Extension Report

Baldwin County Extension Office

302A Byrne Street

Bay Minette, AL  36507

Telephone (251) 937-7176 or

928-0860/943-5061 ext. 2222

FAX (251) 937-7285

Beau Brodbeck

Regional Extension Agent

Forestry, Wildlife & Natural Resources

December 7, 2010

Mulch; The Silver Bullet for Healthy Urban Trees

Why mulch around trees?

First, consider the natural setting of trees, the forest.  Forests have evolved overtime and developed a complex system of nutrient cycling.  As forest trees grow, they draw nitrogen and other nutrients from the soil, which are returned as leaves, twigs, branches, bark and dead trunks that fall to the forest floor.  Over time this develops a thick mat of compost rich in microorganisms working to decompose this material forming the top organic layer. The organic layer and the associated microorganisms help aerate the soil and create an optimal situation for tree roots and healthy trees. 

Second, consider the urban environment.  Streets, sidewalks, driveways, grass and houses are the dominant features.  Unfortunately, the installation of most structures and features often results in the removal of the organic layer.  The construction process with its heavy equipment, stockpiling of materials and importing of sandy clays, ideal for solid home and road foundations, cause compaction that result in root death for existing trees and a hostile environment for newly planted trees. 

The complex nutrient cycle is broken.  The rich forest floor is transformed into a soil low in available nutrients, organic matter and the vital microorganisms needed to make soils well aerated and accommodating to tree roots.   Your best tool to reverse this situation is mulch. The process of mulching is nothing more than an attempt to mimic the natural forest floor by layering organic material under trees to create an environment that helps root growth and overall tree health.

What are the benefits of mulching?

Other than mimicking the natural forest environment, mulch has many positive attributes that contribute towards healthy trees in the urban setting.  The following are benefits achieved through regular mulching:

• Moderates temperature of the root zone.  Mulch insulates the soil from extreme cold and heat providing better rooting conditions.
• Contains soil moisture by keeping down evaporation of water from the soil.
• Improves soil aeration and drainage by preventing the soil from crusting over and encouraging micro biotic activity.
• Prevents soil compaction by dispersing weight of foot or vehicular traffic.
• Adds organic matter to the soil and can improve soil fertility over time.
• Controls weeds by reducing the germination and growth of weeds
• Facilitates maintenance by reducing likelihood of injury from lawn mowers and string trimmers commonly used for weed control around the base of young trees.
• Helps prevent soil erosion by stabilizing the soil, reducing the impact of raindrops, and evenly dispersing rain water.

What type of mulch should I use?

There are many types of mulches available at most garden centers today.  There are two main distinctions to consider, inorganic versus organic mulches.   Inorganic mulches such as pulverized rubber, lava rock, stone, geotextiles and other products are advantageous only in that they do not have to be replenished over time.  These products do not decompose like organic mulches meaning they contribute little in the way of organic matter, soil nutrients, or improving aeration or water infiltration. 

Organic mulches, preferred by most arborists and urban foresters, include wood chips, pine straw, bark, leaves, compost mixes and various other products generally derived from plants.  Organic mulches will decompose over time and depending on the type of mulch and climate will need to be replenished generally about once per year.  It is important to not apply 3 inches every year as overtime this will lead to over mulching.

The most important factor when selecting organic mulch is to ensure that they have been properly composed or aged.  Newly harvested wood chips or grass clippings for example do not have sufficient nitrogen to decompose the organic material.  The use of these un-composed products will often rob nitrogen from the soil and thus reduce soil fertility.  Additionally, these products can also produce heat during decay and injure young trees or plants.

Pine straw, woodchips, and bark are all commonly sold in this region and will all have positive impacts on your landscape.  While, some mulches may have some minor impacts on soil PH (pine straw and oak leaves are more acidic while hardwood chips or bark are more alkaline), overall they will improve soil properties and overall tree health.  For the most part, mulch selection is a factor of aesthetics and availability. 

How should mulch be applied?

When discussing proper mulching techniques it’s important to remember that the root systems of trees are not mirror images of the top of the tree.  Rather, the root systems of most trees are shallow, often in the top six inches of the soil, and also widespread.  It is not unusual for tree roots to far exceed their drip line, often by twice the width of the canopy.  Remember roots need water, air and nutrients all of which are generally found closer to the soil surface where rain, air and organic compost are prevalent.

Keeping this in mind, mulch should be applied in a layer between 2 to 3 inches thick and at least to the drip line of the tree.  This allows the mulch to give maximum benefit to the trees root system.  Applying mulch in a thicker layer may have negative effects on trees by reducing air and water infiltration to the soil and root system.  It is important to not pile mulch around the base of the tree in a volcano fashion. 

Moist mulch in continuous contact with the tree’s trunk can allow the introduction of fungus and insects into the thin bark of young trees.  Also, deep mulch piled around the base of trees can cause advantageous roots to grow up into the mulch and form girdling roots that slowly strangle the tree.  Apply mulch so that it does not touch the tree and allow for deeper mulch (not to exceed 2-3 inches) as you move away from the trunk. 

Over time the mulch will decompose.  It is important before replenishing mulch to check how much last year’s mulch has decomposed before adding another 2 to 3 inches.  If the mulch is decomposing slower, it may only be necessary to add 1 to 2 inches.  Burying trees with too much mulch over time can also be a problem.

For additional information on mulching and mulch selection, please review the Alabama Cooperative Extension Publication on landscape mulches found at the following site http://www.aces.edu/pubs/docs/A/ANR-0385/ ) or contact Beau Brodbeck by email (brodbam@auburn.edu).

Email address: brodbam@auburn.edu

Phone: 937-7176 or 943-5061, ext. 2222

The Alabama Cooperative Extension System (Alabama A&M and Auburn Universities) in cooperation with the U.S. Department of Agriculture is an equal opportunity educator and employer.