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

Tree Roots Cause Problems with Mowing

Tree Roots Cause Problems with Mowing

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

Roots of large shade trees such as silver maple, sycamore and others often create problems when mowing the lawn.  Does it hurt the tree to cut the big roots off?  Would it be better put some soil over the top of the roots to make the surface nice and smooth?

Neither of these is a good solution to the mowing problem.  Severing of large, major tree roots will significantly impair that tree's ability to absorb water and nutrients from the soil.  Removal of large roots also weakens the tree structurally.  Remember the large woody roots help to anchor the tree.

Raising the grade around the tree more than one or two inches can smother the fine feeder roots on a tree.  Roots can't live without oxygen and even several inches of soil can hurt the roots of certain trees.  Deeper soil piled around the trunk of the tree can lead to rot at the base of the trunk... leading to tree decline and death.

Unfortunately, there is no good or easy solution to your problem.  You might try killing the grass and mulching the area with bark mulch, being sure to keep the mulch several inches away from the tree trunk. 

What are Girdling Roots?

What are Girdling Roots?

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

There are many diagrams that show trees with a big tap root directly beneath the trunk. This concept has been proven wrong by many different research studies of tree roots. Most trees do not have tap roots, especially when the soil is compacted or where the there is a high water table. You will not find tap roots on most of the trees in your yard. The exception to the rule might be pines and oaks which as young trees may have tap roots in sandy, well-drained soils.

If there aren't deep tap roots in the soil, where are the roots? Most tree roots are in the top 6 to 24 inches of soil. Tree root systems consist of larger, woody perennial roots and smaller, fine feeder roots. The woody perennial roots grow mostly horizontally in a network that radiates outward from the base of the trunk. Most of these roots will be found in the top six inches of soil, but vertical anchoring roots along this horizontal network may grow several feet deep or more. The function of the woody perennial roots is anchorage, water conduction, mineral transport, and food storage.

The fine feeder roots are much smaller than the perennial roots. They grow outward and upward from the perennial roots and are predominantly located in the top several inches of soil. Feeder roots die and are replaced on a regular basis. Their function is water and nutrient absorption.

It’s not unusual to find young nursery trees with kinked, entwined or circling woody perennial roots.  Once planted in the landscape, these roots grow in girth and can "girdle" or strangle the tree, preventing water and nutrients uptake. Girdling or circling roots also compromise the structural integrity of a root system and can lead to wind-throw in severe winds.

Sometimes circling roots on established trees can be cut with a sharp knife or chisel, but often the problem is noticed too late for this corrective action. If you suspect that your trees have girdling or circling roots, consult an arborist before taking any action. Keep in mind that large woody roots are your main framework of the root system. Cutting large roots can further compromise their anchoring ability.

Avoid girdling root problems when purchasing trees. Check the root systems before planting and only accept trees with well-distributed root systems, ones without kinked or circling roots.

Avoiding Girdling Roots and Planting Problems

Avoiding Girdling Roots and Planting Problems

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

How long will my tree live?  If it's in a protected place on a college campus, it may live 40 to 80 years.  If it's located in a city park, with luck it will survive 25 to 30 years; and if it’s growing in a suburban street right-of-way, it might last 12 to 18 years.  In cities, trees last only about 3 to 4 years.  Why?  Experts indicate that it's usually not insects or disease that ravage these trees... it's most often related to problems with the tree roots or the soil.

The same holds true for the sick or unthrifty trees.  The problem is usually not an insect or disease, it's most often related to a problem with the roots...  improper planting techniques; girdling or encircling roots; poor soil conditions; and watering difficulties.  Many young trees die before they ever develop a good root system. 

Let's talk a little about roots. Roots are alive. They need oxygen for respiration and normal plant metabolism.  It's easy to forget the importance of roots to a tree.  They enable a tree to take up water and nutrients in the soil.  Roots store carbohydrates and synthesize organic compounds involved in regulating plant growth.  For a tree to be healthy, the roots must be healthy.  According to Dr. Rita L. Hummel, Washington State University Horticulturist, "The importance of healthy roots and a favorable root environment cannot be overstated."

