Some horticulturists and arborists call them “mulch volcanoes”. I call them “cones of death”, since prolonged exposure to this overzealous use of mulch will eventually weaken and kill the occupant.
Mulch can be a useful tool in preventing weeds, preserving soil moisture, improving soil tilth (as the mulch breaks down), and even providing a more manicured landscape appearance—but only when used in moderation. Mulch used to protect plants should always be organic, non-toxic, and applied at a level depth no greater than 2”-3”.
Mulch needs to be kept well away from the stems or trunks of plants to prevent injury from bark rot and the fungal diseases that thrive in the mulch’s moist environment. Mulch should be placed at least 3” from the root collar of smaller trees and at least 8” from the root collar of larger trees.
What is the “root collar” of a tree? The Ohio DNR Division of Forestry explains:
“The part of a plant where the stem and root system meet is called the “root collar” which has a “flare” where it transitions between the stem wood and the roots. Four to eleven large roots called “transport roots” radiate from the root collar and function to move water and minerals that are absorbed by smaller roots. Transport roots contain cells that synthesize a variety of substances essential to the normal functioning of the top of the tree as well as help to stabilize the tree. The smaller roots called “absorbing” or “feeder roots” emanate from the transport roots. They have a huge surface area and function to absorb water and minerals from the soil in addition to providing sites for chemical synthesis.”
The photo below shows a tree planted at the proper depth with the root collar flaring just slightly above ground level.
In addition to providing support for the plant, tree roots are responsible for obtaining and transporting water and nutrients to the phloem and the xylem to be translocated throughout the tree. This activity, combined with sunlight, enables the tree to produce its own food (photosynthesis). Most tree feeder roots are located in the top 12” of soil, and they will not function properly if buried too deeply under mulch or soil.
The effect of piling mulch at the base of a tree and/or up the tree’s trunk is equivalent to planting a tree too deeply: root rot may set in, or existing roots will struggle to reach the surface, girdling the stem of the plant or interfering with other roots. Compressed stems can lead to breakage during storms. The photo at right shows the remains of a neighboring, identical tree to the one shown in the topmost photo—a tree also surrounded by a cone of death—following a recent storm. The trunk broke off right at the area covered by the mulch cone.
As if weak trunks and disease aren’t enough, the heat created by heavily applied mulch composting itself can cook both the trunk of a tree and the roots it’s supposed to be cooling. Thick applications of mulch can also provide a haven for insects and rodents that feed on tree bark.
Cones of death are just one method of gradually weakening a tree. Any construction that interferes with the properly planted depth of the tree, such as the example shown at left, can cause similar long-term stress for trees. The owner of this tree undoubtedly thinks the stone construction protects the tree from lawnmower damage while appearing decorative. Notice how the root collar flare is completely covered by heat-producing stones at least one foot deep. Unfortunately, by changing the planting depth, instead of accommodating the architectural design to the tree and its landscape, the owner is contributing to weakening the tree.