Because many plant samples show decline and death from effects of deep planting, we discussed proper planting depth in issues 13 and 14 of this newsletter. Adding too much mulch around the base of a tree or shrub can have the same effect as planting too deeply. The mulch can actually kill the plant it is intended to help, essentially smothering it with kindness.
A mulch is a material that is applied to the surface of the soil around a plant to maintain favorable soil conditions. We generally suggest using an organic mulch such as compost, leaves, bark, various hulls or shells, and pine needles. You can find out more about specific materials at the University of Illinois Extension Web site http://www.extension.uiuc.edu/IPLANT/. Inorganic mulches, such as stone or brick chips, are materials that do not decompose. Inorganic mulches often serve a purpose in design but are usually more expensive, do not improve the soil, and can be costly to remove if a design change is desired.
Mulch helps to insulate the soil. Certainly the soil becomes hot or cold with time anyway, but mulch helps make this change more gradual. We all know too well that sudden temperature drops can be extremely damaging to trees and shrubs, predisposing them to infection by canker fungi and other pathogens such as Verticillium. Mulch also has been shown to keep soil temperatures as much as 10o cooler in the summer.
The National Arbor Day Foundation recommends removing grass in the area to be mulched and mulching an area from 3 to 10 feet in diameter around a tree, depending on the tree size. You may see differences in depth recommendations, but we tend to advise 2 to 4 inches of mulch. If you add more mulch–thinking that more is better–you may be causing other problems. Roots need oxygen to grow. If soil is always saturated with moisture, roots begin to decline. In a wet season, planting beds with very thick mulch do not dry out. This is especially important on clay soils or in newer subdivisions where soil is compacted or has poor drainage. White pine is a good example of how too little or too much mulch can be a problem. We have been seeing white pine problems for over 20 years in Illinois. They do not grow well in alkaline, clay, poorly drained, hot soils. Because we tend to plant them as windbreaks or in exposed sites, we make many mistakes right from the planting date. White pines benefit greatly from mulch, especially due to the advantage of insulating roots from high soil temperatures and maintaining soil moisture. If mulch is used at an excessive depth, however, roots are stressed, and wet conditions promote a root rot called Phytophthora.
The most important message concerning mulching is to keep the mulch away from the tree trunk by at least 4 inches. Physical contact with the tree is not lethal. Problems occur when the mulch is several inches thick against the trunk. This collar area of the tree needs air exchange. Moisture held up against the trunk prevents this and results in tree decline. In addition, the mulch may serve as an overwintering site for rodents, while the bark of the trunk provides a good food source.
The Plant Clinic frequently fields questions on mushrooms or fungi growing in mulch, especially bark mulches. These fungi are not harmful to plants. They are growing in the mulch because it is an organic source of nutrients. The fungi also must have moisture to grow. In dry spells, we tend to water our planting beds, so we see these fungi all summer long, rain or no rain. I am not advocating that you remove the mulch nor that you stop watering your plants. Don't look at the fungicide shelf as a solution to mushrooms in your compost. Most fungicides won't have any effect on these mushrooms. Instead, get out the rake and mix up the bark mulch. This helps it dry out and keep mushrooms under control.