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