Organic mulches serve several important functions in gardens and landscape plantings. Mulches help control annual weeds and conserve soil moisture. They reduce soil erosion by reducing the impact of raindrops and water runoff. Mulches can help reduce the severity of some diseases, such as blights on tomatoes. They help keep fruits, vegetables, and flowers free of rain spattered soil. They also reduce fruit and vegetable spoilage. Organic mulches moderate soil temperatures. During the hot summer months, they help maintain cooler soil temperatures. Mulches also provide an attractive background for landscape plantings.
Several organic materials may be used as mulches. The characteristics of several mulches are discussed below.
Allow lawn clippings to dry before applying to garden areas. Fresh, green material will settle and form a dense mat. It may also produce an unpleasant odor. If the lawn has been treated with a broadleaf herbicide, don't use the clippings until the lawn has been mowed two or three times after the application. The best source of lawn clippings is a well-maintained lawn. Grass clippings from a weed-infested lawn will undoubtedly contain a large amount of weed seed, such as dandelion, plantain, and crabgrass. Grass clippings are not long-lasting and are best used in the vegetable garden or annual flower bed.
Straw that is free from crop and weed seed is an excellent mulch for the vegetable garden and strawberry bed. Suitable materials include wheat, oat, and soybean straw. Urban gardeners can often buy straw at garden centers. Straw may provide a winter habitat for mice and other rodents. Therefore, caution should be exercised when using straw around trees and shrubs.
Leaves should be shredded or composed before applied as a mulch. Shredded or composted leaves do not mat down as readily as whole leaves, are less likely to blow away in the wind, and decompose more quickly. Shredded or composted leaves are an excellent mulch for vegetable gardens, raspberry plantings, perennial flower beds, and around trees and shrubs. While the leaves of some trees, such as oak, are acid in reaction, they can be safely used in the yard and garden. The small amounts used by home gardeners have little effect on soil pH. Leaves are a poor winter mulch for strawberries and herbaceous perennials.
Pine needles are light, airy, decompose slowly, and make an attractive mulch. They may last several years and may be easily removed if necessary. Pine needles are acid in reaction and are excellent mulches for acid-loving plants. However, they can also be safely used in the vegetable garden and elsewhere in the home landscape. The best source of pine needles is a large, established windbreak.
Cocoa-bean hulls are a by-product of chocolate production. Cocoa-bean hulls are light, easy to handle, and have an attractive brown color. They also have a delightful aroma. (Unfortunately for chocolate lovers, the aroma lasts for only a few days.) While cocoa-bean hulls are somewhat expensive, a mulch depth of 1 to 2 inches is sufficient. Cocoa-bean hulls are excellent mulches for annuals, perennials, and roses. The material generally lasts only one growing season.
Shredded newspapers or whole sheets may be used in the vegetable garden. Most newspapers use organic inks so gardeners need not worry about lead contamination. When using newspaper sheets, place a layer of 6 to 8 sheets between plant rows in the garden. Water the sheets so they stick to one another and to the soil surface, then weigh them down with soil to prevent them from blowing away in the wind.
Ground corncobs gradually darken as they age, becoming an attractive mulch. They are generally weed free, lightweight, and easy to handle and apply. Corncobs decompose slowly and are most often used around trees and shrubs. They are also an excellent mulch for raspberries.
Sawdust is easy to apply, weed free, and decomposes slowly. Generally, sawdust should be allowed to age or weather for a year before being applied. If fresh sawdust is used, apply only a 1 inch layer and make sure the sawdust doesn't cake. Do not use sawdust from treated lumber in the yard and garden.
Wood chips are an excellent mulching material which may be available from local arborists. The material is obtained by passing tree and shrub trimmings through a mechanical chipper. Municipal or private yard waste sites are other possible sources of wood chips. They are best used in landscape plantings, such as around trees and shrubs and in perennial beds.
Commercially packaged bark mulches are available shredded or as chips, nuggets, or chunks. They are often available in bulk quantities from nurseries and landscape companies. Bark mulches are attractive, weed free, and decompose slowly. Bark mulches are best used around trees and shrubs and in perennial beds.
The type of mulch determines the depth of the material. Apply most organic mulches 2 to 4 inches in depth. The selection of the mulch should be based on availability, cost, appearance, function, durability, and other factors.
This article originally appeared in the May 12, 2000 issue, pp. 49-50.