If so many tree problems involve roots and problems with root systems, where do these problems originate?  Some are due to faulty root systems that developed in the nursery where the trees were first propagated.  It is there that roots develop kinks or sharp bends.  This is due to the handling of the seedling in the nursery.  With mass production and mechanization of the nursery industry, seedlings may be jammed into pots that don't have adequate room for their root system.  In other cases, the seedlings are left too long in smaller pots before they are planted into larger pots.  When planted into the larger pots, the roots continue to grow in a circle and never fill out into the larger root area.

"Once formed, kinks, circles, and girdles in woody root systems have three possible fates: 1) the root dies; 2) the kinked or circled portion of the roots is pruned out at transplanting; or 3) the root lives... and grows in circumference until at some point the root girdles itself, the stem, or another main root.  This can restrict the flow of water and nutrients in the plant and compromise the ability of the tree to support itself," says Hummel.  It may take many years for the root problem to become apparent, evidenced only by the gradual decline of a tree or a tree may fail to grow well from the time of planting.

To diagnose a tree problem as a "root or soil problem," the owner must check the root system. It's easy to check the leaves or trunk of a tree for a problem, but how do you check the roots? You can look for girdling roots by inspecting the tree where it enters the ground.  In some cases you'll be able to see roots that are twisted around the trunk and are girdling or "choking" the tree.  However, girdling roots can also be under the surface of the soil.  A clue that the girdling root is a problem will be a lack of the normal flare to the trunk on one side of the tree.

If you suspect that there are girdling roots below the soil, you'll need to carefully excavate around the trunk.  You can do this by gently removing the soil from around the tree base until you find the main roots.  A watering can or hose can be used to help clean the roots for your scrutiny.  If the main roots radiate out from the trunk unimpeded by other roots, girdling roots are not the problem.  Girdling or encircling roots restrict the growth of the trunk and roots.  If girdling roots are found, a trained arborist can help you decide what action to take.  In some cases the offending root can be cut, solving the problem.  In other cases, the problem may require removal of the entire tree for safety. 

Younger trees that fail to thrive and grow after a year or two, should be checked to see if the roots have grown out of the original root ball.  This should also be done very carefully, trying not to sever roots.  Check is the depth of planting.  The top of the root ball of a mulched tree should be just below the soil surface.  Anything deeper than an inch or two is too deep.  In this situation, the roots can't get air and will gradually die... and the tree will die too. 

When you dig down to check the root depth, don't look for the fine feeder roots proliferating at the top of the root ball and assume that everything's satisfactory.  In some cases, this is the only place where roots have grown, because it's the only place that they could get some air.  You should find the main woody roots radiating out from the trunk just below the soil surface.  Ifthese main roots are deeper, you should replant the tree at the proper depth.  Do this in early spring before new growth begins or in the fall after the leaves drop.

Another "root" problem that may also be revealed in a root ball excavation... is the failure to loosen and cut encircling roots of container grown trees at planting time.  Locally, we've seen trees and shrubs that have been in the ground for five years or more which seem to do fairly well and then gradually die.  Upon removal, their owners have found that the root ball kept circling in the same pattern of the original pot and the roots never moved out of the root ball.  In fact, the root system even fit back in the original pot several years or more after planting!

Yet another "root" problem that shows up when tree root balls are excavated... is the failure of trees roots to grow beyond the original root ball with balled and burlapped trees.  While landscape contractors' standards often specify leaving the bottom portion of burlap around a root ball, there are problems with this practice.  In many cases the burlap fails to rot and roots don't grow beyond the burlap, even three years after planting.  To avoid this problem, remove the burlap once the tree is situated in the planting hole at the correct depth. Cut the burlap away from the root ball, moving of the root ball as little as possible.

Finally the old adage of "hindsight is better than foresight" doesn't necessarily hold true when considering girdling tree roots and related root problems.  Most of these problems are better avoided in advance by checking trees for kinked and girdling roots before planting and then following proper planting procedures.  In this case, hindsight is just too late to save many of the trees.  Pay attention to the roots before you plant and help your trees live longer!  

