Friday, July 25, 2008

by Susan Hansen

Extension Educator

Mulch comes in two varieties, organic and inorganic, and it is important to know the advantages and disadvantages of each one.

Organic mulches are derived from plants or animals. They break down over time, add organic matter to soil and improve soil structure. In sandy soils, they may increase water-holding capacity. In clay soils they can improve water drainage.  Organic mulches may be incorporated into the soil when beds are renovated or removed, but nitrogen may need to be added to the soil to prevent nitrogen deficiency in surrounding plants.

Some consumers don’t like organic mulches because of their tendency to move out of landscape beds during heavy rainfalls or gusty winds. Watering organic mulches after installation will increase their ability to hold together and minimize movement.  Inorganic mulches are materials that come from nonliving sources. They rarely break down or they break down slowly. Because inorganic mulches don’t break down quickly, they only need to be replenished when they are worked into the soil or are moved off site.  Inorganic mulches are difficult to remove when beds are planted, renovated or eliminated. Many consumers prefer inorganic mulches because of their longevity; however, inorganic mulches will increase reflexive heat and soil temperatures. As a result, landscapes may be uncomfortable to be in or view on sunny days, and increased temperatures may damage plants.

For good plant health, organic mulches are generally preferred. Organic mulch types include:
• Straw -- Straw mulch is unattractive and as a result is primarily used for short-term projects such as seeding turfgrass areas. It will decompose quickly once the turf has been established; however, some straw mulch contains high levels of weed seed that may germinate and compete with the turfgrass.
• Woodchips -- Woodchips can come from branches, bark or recycled wood pallets. Fresh woodchips may contain seeds of trees that can become a weed problem in landscape beds. Free woodchips are available from many cities. Dyes are available to color woodchips any desirable color. Woodchips may create access for an existing termite population if woodchips are placed against a foundation. 
• Corncobs -- Chopped corncobs generally are weed free, lightweight and easy to handle. They decompose slowly and can be used around perennials, trees and shrubs. They can be dyed various colors to match other landscape features. • Grass Clippings -- Grass clippings typically are used in perennial and vegetable gardens. Only clippings from a well-maintained turf should be used. Clippings from turfs that have a lot of weeds will contain weed seeds and may cause future weed problems.
• Grass clippings should be dried before use. Fresh grass clippings are high in water and nitrogen and will readily ferment. The heat and ammonia released from fermentation can damage plants.
• Leaves -- Leaves are readily available. They should be shredded and partially decomposed before placed around plants. As with grass clippings, thick layers of leaves may interfere with water and air movement.
• Pine Needles -- Pine needles are attractive and add fragrance to a landscape. They decompose slowly and therefore do not change soil pH quickly. Thick layers may mat down. Bales of pine needles are becoming more readily available.
• Sawdust -- Sawdust is readily available but does not make a good landscape mulch. It breaks down slowly and will pull nitrogen from the soil. This may result in nitrogen deficiencies for surrounding plants.

Inorganic mulch types include:
• Plastic -- Plastic is available in different thicknesses and colors. Plastics may be used early in the season to increase soil temperatures and extend the growing season or to help ripen fruit. Plastic is not recommended in landscape beds. It inhibits weed growth but will also inhibit the growth of desirable plants. Plastic restricts air movement, water and nutrients to and from the soil, affecting surrounding plant health.

•Landscape fabrics -- Landscape fabrics typically are a black, woven fabric that contains small holes in the surface to allow air, water and nutrients to move to and from the soil. They are not aesthetically pleasing by themselves and typically are used with mulches placed on top.

        While they are purported to inhibit weed infestation, many species, such as  nutsedge and filed bindweed, can grow  through the fabric. Seed deposited on the surface also may still germinate. Landscape fabrics are more suitable for inorganic mulches than organic types.

• Crumb Rubber -- Crumb rubber is a product primarily generated from recycled car tires. This type of mulch typically is found in turf areas subjected to high traffic areas, such as near a cart path or on sports turfs or playgrounds. In high traffic areas, it helps reduce soil compaction, improves wear tolerance and provides padding for those using the turf. Its use as a landscape mulch is limited and not recommended.

• Rock (Lava, River, White, Pink) -- Rock mulches increase temperatures around plants, resulting in increased plant stress and greater vulnerability to insect and disease problems. Rock mulches also may increase the temperatures in outdoor living areas, the cost of air conditioning within adjacent buildings and glare if mulch is light-colored. Using rocks also can be dangerous because mowers and children can throw them.  Rock mulches are a good choice under down spouts or in dry bed streams to help slow water down and reduce soil erosion and the washing of organic mulches into turfgrass areas. In areas where rock is used, select a color that matches other elements of the house or hardscape.