This article is part of the “Tree News You Can Use” series and is provided by the UTIA Residential and Community Forestry Workgroup.
Mulch is any material that is spread or laid over the surface of the soil as a covering to reduce moisture evaporation, allowing more water to remain in the soil that is available to plant roots. When properly applied, mulches help to reduce weed pressures by inhibiting their growth and blocking access to sunlight.
Properly applied mulches provide substantial benefits to urban trees, including the slowing of water loss through evaporation, soil temperature regulation, improvement of soil structure and nutrient content, and inhibition of weed growth. Additionally, there are subjective benefits to tree aesthetics that mulch provides by giving an appearance of uniformity and tidiness. However, when applied improperly, mulch may actually be a detriment to a tree’s health.
Types of Mulch
The first step to ensuring that your trees enjoy the benefits of mulching is to select a high-quality mulch. The most common organic material used for mulch is woodchips. Hard and softwood barks, pine needles, and other plant materials are commonly used. Woodchips may be dyed for uniform coloration; if raw (untreated) woodchips are selected for mulching purposes, it’s best to ensure that the trees chipped were not killed by any sort of pathogen!
How Much is Too Much?
When mulch is applied, it must be done properly. One of the most common errors is using too much! Proper mulching depth is only 2-4 inches, and mulches should (ideally) extend to the dripline of the tree. If too much mulch has been applied, make mulch rings a larger diameter rather than deeper. Avoid mulch “volcanoes!” These piles of mulch around the tree trunks lock moisture around the stem and may contribute to tree decay or disease. In fact, properly mulched trees will have space between the trunk and the mulch ring. The entirety of the trunk flare should be visible after mulch is applied. For more information, visit: https://trace.tennessee.edu/cgi/viewcontent.cgi?article=1073&context=utk_agexfores.