by: Robert Smith

(The following is an article compiled by Ellen Bauske, Program Coordinator, Center for Urban Agriculture, and Willie Chance, Outreach Coordinator, Center for Urban Agriculture, that covers a landscaping topic relevant to our area).
    Landscapers often apply a thick layer of mulch around trees in a ring. When this mulch is piled against the trunk, this is called a mulch volcano. Though this is better than having no mulch at all, mulch volcanoes can cause several health problems for trees.
    Mulch helps trees in several ways:
    •  Controls weeds and keeps soil moist
    •  Moderates very high or low soil temperatures.
    •  Improves tree roots and overall tree health.
    •  Reduces trunk injury and soil compaction from equipment.
    However, according to Chris Starbuck at the University of Missouri, mulch volcanoes can severely drought stress newly planted trees.
     1. Mulch volcanoes encourage poor root growth. Roots buried by the mulch volcano die due to lack of oxygen in the water-logged soil. At the same time, growing conditions in the top of the mulch are temporarily favorable for root growth. This causes roots to grow upward instead of downward. Mulch does not hold water as well as soil and eventually dries out. This stresses the tree’s roots and puts the tree into severe drought stress.
     2. The mulch volcano can also act as an umbrella, shedding water to the surrounding soil.  Fungal activity in the surface of mulch volcanoes can make the mulch repel water. Water then runs off the volcano, rather than moving into it.
    This usually happens more with mulches that are high in carbon like ground wood, wood chips or sawdust. It also happens with bark mulches. Since new trees have few roots the root ball must be kept moist. Volcano mulching can keep irrigation and rain from the root ball and lead to severe drought stress even if the tree has a regular water supply.
     3.  Mulch volcanoes encourage fungal canker diseases. Starbuck also notes that mulch volcanoes keep the base of the tree constantly moist. Trees are also stressed because the cells in the bark of the tree cannot get enough oxygen. This can cause bark decay. Finally, volcano mulching harbors termites and rodents that may attack the tree.
     Once the trunk is damaged, there is usually little that can be done to reverse the damage.
    Avoid mulch volcanoes when mulching. Do not pile mulch around the base of the tree. Apply a two to three inch mulch evenly around the tree, preferably out to the edge of the drip line.
     For more information: Call your local Extension Agent at (800) ASK-UGA1 or locate your local Extension Office at http://www.caes.uga.edu/extension/statewide.cfm

Published in the January 22, 2009 edition.