Anything in excess is detrimental to the development of a plant. So like sunlight, water, and fertilizer, mulching should be applied effectively to avoid negative consequences.
Mulching is done by covering the surface of the soil with material such as bark, wood chips, leaves, or plastic. The process helps deter weed growth while keeping the soil cool and moist. It also prevents soil erosion and if the material used for mulching is biodegradable, it also helps provide nutrients in the soil.
Overdoing mulching, however, is not only a waste of material, but it negatively affects plants in many ways.
Overmulching traps excessive moisture which makes plants more susceptible to diseases. Overly dense mulching also prevents moisture and oxygen from reaching the soil. This can over-insulate the soil, encourage mold growth, and result in awful odors. Overly large humps of mulch can also attract rodents and other small animals to nibble on the mulch and then on the plant afterward.
To prevent overmulching, apply about two to four inches of mulch around the base of the tree. Do not cover the stem or leaves with mulch and keep it away from the stem by four to six inches.
The mulching material should be light enough to let rain and air pass through it but not too light that wind easily blows it away. It should also be dense enough that it is capable of holding water.
Another consideration is that mulching material should be free from contamination. If a tree was sprayed with pesticide, its leaves should not be used as mulch. For biodegradable mulch, its decomposition rate should also be studied to avoid needing to re-apply it too frequently.