Using Mulch in the Landscape - July 27, 2011
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Many woody landscape plants evolved in forests where the soil is covered by a moist layer of decaying leaves, twigs, and branches. These natural mulch layers recycle nutrients, reduce competing vegetation and reduce evaporation from the soil surface. Mulching trees and shrubs can recreate some aspects of a forestís soil environment, even in sun-baked landscapes far from any forest. Organic mulches also can improve the soil structure and increase the fertility and permeability of landscape soils, which often are compacted and lacking in organic matter, especially around newly constructed buildings.
There is often confusion about the differences between compost and mulch. Compost is organic material that has been decomposed by soil microbes and is used as a soil amendment. Mulch is material applied to the soil surface in order to moderate soil temperature and conserve soil moisture as well as minimizing weeds and soil erosion.
Common organic mulch materials include 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 often used in permanently landscaped areas, especially in commercial settings.
Organic mulch layers should be at least 2-3 inches thick. Sawdust, wood chips, or straw mulches 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 a small amount of nitrogen fertilizer when nitrogen-consuming crops are present (annuals, vegetables, and fruit trees). Bark-based products are very resistant to decay and will not usually induce a nitrogen deficiency. Green mulch materials, such as alfalfa hay and grass clippings contain nitrogen which becomes available to plants after decomposition.
Grass clipping mulches can also provide good weed control. Build up the layer gradually, using dry grass. A thick layer of wet, green grass may give off excessive heat and foul odors as it decomposes. 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 clippings as they may have seeds or stolons that will become weeds. Also, do not use clippings from lawns which have been treated that season with herbicides or fertilizer/herbicide combinations.
Leaves, especially pine needles, are an excellent mulch material. Leaves are easy to obtain, attractive, and will improve the soil as they decompose. Where strong winds are present, you may need to allow the leaves to decompose partially in a compost pile before using them
Newspaper, cardboard and shredded paper decompose within a season, are readily available, and cheap. While not as attractive as other materials, paper products can be used in vegetable gardens and other areas where aesthetics are not as important. Heavy metals in printers' ink have been a concern of some gardeners desiring to use newspaper for mulch and compost. Today, most newspapers use soy-based inks.
Flammability is also a concern with organic mulch materials. In general, straw, pine needles and newspaper are extremely flammable. In areas where wildfire defensible space is necessary, it is best to avoid thick layers of any organic mulch within 10 to 15 feet of any flammable structure (decks, wooden fences, propane tanks, etc.). Keeping organic mulches away from structures will also reduce the potential for termite activity.
Landscape fabrics, also known as geotextiles or weed barriers, have become widely available for mulch applications. The fabric is usually used as a base and covered with another mulch product such as bark, chips, or gravel. While fabrics seem like a good idea, soil material often blows in and weeds often start growing on top of the fabric after a few years. My recommendation for residential landscapes is to forgo the fabric, prevent/remove weeds, occupy the growing space with desirable plants (consider natives and drought adapted species), donít overwater and keep adding new mulch over time. Do not use black plastic as it repels water and reduces oxygen in the soil.
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Last Updated: November 6, 2014
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