Around town, piles of mulch are built up like volcanoes around the bases of trees. Perhaps it is because there is extra mulch: If a little is good, than a lot must be great. Perhaps it is an artistic statement. Perhaps it is learned from watching others in the neighborhood.
The reasoning dumbfounds horticulturists because the practice kills the trees slowly.
University of Illinois Extension Horticulture Educator Martha Smith explains the slow death. "Problems can emerge because of this practice. Tree bark is meant to protect the trunk. It works best in the air and light. If you pile mulch onto the bark, it is now exposed to dark and moisture. Bark will begin to rot, and rotted bark cannot protect the tree from insects and diseases. In fact, diseases grow better in this type of environment."
Mulch volcanoes build up heat, killing living tissue and causing roots to grow in the mulch rather than in the ground. The latter occurs most often with newly planted trees.
Mulching your trees conserves moisture, prevent weeds, moderates soil temperature and adds organic matter slowly as it breaks down. Mulch can also indicate the boundaries of the yard.
However, your mulch ring should look like a bagel and not a muffin. Apply mulch 2- to 4 inches deep, as wide as the drip line, and keep the mulch at least two to three inches from around the base of the trunk. The larger the mulch ring, the better. Mulch breaks down at different rates according the kind you choose, so make sure it is not too deep before you decide to add a fresh layer. Use a medium coarse mulch, like hardwood chips; fine mulch can conserve too much moisture. Plastic or weed barriers prevent oxygen from getting to the roots.
A yellow fungus may grow on mulch during warm, wet periods. This is called dog vomit fungus, and occurs as fungus breaks down the mulch. It will not harm your plants. If it looks bad, break it up and let it dry out.
Mulch is of great benefit to the health of trees but covering the base of the trunk is an inevitable death sentence.