Mulch: too much of a good thing may be killing your trees!

By VJ Comai

Despite the lingering snow cover and below-freezing temperatures, spring is just around the corner, and according to the calendar, it has already arrived. Once the snow has left and we begin seeing moderating temperatures and that slow-but-welcome greening of our world, many of you will begin the annual ritual of cleaning up your yards and tending to your landscape trees and shrubs.

Part of this ritual often includes applying a fresh layer of bark mulch on shrub beds and around trees in the landscape. The smell of fresh mulch along with the look often leaves homeowners with a feeling of satisfaction and the sense that they are taking good care of their property, and to forgo this annual ritual would be a sign of neglect.

The benefits of covering the root zone of trees and landscape plants with a thin layer of mulch are many. A proper layer of mulch simulates the effects that the buildup of decaying leaf matter and woody debris has around the base of trees in their natural environment. This covering of organic matter around plants in the home landscape helps to retain moisture for plant roots, moderates soil temperature favoring roots’ development and function, reduces competition from weeds and turf, reduces soil compaction, and keeps lawnmowers and weedwackers from damaging the base of trees. As the mulch slowly decomposes over time, it helps to improve soil quality by increasing organic matter and providing a more favorable environment for beneficial microorganisms.

Mulch should not exceed a depth of three inches, and should not be in contact with the bark and stems of trees. To clarify, three inches should be the total depth of the mulch, not the amount applied on an annual basis, and therein lies the problem.

Each spring, the annual ritual ensues and I watch in horror as homeowners and landscape maintenance laborers alike pile mulch around the base of trees where it already exists in excess and is contributing to the premature decline and death of the tree. It is truly an epidemic and is one of the leading killers of trees in our urban landscapes.

Excessive mulch can actually suffocate plant roots by inhibiting air exchange from the soil and creating an excessively water-soaked soil that diminishes root function. When roots decline and die, the tree declines. Mulch piled against the bark of trees creates a continuously moist environment that can eventually lead to the death of the inner bark (phloem), which is the vascular tissue responsible for the transport of photosynthates from the crown of the tree back to the root system, further contributing to decline.

Trees stressed and in decline due to the effects of over-mulching are far more vulnerable to disease and insect problems, and by time the tree begins to show outward signs of decline from over-mulching, it is often too late to do anything about it.
This spring, refrain from taking part in the annual ritual of over-mulching and save yourself some time and money. Check the depth of mulch around the base of your trees. If there is more than three inches, remove it and be sure to pull mulch away from the base of trees so that it is not in contact with bark. Lightly raking over existing mulch will give it a fresh appearance and will help to increase air exchange to the roots while aiding in the decomposition of the mulch. If you already have a sufficient layer of mulch around your trees, adding more will be of no benefit and may be detrimental to the health of your trees.

V.J. Comai resides in Charlotte and works as an arborist representative for Bartlett Tree Experts. For nearly 25 years he has managed the South Forty Nursery in Charlotte where he grew field produced trees and shrubs for the wholesale market. He is a certified arborist with the International Society of Arboriculture, (ISA) a past president of Green Works, The Vermont Nursery and Landscape Association.

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