You may not find the phrase "mulch volcanoes" in the dictionary, said a University of Illinois Extension horticulture educator, but the concept has been gaining ground and can create problems
"To landscapers, arborists and gardeners this is a phenomenon that wasn't so prevalent years ago," said Martha Smith. "Hm-m-m, maybe we should call Mr. Webster... "Why do we mulch? Let's review."
Mulching helps maintain moisture.
"Evaporation is reduced, and the need for watering can be minimized," she said. "It also helps control weeds. A mulch layer will suppress weeds from germinating at the soil line. Remember lawn mower clippings blown onto mulch and animals may bring in weed seeds that may germinate on top of the mulch."
Mulch serves as nature's insulating blanket. Many organic types of mulch can improve soil aeration, structure (aggregation of soil particles), and drainage over time as they decompose. "Mulching lowers maintenance needs and can reduce the likelihood of damage from 'weed whackers,' or the dreaded 'lawn mower-itis' when equipment gets too close," she said. "Mulch can give planting beds a uniform, well-cared for look."
Mulch should be between two to four inches deep. Often, when applied, it appears deeper, but after settling you should end up with a two-inch matted layer.
"Organic mulches include wood chips, pine needles, hardwood or softwood bark, cocoa hulls, leaves, compost mixes, and a variety of other products usually derived from plants," said Smith. "Organic mulches decompose in the landscape at different rates depending on the material.
"Those that decompose faster must be replenished more often. Here lies the problem. Some mulches, such as cypress mulch, remain intact for years but they turn a gray-tan color. People prefer the "fresh" look of new mulch and top dress annually, not considering the existing mulch depth."
Deep mulch, she added, can lead to excess moisture in the root zone leading to root rot and insect and disease problems. "If mulch is too heavy you can deprive the roots of oxygen and greatly reduce the soil's ability to dry out." Thick layers of fine mulch can become matted and may prevent the penetration of water and air, and anaerobic "sour" mulch may give off odors.
"These issues have been on-going, but in recent years people have started to mound mulch up around the base of trees-the 'mulch volcano' (pictured below). New problems have emerged because of this practice," said Smith. "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 breakdown can produce heat. Think of your compost pile reaching temperatures above 140°F. Mulch piled high around the tree trunk can get hot. This heat may directly kill the inner bark/phloem layer of young trees, or may prevent the natural hardening off period that plants must go through in the fall in preparation for the winter.
"Mulch piled around the trunk promotes the growth of secondary roots, which can encircle the trunk and choke off the trees main roots," she cautioned. "Some trees have shallow roots such as maples, and deep mulch encourages these roots to grow into it.
"A mountain of mulch, piled high against a tree trunk will not kill the tree immediately - it results in slow death. Homeowners don't associate their actions with tree decline several years after they over mulched a tree."