Mulches in the Garden and Landscape - August 3, 2005
Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Mulch is 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, but not always.
Effective mulch layers should be at least 2-3 inches thick. Bark, sawdust, wood chips, straw, cardboard, and newspaper can cause localized soil nitrogen deficiencies. This occurs because soil microbes colonize organic material and utilize available nitrogen for their growth and reproduction. Deficiencies can be as counteracted by adding fertilizer when nitrogen-consuming crops are present (annuals, vegetables, and fruit trees). Green mulch materials, such as alfalfa hay and grass clippings contain adequate nitrogen to counteract this effect.
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. 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. Additional nitrogen is not usually required with pine needles because of their slow decomposition.
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 crabgrass and grass full of seed heads. Also, do not use clippings from lawns which have been treated that season with herbicide or a fertilizer/herbicide combination.
Leaves provide good annual weed control. Leaves are easy to obtain, attractive as a mulch, 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.
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, though colored inks may contain lead.
Black plastic provides excellent weed control, is relatively slow to decompose, but will be 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.
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 light waves from sunlight. Other colored plastic mulches have also been investigated.
Highly 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 easy maintenance and clean appearance.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at email@example.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: July 28, 2005
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