  Preparing the soil in the entire bed for planting eliminates the bathtub effect that can occur with planting holes.

Compact Soils

In planting sites around new homes and buildings, the soil is often very compacted.  This is especially true on commercial sites where the soil had been compacted with heavy machinery to provide a suitable area for paved parking.  Roots of trees, shrubs, and other plants have a hard time growing in compacted soil.  Water doesn’t move well into and through compacted soil.   There isn’t as much oxygen available to plant roots in a compacted soil.  Because the soil is tight, roots have a hard time penetrating the soil.

The best thing that can be done in a new planting area to relieve soil compaction is to loosen the soil by physically disturbing the soil by deep tilling or digging.

Proper Planting Techniques

1. Dig the hole deep enough to accommodate the root ball.  Keep in mind that the top of the root ball should be level with the soil or slightly above it.  The planting hole should be at least twice the diameter of the root ball.  The wider the hole, the better. Note: Always lift your tree and shrub by the root ball, not by holding onto the trunk. 
2. For balled and burlapped plants, first situate the plant in the hole and then remove all twine, string, or wire from around the root ball and stem.  Cut the burlap away from the root ball, removing as much as possible from the hole. For container-grown plants, cut and loosen roots if needed before placing the root ball in the hole.
3. Add appropriate amounts of slow-release nitrogen fertilizer to the planting hole. Backfill with the native soil you removed from the hole. Do not amend the backfill soil with organic matter. Gently firm the soil around the roots and definitely don’t tamp the soil down around the roots with your feet.
4. Water the plant thoroughly to help settle the soil around the roots.  Mulch the entire root zone area with bark mulch.
5. Keep the soil around the roots moist but not wet and saturated.
6. Prune to remove only the broken branches.  

Chlorosis — Yellow Leaves

Chlorosis — Yellow Leaves

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

Leaves turning yellow is called chlorosis. Chlorosis is an abiotic (not caused by a living organism such as a fungus or virus) disease. It's characterized by greenish-yellow to yellow leaves. What causes chlorosis? The answer isn't easy because the cause is probably not the same in every case... but there are some very likely possibilities.

Much of the chlorosis seen in this region on trees during the summer is actually iron chlorosis or chlorosis caused by a lack of iron in the plant tissues. Iron is needed for the formation of chlorophyll, the green pigment in leaves. Without the green pigment which allows a plant to utilize sunlight for production of food and energy, the plant will eventually die.

Plants with iron chlorosis first turn yellow-green to yellow between the veins, with the veins remaining a darker green. With more severe chlorosis the leaves become pale yellow and develop brown spots between the main veins. Leaf margins may also turn brown with the leaves later drying up and falling off. Tree growth slows to a stop and dieback of branches can occur when iron chlorosis is extreme.

Iron chlorosis is quite common in our area because we tend to have alkaline soil. Alkaline soils are characterized by a pH above 8.0. While our soils actually contain adequate amounts of mineral iron, its in a chemical form unavailable to the plants due to the high pH of the soil. The yellowing or chlorosis can involve the entire tree, or may be restricted to one side or even just one branch. Within the same yard, there may also be perfectly healthy green trees growing right next to ones with iron chlorosis.

Certain types of trees and shrubs are more prone to iron chlorosis than others because they're more sensitive to high pH soils. Those trees most likely to show symptoms of iron chlorosis include pin oak, flowering dogwood, sweet gum, silver maple, tulip tree, magnolia, catalpa, white oak, holly, and white pine. Acid-loving shrubs, like azalea, blueberry and rhododendron, are also prone to iron chlorosis. These types of trees and shrubs should be avoided when planting in soils where the pH is extremely high.

While it's common to encounter highly alkaline, calcareous soils in this region, a high pH is not the only cause of iron chlorosis. First of all, it may not even be iron chlorosis. Chlorosis can be confused with similar symptoms expressed by mineral deficiencies such as magnesium, manganese or boron deficiencies.

Cultural factors can also lead to symptoms of chlorosis. Over watering is probably the most common cause of chlorosis, in fact iron chlorosis can be induced if soils are kept excessively wet as a result of over watering, compacted soils, or poor drainage. Tree and shrubs in this region often develop "lime-induced chlorosis" as a result of over watering.

Chlorosis can also be the result of root damage, girdling roots, or trunk damage from mowers and weed eaters. This is because root restriction, root injury and trunk injury all impair a tree's ability to take up and transport soil nutrients.

