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

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. 

Fertilizing

Fertilizing

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

Poor fertilization practices can lead to problems with nitrates finding their way into surface water... streams, lakes, rivers... and into ground water. According to Mary Robson, WSU Area Extension Agent in the greater Seattle area, it’s important to apply fertilizers correctly to both provide needed nutrients to plants and to prevent runoff and leaching.

Robson directs her comments, not to the agricultural producer, but to home gardeners. She points out that at this time of year, many gardeners start thinking about applying fertilizer to their landscape and garden plants. Gardeners should apply fertilizers right before the plant buds break and start to grow. This time for our area would usually be in March and early April.

Trees, shrubs, and perennials plants utilize stored nutrient reserves during the first flush of spring growth. Fertilizer is applied to replace nutrients needed by plants and keep the plants healthy. Keep in mind that fertilizer is not plant “food.” Fertilizer simply provides the nutrients that a plant needs so that it can make its own food using energy from the sun by the process of photosynthesis

The following are tips that Robson offers gardeners regarding environmentally friendly fertilization of landscape and garden plants:

ESTABLISHED TREES & SHRUBS   Not all trees and shrubs will need fertilization. Large established plants will often get along fine with only a spring compost mulch or another organic mulch and no other supplemental fertilizer. What are the signs that a plant is growing well? If the plant develops good leaf color, puts on an average amount of new shoot growth and length per season, and generally appears healthy, fertilizer is probably not be needed.

YOUNG TREES & SHRUBS   A young landscape plant does need spring fertilization. If the garden has newly planted trees and shrubs, installed within the last two to four years, be sure to fertilize these to ensure the best growth possible.  Roots that are just getting established need extra nutrients in their second year.

WHEN TO FERTILIZE   It’s easy to over-fertilize, applying too many nutrients too often by assuming that landscape plants need lots of “plant food.”  Be sure to follow label directions for quantity and don’t add fertilizer to tree and shrub plantings after active spring growth has finished.  This means eliminating fertilizer applications to landscape plants after the middle of summer.  If you apply fertilizer too late in the season, the plant may fail to go into normal fall and winter dormancy and can be harmed by winter freezes.

NEWLY PLANTED TREES & SHRUBS   Robson recommends mulching newly planted trees and shrubs with two to three inches of mulch and then waiting to fertilize six months before applying any fertilizer to the newly installed plant.  This is good advice for the Seattle area.  However, many of our local soils contain so little nitrogen, that fertilization at planting time will greatly benefit plant growth.  Moderate amounts of slow-release fertilizer or tablets added to the planting hole can provide the needed nutrients as soon as the roots start to grow out of the original root ball.  If planting in a landscape or perennial bed, slow-release fertilizer and organic matter should usually be added to the soil when preparing the bed prior to planting.

Fertilizer bags often have confusing labels with different numbers.  Nitrogen is the main nutrient needed for good spring growth of woody plants.  Nitrogen is represented by the first number in the set of numbers on the fertilizer label.  Since that’s the primary nutrient needed by your trees and shrubs, you should look for a fertilizer where that number is larger than the other two numbers that represent the amounts of phosphorus and potassium.  Slow-release formulas are more expensive, but they’re best for your plants because nutrients are released gradually rather than all at one time.  Slow-release fertilizers generally lead to less waste and leaching of nitrogen and promote better plant growth.

If your trees and shrubs are situated in or next to the lawn area, they’re probably getting more than enough fertilizer through your regular lawn fertilization, especially if you aren’t using a slow-release material.  This can explain why you don’t usually need to fertilize these plants!  Yet, it’s important to note that the best times of year to fertilize lawns are not always the best time to fertilize trees and shrubs.  Washington State University Cooperative Extension recommends fertilizing lawns in September, early November, June, and May.

This dichotomy in fertilization times causes us a dilemma.  Should we fertilize the lawn at the right time or fertilize our trees and shrubs at the right time?  Fall fertilization can lead to problems especially when tender plants are fertilized late in the season.  Fall is the best time to fertilize our lawns to keep them healthy and thick enough to keep weeds out.  What should we do?  There may be no good solution.  However, try to avoid applying fall lawn fertilizers to areas above tender trees and shrub roots.  Slow-release fertilizers would help avoid a late flush of growth on woody plants that might be stimulated with a quick-release type of fertilizer.

How much fertilizer should be applied to individual trees and shrubs?  That’s one of those questions that’s hard to answer directly.  The amount of fertilizer needed depends on the size of the plant and the type of fertilizer applied.  To help you determine the amount of fertilizer you should apply to trees and shrubs, Washington State Unversity Cooperative Extension has a handy bulletin, “Fertilizing Landscape Trees and Shrubs EB1034," with the recommended rates and methods of application.  You can purchase one at your local county extension office.

How can you tell if your plants need fertilizer?  Look for plants that aren’t putting on good growth, where leaves are undersized and chlorotic or yellow.  Here again we have another dilemma, because these are also symptoms of other plant problems.  In many cases the symptoms of poor growth and yellow leaves turn out to be a root or soil moisture problem.  If the tree or shrub has trunk injury, root damage, girdling roots, root rot or excessive soil moisture or drought, the symptoms will be pretty much the same.  Fertilizer will not help these plants or solve the problem of poor growth.

How can you tell if this is a root problem or a lack of nutrients?  If most plants in the yard and garden are growing well without any special fertilizer applications, this is a hint that the affected plant has a problem unrelated to soil fertility.  If the problem happened over a fairly short period of time, this is a hint that a root or soil moisture problem is involved.  If you suspect a root problem, you can check it out by examining the base and roots of the plant.  This involves a process of gentle excavation.  Soil moisture in the root zone and irrigation practices should also be reviewed.

Robson wants home gardeners to know that fertilizing landscape plants isn’t just a matter of buying a bag of fertilizer and applying it.  Responsible, caring gardeners should observe the health and stage of growth of the plant first.  “Apply the right amount at the right time!”

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.