What is the Best Mulch? - October 2, 2013
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Mulch is a material applied to the soil surface to prevent weed growth, protect the soil from erosion, and reduce evaporation from the soil surface. Mulches are often organic material such as bark, sawdust, wood chips, grass clippings, straw, cardboard, newspaper, pine needles, or other leaves. Inorganic mulches are plastics, landscape fabric, gravels, and other materials that are non-living and slow to decompose. Organic mulches are most often used in regularly cultivated areas such as gardens, flower beds, and orchards. Inorganic mulches are usually used in permanently landscaped areas and for specialized situations. Effective mulch layers should be at least 2-3 inches thick.
Straw and hay make good mulch for vegetable and fruit plantings. A 6- to 8-inch layer of hay or straw provides good annual weed control. These materials decompose quickly and must be replenished to keep down weeds. Once moistened and settled, they stay in place and will improve the soil as they decompose.
Pine needles make excellent mulch around shrubs, trees, and in other areas where long-lasting mulch is desired. Pine needles are very slow to decompose.
Grass clippings also provide good weed control. Build up the layer gradually, using dry grass. A thick layer of green grass will give off excessive heat and foul odors rather than decompose. However, in limited quantity, clippings will decompose rapidly and provide an extra dose of nitrogen to growing plants, as well as adding humus. Avoid bermudagrass and other weedy grasses full of seed heads. Also, do not use clippings from lawns which have been treated that season with herbicide or a fertilizer/herbicide combination.
Dried leaves provide good annual weed control. Leaves are easy to obtain, attractive, and will improve the soil once decomposed. To reduce blowing of dry leaves, allow to decompose partially in a compost pile. Avoid using black walnut leaves as garden mulch. They contain a compound, juglone, which inhibits plant growth.
Bark and wood chips provide good weed control, but are slower to decay than other organic mulches. They very effective when used as a pathway material and between raised beds. These are often available for free at transfer stations and from arborists and utility companies that maintain line clearances.
Newspaper and cardboard decompose within a season, are readily available, and cheap. Newspaper should be covered with an organic mulch, such as sawdust or hay, to hold in place. Lead in printers' ink has been a concern of some gardeners desiring to use newspaper; however, printers no longer use lead compounds in ink for black and white newsprint. Slick pages of any paper product are not recommended because some inks in these materials may still contain heavy metals.
Black plastic provides excellent weed control, is relatively slow to decompose, but will breakdown after exposure to sunlight and must be replaced every two years or so. Black plastic mulch may increase the soil temperature in the spring. It is not recommended for use under other mulches because it prevents water and oxygen from getting into the soil. Many small-scale growers use it for annual vegetable crops
Clear plastic will provide little weed control; in fact, it makes an excellent environment for growing weeds. This material is most often used to warm the soil temperature early in the spring to prepare an area for planting. It will raise the soil temperature by 10°F or more. It can be used effectively to solarize soil for weed and disease control (for more information on solarization, see the May 21, 2003 Backyard Gardener).
Red plastic mulch has been shown to boost tomato yields in research plots up to 20 percent, while conserving water and controlling weeds. Red plastic mulch reflects onto plants higher amounts of certain growth-enhancing wavelengths of light. Other colored plastic mulches have also been investigated.
Reflective mulches, such as aluminum foil, have been shown to repel aphids and whiteflies. They also keep the soil cooler and have been shown to increase potato yields.
Rock or gravel mulches collect heat, cause temperature increases and therefore increase cooling costs of the home and irrigation costs to the landscape during summer. Weed seeds are easily blown into gravel or stone and soon establish themselves (even if you use landscape fabric underneath). Even so, they are widely used because of their persistence, easy maintenance, and clean appearance.
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Last Updated: September 26, 2013
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