The Homeowners Column

The Homeowners Column

Mulching Trees Improves Growth

June 21, 2003

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

Mulching trees isn't particularly new and innovative. Mother Nature has been doing it for years. Most of our landscape trees grow in a forest in their native areas and are naturally mulched by leaf litter and plant debris. The growth and health of landscape trees can be dramatically improved by the proper use of mulches.

Mulches are any materials used on the surface of the soil. This broad definition could include organic materials such as wood chips, pine needles, straw, peat moss, corncobs and lawn clippings. Inorganic mulches would include river rock, ground tires, volcanic rock and synthetic fabrics. However, not all mulches are good for plants.

Organic mulches are preferred for plant growth because of their ability to improve soil structure and provide a more natural environment for good root growth and, therefore, good top growth of the plant or tree.

Rock and stone mulches can cause real problems for trees and people according to Bill Vander Weit, City Forester for the City of Champaign. While trees prefer an acidic soil, most rock is alkaline which can lead to micronutrient deficiencies. Rocks also retain heat that can translate into moisture stress for trees.

The presence of rocks also creates difficulties when a tree needs to be removed. Despite the best efforts to remove rocks, the high-speed cutting wheel of a stump grinder can hurl buried rocks long distances. Rock can also become embedded in trees as they grow.

Vander Weit also states that placing rock or stone within the dripline of any tree on city right-of-way (those trees generally between the curb and sidewalk) is prohibited without a permit. Typically the use of rock is only permitted if the tree is surrounded completely by sidewalk, as in downtown or campus plantings.

Unfortunately even organic mulching can be overdone, witnessed by the all too common mulch "volcano" piled around trees. Overmulching results in excessive moisture in the root and trunk areas leading to root or crown rot. Deep mulch can also impede water and air penetration and may create habitat for bark chewing rodents. Tree Mulching recommendations:

  • Use organic mulch such as wood chips or compost. Use wood chips that have been composted for at least three months.
  • Apply composted material to the soil surface and top with coarser and fresher material. Incorporation into the soil is not recommended or necessary.
  • Mulch should be about 2-4 inches deep. Two inches for poorly drained soils. Do not exceed six inches in depth.
  • The larger the mulched area the better, but it should reach at least to the tree's drip line (the outer perimeter of the branches).
  • Do not use geotextile landscape fabrics beneath the mulch, as this prevents the decomposing mulch from mixing with the soil. Plus the weeds just grow on top and through the fabric.
  • Keep mulch at least six inches away from the tree trunk.
  • Avoid disrupting the mulch with annual flowers. If plants are desired, use perennial ground covers such as vinca or English ivy.

Check out the Landscape Recycling Center at 1210 East University Avenue in Urbana for a variety of mulches. Call 217-344-5323.

The International Society of Arboriculture has an excellent brochure, Proper Mulching Techniques as well as lots of information on tree care. Just go to www.isa-arbor.com, select Publications, and then select Tree Care Consumer Guide.

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