When Strong Winds Uproot Trees

When Strong Winds Uproot Trees

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

It’s not unusual in this region for strong winds to sometimes blow trees over and or at least cause them to lean a bit more than they had been before the windstorm. Why do trees fall over in the wind? It’s usually due to the failure of the root system to keep the tree upright, not the size of the tree.  In addition to absorbing water and nutrients, roots have the task of job of anchoring the tree along.

There are many diagrams of trees that show large trees with a big tap root directly beneath the trunk. This concept has been proven wrong by many different studies of tree root systems. Most trees do not have tap roots, especially in urban situations where the soil is compacted or where the there is a high water table.  You will not find tap roots on most of the trees in your yard. 

Most tree roots are in the top 6 to 24 inches of soil.  Tree root system consists of larger, woody perennial roots and smaller, fine feeder roots.  The woody perennial roots horizontally, radiating outward from the trunk.  Most of these roots will be found in the top six inches of soil, but vertical anchoring roots along this horizontal network may grow several feet deep or more.  The function of the woody perennial roots is anchorage, water conduction, mineral transport, and food storage. 

The fine feeder roots are much smaller than the perennial roots.  Their function is to absorb water and nutrients. They grow outward and upward from the perennial roots and are predominantly located in the top several inches of soil.  Feeder roots die and are replaced on a regular basis.  These feeder roots are only about 1/16 of an inch in diameter or smaller and increase the root system's surface area.  This is important because, the greater the surface area, the greater their capacity to provide water and nutrients to the tree.

Major perennial tree roots may not be as deep as we've imagined in the past but they are longer than most people think. The framework of roots often grows outward to a diameter one to two times the height of the tree.  This is much further than the simple branch spread or "drip‑line", where they were once thought to terminate.  On average, tree roots spread about three times the spread of the branches.  Studies have indicated that over 50 per cent of the roots are outside the drip‑line.

The major perennial roots have the job of anchoring the tree.  In order for a root system to support a large, long‑lived plant like a tree, it must be free of structural defects and the main structural roots should be distributed evenly around the trunk.  Wind‑throw results most often from poorly distributed roots or poorly developed root systems. 

Poorly distributed roots can result from kinks and circling roots; improper planting practices; physical damage or severing of roots; restriction of root growth; or death of roots from cultural or climatic factors.  Poorly developed root systems can result from improper planting practices; compacted and shallow soils; and poor growing conditions.

Roots fail to hold a tree upright when there isn't a healthy, well‑distributed root system.  When a tree uproots and falls over in the wind, we have to ask ourselves what caused its failure.  This takes examining the roots and trying to detect the cause.  It also involves reviewing the trees situation and history.  Were major tree roots severed recently or years ago?  Were there any girdling or kinked roots evident?  Was there a poorly developed system?  What might have been restricting root growth on one or more sides? 

Can fallen trees be uprighted and saved?  Large trees, over eight feet tall, can't and shouldn't be saved.  Removal is the only option.  Even if you can successfully upright the tree, it may become a hazard because of the damage that has occurred to woody roots anchoring the tree.  Uprighted trees can pose a serious hazard, especially as they grow in size and girth.

For trees smaller than eight feet, it may be possible to upright the tree and save it.  However, it will probably only be successful if one‑third to one‑half of the roots are still in the soil and not exposed and if the roots which are exposed are fairly compact and undisturbed. 

The process of “uprighting” the tree may require some type of lift equipment to pull the tree upright.  Before pulling it upright, you should remove some of the soil beneath the exposed root mass to allow the root mass to be situated at soil grade level.  Once you have the tree back in place, fill in the soil around the roots as needed, being sure to keep the tree at the same soil level as it had been growing.  Water the tree thoroughly to get rid of air pockets and settle the soil around the roots.

Since the tree has lost a portion of its anchoring roots when it blew over, you will need to provide some support in the form of staking.  Drive two stakes into firm ground about six to eight inches away from the trunk.  Then tie the trunk to the stake at the lowest height which stabilizes the tree in an acceptable upright position.  The stakes should be removed when the tree can stand by itself.  Check the tree every year to make sure it's still stable.  If it's not, removal may be needed to maintain safe conditions.

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.

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.

