Q. I see mulch piled high around the base of all the trees planted within my condominium complex. Does this really help the tree?

A. Mulching trees and shrubs is a recommended maintenance method with many benefits, yet it can literally kill plants if applied improperly. A mountain of mulch, piled high against the tree trunk, does not kill a tree immediately – it results in a slow death. Over-mulching is a waste of mulch (and money!). It is a leading cause of death of azalea, rhododendron, dogwood, boxwood, mountain laurel, hollies, cherry trees, ash, birch, linden, spruce and many other landscape plants.

How does over-mulching kill trees and shrubs? One of the most common causes is oxygen starvation. Oxygen starvation is the suffocation of the tree roots. Symptoms may take several years to appear, depending on the plant and soil type. One of the symptoms is off-color, yellowing foliage (chlorosis). Unfortunately, by the time any symptoms are noticed, it is generally too late to correct the problem. At this point, the plant is usually in a state of irreversible decline, and will most likely die.

Piles of mulch next to the trunk may also 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, which then girdles the trunk.

Mulch may thus be one of the best or worst things you can do for your plants. Mulch depth should generally not exceed three inches. However, on poorly drained soils, such as clay, mulch depths should not exceed two inches. Wet soils may not need any mulch.

If you have a problem with excess mulch, dig through the landscape and see how deep the mulch really is. A light raking of existing mulch may be all that is needed to break through the crusted or compacted layers that repel water. Pull mulch back from plant stems and trunks – a rule of thumb is 3-5 inches away from young plants, and 8-12 inches away from mature tree trunks. Visually look for the presence of a root flair; if not visible, at least some may be partially buried and must be exposed. Remove all soil or mulch up to the junction of the roots and trunk collar (taking care not to damage the tender bark) to expose the root collar. Leave the resulting well open and exposed to air. Research shows that an amazing number of plants have rapidly improved in color and vigor within months of root collar excavations.