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Mulching your trees and shrubs is a smart thing to do; the benefits include keeping down the growth of weeds and holding in moisture.
However, over-mulching can kill plants, warns Bob Mulrooney, a plant pathologist for University of Delaware Cooperative Extension.
“A mountain of mulch doesn’t kill a tree immediately,” says Mulrooney. “Instead, it’s a slow death sentence. Over-mulching is a leading cause of loss in azaleas, rhododendrons, dogwood trees and hollies.”
Over-mulching kills because tree roots suffocate from lack of oxygen. “Roots need oxygen for respiration,” explains Mulrooney. “When soil oxygen levels drop below 10 percent, root growth declines, resulting in dying roots.”
Symptoms include off color, yellowing foliage, abnormally small leaves, poor twig growth and dieback of older branches. These symptoms may take several years to appear and by the time they do, it is generally too late to correct the problem.
In addition to the amount of mulch, the placement of mulch matters. When mulch is placed directly against the trunks of trees and shrubs, it stresses and ultimately kills the inner bark tissue. This leaves the trunk open to fungal and bacterial diseases, which need moisture to spread and reproduce. Once established, the disease organisms invade the inner bark. Then, bark beetles and borers come, attracted to stressed plants. These insects expedite the plant’s decline.
Piles of mulch next to the trunk also may provide cover for chewing rodents, such as mice and meadow voles. “These rodents live under the warm mulch in the winter and chew on the nutritious inner bark, an activity that often goes unnoticed until the following spring,” says Mulrooney.
So how does one mulch right? Here’s Mulrooney’s advice:
• Do not exceed a 3-inch depth in well-drained soils.
• Do not exceed a 2-inch depth in poorly drained soils.
• Coarse-textured mulches offer better oxygen diffusion and thus can be applied more heavily.
• Fine-textured mulches, such as double-shredded varieties, should be applied lightly — just 1 to 2 inches deep.
• Taper mulch to the soil level at the trunk so no mulch is piled against the tree or shrub trunk.
“Before applying a new batch of mulch, dig down to see how deep the current supply really is,” says Mulrooney. “A light raking of existing mulch may be all that is needed to break through the crusted or compacted layers that repel water.”
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