Mulching More is not Always Better!

Carol Morton
Adams County Master Gardener

While traveling around Adams County this spring, Ive noticed pyramid-shaped mulch piled up around trees and shrubs. It is especially noticeable with the newer red mulch that has become so popular. Mulch should only be 2 to 4 inches deep over the entire area and always scooped back from the trunk of the tree about 2-3 inches.

The roots of plants need a constant supply of oxygen. When mulch over 4 inches deep is placed over the root system, the mulch may smother the roots by reducing gas exchange and the plant may die. This is particularly true with young plants. Death from over mulching is gradual, starting with a decline in plant vigor and rate of growth.

Symptoms of too much mulch are chlorotic foliage (resembling iron deficiency), abnormally small leaves, poor growth and dieback of older branches. When excessive mulch is applied around the trunk of the tree, cankers, bark decay and other health problems may develop.

Some species send out new roots from the stems into the mulch rather than expanding the root system into the soil. This may result in drought and freeze damage as the roots are exposed. Disease-causing organisms also thrive in the cool, moist environment of heavy mulch often killing the tree.

Deep layers of mulch will also absorb all the water from light rains, keeping it above the root system. During wet seasons the heavy mulching may lead to waterlogged soil, especially in heavy clay loam soils. The extra heavy layers in the fall can be a nesting area for rodents, which may lead to girdling of selected plants when wildlife food sources are scarce. Pull all mulch back approximately 6 inches from the trunk in the fall to avoid rodent nesting.

Heavy mulch may be a result of repeated applications when the original mulch loses its color as it decomposes. Try to keep the mulch level at 2-4 inches by pulling back the old mulch and mix with the new or use it in other areas. Shallow raking of the existing mulch will spruce up the appearance until new mulch is required.

There are many recommended mulch materials available at varying costs from local garden and landscape businesses. Composting yard wastes and brush can also be chipped and used as mulch. Never use black plastic to mulch around trees or shrubs.

Mulching is very important. Tests have shown that mulched soil can be as much as 30 degrees cooler than bare soil. Mulch helps to prevent weed growth, conserves moisture in the soil, reduces heaving of small plants when freezing and thawing of the soil occurs, reduces soil erosion on slopes, improves aesthetics of a landscape and adds to the property values.

So, keep on mulching; however, remember to only mulch 2-4 inches and keep the mulch 2-3 inches away from the trunk.

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