Large Trees and Surface Roots

Large Trees and Surface Roots

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist  

Have you ever noticed those big, gnarly roots of trees that come to the surface and create all sorts of problems with sidewalks, driveways, and lawns?  They can make mowing almost impossible.  What can you do to solve the problem... short of cutting down the trees?

How do those roots get there? The woody roots of most trees and shrubs are only about 12 to 18 inches deep in the soil.  These roots grow and radiate out from the tree trunk in a horizontal network. 

As a tree grows older and larger, their branches and trunks grow in girth or diameter and so do their roots.  As they grow in diameter, the roots “come to the surface,” but actually they’ve always been there, they’ve just gotten bigger.  Surface roots become a problem sooner or later on most large trees.  However, they become a problem more quickly with fast growing tree species, such as silver maple, Norway maple, cottonwood, sycamore, elm, and willow.

So what can be done about these roots?  Cutting them all off right at the trunk isn’t a good solution, if you want to keep the tree.  These larger roots are the major “pipelines” for the transport of water and nutrients to the tree.  When you sever the roots, you are preventing the uptake of water and nutrients.  We should also keep in mind that the roots help to anchor the tree.

How about covering the roots with soil?  Most tree species will not tolerated a significant change of grade.  Extra soil placed on top of the roots leads to the suffocation of roots and the eventual death of the tree.  The least drastic approach to take with “surface roots” is to mulch the problem area with a three to four inch layer of mulch.  A coarse mulch that allows for the free flow of air and water into the soil is best.  Some good choices include coarse bark mulch, wood chips, coarse compost, or pine needles.  Keep the materials six inches or more away from the trunk to deter rodents and to prevent damage from freezing and thawing.

Another option would be to cover the area with a shade tolerant groundcover.  Many times the root area directly beneath a tree is bare because the dense shade is not conducive to grass growth.  Soil erosion from wind and water often follows, making the area increasingly difficult to mow and quite unattractive.  Groundcovers adapted to shady conditions can be used to hide the roots and to avoid the need for mowing.  Consider the use of ajuga, vinca, pachysandra, or other shade tolerant groundcovers. 

It’s okay to plant a groundcover, but don’t come in and mound soil around the base of the tree so you can plant flowers or shrubs.  This soil around the base of the tree can lead to rot in that area and cause the gradual death of the tree.

Mulch or groundcovers aren’t going to solve the problem of roots that are damaging your foundation, driveway, or sidewalk.  Something has to be done! If the tree is already large you may want to consider the use of root pruning.  Root pruning will hurt the tree, but if done properly you can attempt to keep the damage to tolerable levels. 

WSU Cooperative Extension of Spokane County recommends pruning the roots over a period of at least three years.  They suggest in their bulletin “Surface Roots” to slice straight down into the soil at regular intervals along part of the length of the encroached structure or feature, marking where the cuts were made.  The following year make more cuts and continue yearly until the entire length has been pruned.  “If more than 1/3 of the tree’s roots are severed at any one time, there will likely be noticeable damage to the tree, such as dieback of limbs and branches, stunting of growth, and leaf drop.  Every year, you should also re-cut the previous years’ cuts to keep the roots from reentering the area.”

Keep in mind that you are compromising the structural integrity of the tree when you cut any of it’s major roots off, especially when they’re close to the trunk.  The more roots you cut, the bigger the compromise.  If a major portion of a root system must be pruned to save a structure or feature, it may be better to simply remove the tree. If you don’t remove the tree, it may come down in a windstorm.

If you have a younger tree that you realize was planted in the wrong place and will eventually be the cause of a root and structure clash in the future, you can perform root pruning earlier to prevent damage from occurring and to avoid significant damage to the tree later in life.  This type of root pruning is done by slicing straight down into the soil about 12 inches away from the structure or feature.  You are slicing through small roots instead of large major roots of mature trees.  To prevent encroachment in the future, the roots must be cut along the same line every year.  This same procedure can be used if tree roots are encroaching on garden areas.  The earlier you start and the smaller the roots cut, the less damage that will occur to the tree.

The real cause of troublesome surface roots is planting a large tree species that is very fast growing and placing it where it will become a problem.  Select the right size tree and situate it where it won’t be a problem as it grows larger.