A mulch volcano, which is the term used to refer to mulch piled around the base of a tree or shrub, is very harmful to the health of the tree. They look innocuous enough, just simple mounds of mulch around a tree. But underneath lurks a hotbed of trouble waiting to slowly erupt. They form under good intentions, often to help protect trees from lawn mowers and weed whackers. When the mulch is too thick, those well-meaning deeds lead to more stress for the tree.
The concept is simple: trees and shrubs have different parts that serve specific purposes, and each part has adapted to certain conditions. To be specific, the roots of a plant are designed to grow in the soil and soak up moisture. The trunk, in contrast, is designed to grow above ground where conditions are dry. But piling mulch around the base of a tree or woody shrub keeps the trunk moist, thereby suffocating cells due to water saturation.
Roots respire, meaning they take in oxygen and release carbon dioxide. This exchange of gases, as it is known, takes place in the upper 18 inches of soil. It’s no accident that roots proliferate in the top 18 inches because this is where oxygen is most readily available. Bring in a boatload of mulch and suddenly the lower roots no longer have access to oxygen. This leads to stressed and dying roots which in turn stresses the tree. A stressed tree has fewer defenses than its properly mulched counterpart, leading to susceptibility to otherwise-minor insect and disease problems.
This type of mulching also harms the cells of the trunk. As the cells become damaged, they are not able to perform their duties, which include gas exchange and moving food through the plant, just to name a few. Furthermore, a mulch volcano provides an opportunity for the tree’s roots to grow around the trunk. As the roots elongate over the years inside of the mulch volcano, they encircle the trunk. This is called girdling. The girdling action literally strangles the tree and deprives its roots and canopy of necessary resources.
Another detrimental effect of a mulch volcano has to do with the tree trunk. Tree bark is well suited to protecting the trunk from sunlight and wind. Mammoth piles of mulch surrounding a trunk, however, keep bark constantly moist, fostering decay. Over time the bark rots, exposing the conductive tissue beneath the bark to decay as well. This negatively impacts the trunk’s function to move water upward/sugars downward and the structural integrity of the tree.
This can be corrected with a little hard work. First, the mulch must be removed down to the soil line. Care must be given to removing secondary roots found during excavation, especially those encircling the trunk. Then add fresh mulch but aim for more of a flat doughnut shape and avoid the lava dome look.
Ideally, mulch should be no more than 2-3” thick, spread about 4-6’ in diameter around the trunk, and not touch the trunk. This amount benefits trees greatly by acting like a blanket by moderating soil temperatures to keep summer temperatures down and winter temperatures up. Soil structure improves as well – aeration is better, erosion is lessened, and water can better penetrate the soil surface. Surface evaporation is also reduced, helping maintain soil moisture. Our objective is to imitate what occurs in nature. When walking through the woods, you will not see a tree or shrub with a buried root collar. Instead, each tree will have a visible root collar that spreads gently into the surrounding grade. In a perfect world, all our suburban trees will also have properly installed mulch and exposed root collars to ensure their health and longevity.
