Most of us are well aware of the benefits of using natural mulches in planting beds and around trees. Remember, too much of a good thing can yield bad results. 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. Keep these ideas in mind as you add mulch to your planting beds.
A mulch is a material that is applied to the surface of the soil around a plant with the intent of maintaining favorable soil conditions. We generally suggest use of 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/ where mulching is discussed. Inorganic mulches are materials that do not decompose so they do not add organic matter to the soil. They include such materials as stones and brick chips. 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 will become hot or cold with time anyway, but mulch helps make this change more gradual. Sudden temperature drops can be extremely damaging to trees and shrubs, predisposing them to infection by canker fungi and other pathogens such as Verticillium. For this reason, mulching can help you avoid some diseases. Mulch also has been shown to keep soil temperatures as much as 10 degrees cooler in the summer, again avoiding heat stress and disease susceptibility.
The National Arbor Day Foundation recommends removing grass in the area to be mulched and mulching in an area around the tree that is from 3 to 10 feet in diameter, depending on the tree size. You will see differences in mulching depth recommendations, but we tend to advise that mulch be not less than 2 inches deep and no deeper than 4 inches. If you add more mulch, thinking that more is better, you may cause 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. Problems with white pine provide a good example of how too little and too much mulch can be troublesome. We have been seeing white pine problems for over 20 years in Illinois. This species does 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 because of 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 about mulching is to keep the mulch at least 4 inches away from the tree trunk. Physical contact of the mulch 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 does not allow this to happen, and tree decline results.
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 removal of the mulch, and I donít think you should stop watering your plants. Donít look at the fungicide shelf as a solution to mushrooms in your compost. Most fungicides wonít have any effect on these mushrooms. Instead, get out the rake and mix up the bark mulch. This will help it dry out and will keep mushrooms under control.