Organic Mulches - July 2, 2008
Jeff Schalau, Associate 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 of landscape soils, which often are compacted and lacking in organic matter, especially around newly constructed buildings.

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 could 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.

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.

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. Most newspapers use soy-based inks, though colored inks in the glossy inserts could contain heavy metals.

Flammability is also a concern with organic mulch materials. In general, straw pine needles and newspaper are extremely flammable. Coarse textured organic mulches such as pine needles, straw, wood chips, and bark nuggets are more flammable than shredded wood products. 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.

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 cottonwoodmg@yahoo.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://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: June 25, 2008
Content Questions/Comments: jschalau@ag.arizona.edu
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