TIPS FROM THE SHADE TREE COMMISSION
MULCH: The Good and the Bad
Horticulturists and landscapers agree: mulch is good. But the kind of mulch you use and how you use it will make the difference between a nutritionally deprived tree or shrub and a healthy, attractive garden. In our area of long, warm autumns and relatively mild winters, a layer of protective mulch should not be applied until AFTER a good freeze, or when the winter cold has set in permanently. Mulch doesn't keep the soil warm. Rather, it moderates soil temperature by holding moisture in -- the higher the soil moisture content, the slower the rate of temperature changes around a plant's roots, in all seasons. In winter it prevents heaving during freeze-thaw cycles. In summer's heat, the mulch keeps the soil cool. Mulching perennials after they've gone dormant gives them the same benefits; remember to rake it away in early spring.
Here are a few mulching guidelines.
Rule 1: Never throw away leaves. When you rake up autumn's falling leaves, pile them in an open, sunny location and let them break down. (It's easier to use a mulching mower, gathering them into a catcher.) Or, simply spread them around your trees and shrubs and into your flower and vegetable garden.
Rule 2: Never throw away grass cuttings, either. Mix them with your leaves. Grass breaks down quickly, heating up as it goes, which encourages the decomposition of the leaves. Eventually, you have compost, either in a neat pile or around your plants.
Rule 3: Shun wood chips. This mulch, often distributed free from municipal sites, contains unknown substances, some of which may be toxic -- treated lumber, for instance, is treated with arsenic. You don't want that anywhere in the soil! In addition, wood chips absorb moisture and can even draw it from the soil. It will not break down easily, but rather tends to sink into the soil. You think it's gone, but it's actually still there, sucking up the water and nutrients your trees need.
Rule 4: Use finely ground bark mulch. This material actually sheds water, which means that rain will pass through and soak into the roots of your trees, where it belongs. The bark has nutrients, it is free of unknown additives, and it breaks down at a good, slow rate, allowing moisture and nutrients through to the soil. Pine needles are nice, too, and will not increase the acidity of your soil.
Rule 5: Moderation, moderation, moderation. Mulch should not be deeper than about 2 inches. More than that will draw off so much water (See Rule 3) that a young tree could die, even if it's been irrigated properly. Too much mulch (say that three times, fast) ties up nitrogen and promotes the growth of molds.
Rule 6: NO VOLCANOES! Don't pile up the mulch closely around the trunk or stem of plants. Keep it several inches away from the area where the roots join the trunk -- the crown, which is a few inches ABOVE the soil. That area needs air. A big volcano piled up around the stem will invite all sorts of critters that do your plants no good at all: fungus, small rodents (who will munch the nice, moist bark right through ), and all sorts of disagreeable diseases.
Rule 7: Go outside and sit under your tree and enjoy it!