Too much of a good thing, mulch volcanoes slowly kill trees

URBANA, Ill. – Mulch volcanoes have not quite made it as a new dictionary entry, but to landscapers, arborists, and gardeners, they are a new phenomenon.

“A mountain of mulch, piled high against a tree trunk will not kill it immediately,” says University of Illinois Extension horticulture educator Martha Smith. “Rather, it is a slow death, and homeowners don’t associate their actions with tree decline several years after they over-mulched a tree.”

Adding mulch around the tree helps maintain soil moisture and control weeds. Evaporation is reduced, and the need for watering can be minimized. A mulch layer will suppress weed seeds from germinating but remember that lawnmower clippings blow onto mulch and animals will bring in weeds that may grow on top.

"Mulch serves as nature’s insulating blanket," Smith says. "Mulch keeps soil temperatures more consistent in the winter and cooler in the summer. It can improve soil aeration, structure, and drainage over time and lowers maintenance needs

Mulch can reduce the likelihood of damage from trimmers and mowers, Smith adds, and can give planting beds a uniform look.

How to apply mulch

Mulch should be 2 to 3 inches deep. When first applied, it may appear deeper, but will settle. Organic mulches include wood chips, pine needles, hardwood or softwood bark, cocoa hulls, leaves, compost mixes, and a variety of other plant-based products. Organic mulches decompose at different rates. Those that decompose faster must be replenished more often. Some mulches, such as cypress, remain intact for years, but eventually turn a gray-tan color.

“People prefer the fresh look of new mulch and top-dress annually, not considering the existing mulch depth," Smith says. Deep mulch can lead to excess moisture in the root zone leading to root rot.

"Insect and disease problems can build up when thick layers of fine mulch become matted and prevent the penetration of water and air," Smith says. "And anaerobic sour mulch may give off odors." 

Mulch volcanoes

Recently, people have started mounding mulch up around the base of trees, a practice called mulch volcanoes. New problems have emerged because of it.

Tree bark is meant to protect the trunk. It works best in the air and light. Piling mulch onto the bark exposes it to dark and moisture. The bark will start to rot, and rotted bark cannot protect the tree from insects and diseases, which grow better in this type of environment, Smith says.

Tree volcanoes cause a variety of issues:

  • If mulch is too heavy, it can deprive the roots of oxygen and greatly reduce the soil’s ability to dry out.
  • Mulch produces heat as it breaks down. This heat may directly kill the inner bark layer of young trees or may prevent the natural hardening off period that plants go through in the fall to prepare for winter.
  • Mulch piled around the trunk promotes the growth of secondary roots, which can encircle the trunk and choke off the tree’s main roots. Some trees, such as maples, have shallow roots, and deep mulch encourages these roots to grow into it.          

For more information on mulching, check out Mulching Landscape Trees or How to Mulch Trees and Shrubs.

University of Illinois Extension is the flagship outreach effort of the University of Illinois at Urbana-Champaign, offering educational programs to residents of all of Illinois' 102 counties and far beyond. Illinois Extension provides practical education you can trust to help people, businesses, and communities solve problems, develop skills, and build a better future. Through our Agriculture and Natural Resources programs, Illinois Extension supports the economic viability and environmental sustainability of natural and managed landscapes and productive lands in Illinois. Horticulture program educators provide research-based information and training about gardening, fruits and vegetables, flowers, insects and diseases, composting, landscaping, and more.

News source/writerMartha A. Smith, Horticulture Educator, Illinois Extension