Mulch and trees

Mulch and trees


Every gardener must, at some time, face the question: “to mulch or not to mulch?”. The general definition of mulching is the addition of a thin layer of material, most often organic, to the soil surface. The benefits of mulching are to reduce soil moisture loss, block weed growth, reduce soil temperature, slow surface runoff, improve soil fertility and enhance the visual appearance of an area. The most common type of commercial mulch is 100 % organic and is a by-product of saw mills, consisting of partially decomposed bark, wood chips and sawdust. Tree pruning and removal in urban areas is another source of this type of mulching material. Several other types of organic mulch can be used including leaves, grass clippings, straw, hay and composted vegetable waste. These types of mulch enhance soil fertility, but are messy to handle, may be smelly and not very attractive in an ornamental garden.

The reasons for mulching vary. If weeds are a serious problem, it is suggested that an area to be mulched be first covered with black polyethylene sheets for several months to kill the weeds. The sheets need to be anchored to the soil to prevent movement.   After removal of the polyethylene, mulch can be laid down and will be more effective in weed control. Digging out weeds manually before mulching is also effective.

Organic mulch continues to decompose once applied at a thickness of about 2 inches. It becomes reduced in volume and is incorporated into the soil providing organic matter and soil nutrients available to trees, shrubs and herbs. Mulch decomposition with the aid of organisms produces some heat and releases carbon dioxide, methane and other gasses, giving off an earthy odor which is not objectionable. Reapplication of mulch is necessary about once a year.

Calculating the amount of mulch needed is simple mathematics. For example, if you have a bed that is 6 x 3 feet, and want to add 2 inches of mulch, convert the feet dimensions to inches and multiple the three figures together to obtain cubic inches. Thus: 72 x 36 x 2 = 5,184 cubic inches; then dividing by 1,728 (the number of inches in one cubic foot tells you it is 3 cubic feet. Or, you can use an on-line mulch calculator such as the one found at: Organic mulch is commonly sold in bags containing 2 cubic feet.   Mulch labels may indicate the main source of the material such as cypress, or that they are died black, brown, red or other colors. The dyes are benign and not harmful to humans, pets or the environment. Colored mulch can be used to accent the plants they are associated with.

There is general consensus among nurserymen and gardeners that organic mulching is beneficial to tree root growth and to the tree itself, and to other plants, as well. Use of plastic mulch or mulches containing non-biodegradable materials has the disadvantage that they need to be removed and when their effective life is over disposed of. Using organic mulch avoids that problem.