Mulching is one of the best things you can do for the health of trees, shrubs, and flowers if done properly. If some is good, though, more is not necessarily better. Reasons you should mulch trees and shrubs, and proper techniques, are given in Extension leaflet OH 78 from the University of Vermont master gardeners.
Proper mulching is especially important for plants under stress, or newly planted ones without extensive root systems. To correctly apply organic mulches, such as the common shredded pine bark, to trees, start six inches from the base working out to the desired diameter. Depth should start at one inch at the inner circle, increasing to no more than four inches (two inches for clay soils) at the outer edge of the circle. Final depth may be reduced if landscape fabric is placed under the mulch.
Consider using perennial groundcover plants, such as Vinca, Ajuga, or Pachysandra, as a mulch alternative. Some even plant annual flowers such as impatiens or begonias in circular beds around tree bases.
Annual additions to mulch only should be made to maintain proper depth. Removal defeats one of the purposes of mulch, which is to decay and mix with the soil. Fluffing the old mulch, before adding more, will prevent it from forming a hard surface that deflects water, rather than retaining it.
Excessive mulch material piled up against the base of a tree or shrub, forming a mulch "volcano," keeps moisture in direct contact with the bark. The moisture penetrates the bark and suffocates the cells of the phloem, which is the layer of living tissue that transfers food up and down the plant. When this supply of food from the leaves is limited, the roots die back. This leads to less water being taken up, and the tree or shrub goes into general decline, leaf drop, and premature death.
Secondary problems, like borers and fungi, move into plants weakened by improper mulching. In sugar maples, the fungal pathogen Phytophthora will move in because of the high moisture around the trunk. This may create a canker symptom (sunken discolored and dead area) that girdles the trunk at the base, and hastens the decline of the tree.
On the other hand, such a thick mulch "volcano" may shed water, keeping the soil underneath too dry. This may be caused by an impenetrable layer of fungus forming in such a deep layer. Light watering or rain may not penetrate this thick mulch pile to the soil. In this scenario, small feeder roots may work their way up into the mulch seeking moisture. If many of the tree or shrub roots end up in this layer over time, they can be susceptible to the mulch drying out and dying. Or, with plant roots in the mulch rather than the soil, the plant may become less anchored and so less stable in high winds.
If trees or shrubs have been mulched too heavily, remove excess mulch using a shovel, trowel, or whiskbroom while taking care not to injure the trunk. A hard stream of water may be used to remove excess mulch and soil from the trunk and flared base. Cut off secondary roots that may have grown into the mulch. The trunk and flare should be visible. New mulch can then be applied properly.
Some of the same mulching principles apply to flowers. It's too easy to shovel huge fork loads over plants. Don't cover perennials, or you may smother them. This is especially true for shallow-rooted species such as yarrows and many bellflowers. This is one reason to not mulch perennial beds too early in the spring before shoots emerge. Just as thick mulch will prevent weeds, it will prevent these plants from growing too! Mulching over peonies more than an inch or two may keep them from blooming.
Weed fabrics and black plastic are not useful for perennials, except perhaps in the first year. As perennials grow, they spread or get larger, and such fabrics can kill them. Such materials are useful for annual flowers which will only be in one place for a year.
Many lay black plastic, make holes to plant the flowers, then lightly mulch over the top with bark or other material mainly for aesthetics. A similar method can be used with annuals, or in vegetable gardens, with thick layers of newspapers (they are easier to lay if moistened), then covered with rotted sawdust, wood shavings, or similar. Of course the paper will rot and can then be tilled back into the soil, adding further organic matter.
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