By Mary Francois Deweese – Registered Landscape Architect and Sustainable Site Consultant, Principal of Acorn Landscapes, St. Louis, MO.
Today mulch is a staple of the nursery industry and home gardeners are using it with un-cautioned abandon, presumably for the benefit, though sometimes to the detriment, of their plants. This article takes a look at the proper way to use mulch.
Despite good intentions, there is a great deal of misunderstanding regarding proper mulching techniques. The biggest mistake seen in the garden is allowing mulch to come in contact with the bark of a plant. In fact, mulch should never come into contact with the bark of a tree or shrub. If mulch is in contact with the base of the tree or shrub it can cause damage in several ways. First, it creates an ideal environment for insects, fungi, and rodents. Don’t take my word for it, go look at a tree that has mulch piled up against the trunk, pull back the mulch from the bark and we know you will be surprised at how many insects you see. Secondly, piling mulch against the base of the tree can rot the wood, causing decay, and even death. Mulch keeps in moisture, which is good for the soil, but not for the bark. (Unlike the roots, the bark of a tree requires air circulation around it, not moisture.) Keeping the sensitive bark moist will actually rot the wood at the base of the tree, cause disease, and stress for the tree or shrub. The effects can be dramatic. I have seen large beautiful mature trees snap off at ground level because the base of the tree had rotted through. Imagine the damage a falling tree could cause, not to mention the loss of a mature tree from this easily avoidable mistake.
On the other hand, proper mulching is indeed important to the health and aesthetics of a tree or shrub. There are a multitude of benefits of proper mulching. The correct way to mulch involves covering the soil surface for the entire root zone of a plant. Ideally, a mixture of native or adapted groundcovers below trees will give you the benefit of mulch, along with possible nutrient cycling and self renewing fertility. (More on that in another article).
Newly planted trees are helped by a ring of mulch 6 feet in diameter. Shrubs and perennials also benefit from a well placed layer of mulch. Contrary to popular opinion though, large mature trees actually do not benefit from mulch. Their roots are deep and expansive and it would be impractical to mulch every tree to the extent that its roots extend. Mulching a large tree has but one purpose, it will keep you from damaging the bark with mechanical lawn equipment like a weed whipper.
Another thing to avoid is the yearly addition of too much mulch. If you put a nice 3-inch layer of mulch down the first year, you don’t want to add another 3 inches every year. Very quickly the mulch layer will build up too thick and you will start seeing some stress in your plants. Remember, much of the damage done by mulch takes several growing seasons to show up visually in your plants. Don’t wait until you see damage to correct your technique. The following pictures illustrate how to properly mulch trees and shrubs.