Mulch can be a benefit to plantings, but there is such a thing as too much, experts say.
“People don’t realize how damaging too much mulch is to trees and bushes,” Tree Warden John Pinchbeck said. “One thing to remember is that a moderate rain should be able to penetrate the mulch to reach the soil underneath with moisture.”
Asked for the basic rules on mulching, Mr. Pinchbeck responded by listing three important standards: The mulch should be no more than two to three inches deep; the mulch should be flat — no volcano or mounded mulching; and mulch should not touch the trunk of a tree or the stems of bushes. The first two rules, he said, relate to supplying roots in the soil with adequate moisture and usable nutrition: “When only the mulch is damp, the anchoring roots, instead of growing down into the soil, turn back up into the mulch, which provides neither the proper nutrition for the tree nor the stability it will need as it grows.”
Mr. Pinchbeck explained that keeping mulch away from trunks and stems is important because the bark tissue above the “root flair” (the widening of the truck as it enters the ground) is designed to “breathe” and not to be kept damp, which can lead to rot or insect damage.
“Mulching can be very valuable, but overdoing it can be very damaging,” he emphasized.