What is a mulch volcano? It is a large buildup of mulch, intentionally placed around the root flare of a tree and often up its trunk as well. A common practice visible in parking lots of commercial areas, these mulch volcanoes are done to make trees look pleasing and to suffocate weeds to reduce maintenance over the summer months. This practice creates a “lollipop effect,” where tree trunks appear to rise straight up out of the mulch.
Mulch volcanoes kill trees. This landscaping trend causes stress and difficulty for trees trying to survive in urban and suburban habitats. The popular PBS TV show, “This Old House,” produced an informative video showing what is in a mulch volcano and the damage this practice causes:
Tree experts from Cornell, across New York State, and around the country have condemned mulch volcanoes as extremely harmful to the health of trees.
Consider that mulch does more than spruce up your yard. It could ruin your landscape if not done right. Your yard is an ecosystem, and death of a tree removes something out of the ecosystem, leading to consequences that could drop the value of your home. A walk in the woods will show you tree flares, exposed trunks, and tree roots living above ground naturally.
The base of the trunk of the tree, whether newly planted or established, must be exposed and flare out. This is the basal root flare, the transition between the trunk and the root system. The root flare needs to be exposed to allow excess moisture to escape, preventing growth of fungus and minimizing rot and decay. Mulch can help a newly planted tree by keeping its root system covered, holding nutrients in the soil, hindering weed growth, and holding moisture on the soil line to keep young trees hydrated. Mulch prevents soil compaction, which is bad for root development, and it reduces the amount of freezing and cold exposure to the root system by insulating the ground.
Mulch volcanoes begin as a newly planted tree needs coverage to protect its young roots and base. Placing too much mulch around the flare and trunk creates the first layer of buildup. The following seasons, as more mulch is placed around the tree's flare and trunk, the mulch mats itself together to form a barrier. The constricting layers of mulch bury those areas that naturally need to be exposed.
As mulch volcanoes bury tree trunks and flares, moisture collects that leads to rot around these vulnerable areas. Once rot sets into the trunk and flare, bark and protective structures become weakened and exposed. The bark cracks, peels, or comes off, and internal structures are vulnerable to insects, disease, decay, and malformation. These all move in and slowly kill the tree.
Mulch volcanoes have several side effects, including creating a compost pile, where the material becomes hot enough to kill the inner bark of young trees or prevent the natural hardening off period where trees prepare for winter. These mountainous monstrosities promote growth of secondary roots in the mulch rather than in the surrounding soil. Roots circle around the tree and can eventually choke off the main roots. In periods of heavy rainfall, the tree can drown or be more susceptible to rot due to the sponge-like nature of mulch. Heavy layers of mulch can be colonized by water-repelling fungi, which will actually turn the pile into a hydrophobic area, leaving the tree in drought conditions despite receiving water.
The proper way to mulch around a tree appears more like a doughnut. The depth of the ring should be 2 to 4 inches at most. For soils that are poorly drained such as clay, only 2 inches of mulch is needed. Once the mulch is applied, pull the mulch away from the tree trunk by 5 to 6 inches. You should be able to see the tree trunk and the flare of the tree. The diameter of the mulch should extend to the drip line of the canopy.
An important factor to keep in mind is that although mulch color will fade, this doesn’t mean we should top off the organic matter with a few inches of fresh material. Measure the mulch levels before deciding to add more, or you could end up creating a deadly environment for the tree.