Chlorosis can also develop in extremely dry soil situations because mineral nutrients must be in solution for a tree to be able to absorb them from the soil. In dry soils they can't absorb the nutrients. Chlorosis is often seen on large silver maples in situations where half of the root zone or more is located in an area that isn't irrigated regularly, such as a dry lot, a gravel driveway, or a ditch bank area.

What can you do about chlorosis? If the cause is excessively wet soil, adjust the watering so the it doesn't remain saturated for any length of time. You will still need to water the tree adequately so it doesn't undergo drought stress. Watering should be done slowly enough to moisten the soil to a depth of at least 18 to 24 inches without saturating the soil. If the soil is compacted, aeration may help the water penetrate the soil more quickly. If the soil is too dry, the remedy is simple... water regularly to maintain moist soil to a depth of 18 to 24 inches in the entire root zone of the tree.

If watering or compacted soils doesn't seem to be the problem, check for trunk or root problems. Look for trunk injury from physical wounds to the base of the tree from mowers or weed trimmers. See if the bark on the southwest side of the tree has been damaged from winter injury. Check for girdling roots, constricted roots, or damage to the root system. When possible, these problems should be corrected. In some cases the tissue damage can not be corrected and the tree may eventually succumb to its injuries.

Finally, correcting iron chlorosis in alkaline soils isn't an easy task. Before you do anything, it's a good idea to have a soil test to find out the alkalinity of your soil. Once you've determined that your soil is alkaline, there are several approaches you can take in an attempt to correct iron chlorosis.

One of the most simple approaches is to acidify the soil. This is most easily achieved by adding sulfur to the soil prior to planting, but acidification is a slow process and the pH change will be slow. The easiest sulfur to use is prilled sulfur. This should be applied at the rate of 25 pounds per 1000 square feet of landscape bed and mixed thoroughly into the soil to a depth of 18 to 24 inches. If trees and shrubs are already established you will need to apply acidifying fertilizers, such as ammonium sulfate, instead.

Another method of correcting iron chlorosis is the application of iron chelates to the soil. Chelated iron is less affected by soil pH and more readily available to trees. Iron chelates must be placed in the root zone by drilling holes in the soil or working it into the soil.

Iron chelates in soluble form or iron sulfate can also be applied to plants through a spray to the leaves. These foliar sprays often result in a quick "greening" of the leaves, but the effects are temporary. New growth that develops after application will still be chlorotic.

There are also methods available for injecting iron right into trunk tissues with implants or injections, but these cause wounds to the tree trunk and many arborists advise against using them.

So if your trees are turning yellow, determine the cause and take action.

Winterizing Trees and Shrubs

Winterizing Trees and Shrubs

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

Avoiding drought stress with adequate fall irrigation is also important in winterizing your trees and shrubs. Be sure to give your landscape plants a deep watering before your water supply is cut off for the season.  Don’t neglect watering the trees, especially birches, situated in your in the lawn. 

Fall watering is critical for the broadleaf evergreens, such as rhododendrons, and needled evergreens, such as like pines and arborvitae, in your landscape.  They may not be actively growing, but evergreens still lose moisture through their leaves and needles during the winter.  They’re prone to damage from winter drought... another reason to keep a hose and sprinklers handy during the winter months.  Don’t let them go dry, especially during mild winter weather.

Fall fertilization may also give some protection to trees and shrubs against cold winter temperatures.  If you decide to fertilize your plants, place the fertilizer in the root zone area after they have gone dormant, but before the soil in the beds drops below 45 degrees.

Mulching the root zone of trees and shrubs is frequently recommended for weed management and to help reduce the loss of moisture from the soil, but it can also provide some insulation to tender root systems. This is particularly important for plants that are only “borderline” hardy in this region, as well as for young or recently planted trees and shrubs. 

Loose mulches can be applied to the root zone to provide some insulation from cold temperatures and to moderate the effects of freezing and thawing.  Apply a several inch layer of a mulch material that allows good air and water movement.  Use mulches like shredded bark, pine needles, or coarse compost.  Keep any mulch several inches away from the trunk of the tree or shrub to discourage mice and to avoid problems from excess moisture close to the base of the plant.

Trunks of young or recently transplanted trees can be protected against splits by shading the south and west sides of the trunk.  Some gardeners shade the trunk of their trees with a commercial bark wrap or they simply use a board on the sunny sides.  The shading or wrap keeps sunlight off the trunk, preventing the bark from warming up too much on a cold winter day and reducing temperatures fluctuations that can lead to trunk or bark splitting.