Landscape - Mulch and Mulch Characteristics

The major spring mulching season is behind us. However, new summer and fall plantings will need to be mulched. The following is information on the types and characters of different mulches.

Bark is readily available in bags or bulk. Usually made from pine, cypress, or hardwood trees, it is resistant to decomposition. Research indicates that pine chips last longer than hardwood, but most bark mulches must be reapplied annually. Windblown seeds often germinate in bark mulches and necessitate cultivation or herbicide application.

Wood chips are often available from local sources. Chips larger than 3" are least likely to compact. Caution must be used, as chips that have not been composted or "aged" can be toxic to tender plants during the decomposition process. Wood shavings can also be used for mulching.

Chip and sawdust mulches from some woods, such as walnut and cedar, produce phytotoxic (plant-killing) chemicals and should be avoided. This toxicity phenomenon is called allelopathy.

Pine strawis available in bales or bulk. Pine needles do not compact and add a natural look to gardens. Pine needles do, however, decrease soil pH by adding acidity, and can induce nitrogen deficiency which shows up as poor growth and light-colored or yellowish-green foliage that appears on the older leaves first.

Sawdust is attractive and readily available. It decomposes slowly and can improve the quality of clayey and sandy soils. Sawdust does crust and form a mat, however, and must be broken up frequently to prevent water shed. Fresh sawdust temporarily ties up nitrogen as it decomposes, and therefore nitrogen fertilizer should be incorporated before mulch application. Some rotted sawdusts are very acidic and may need liming. A 3"-6" layer of sawdust is useful for mulching pathways.

Hulls from cocoa, buckwheat, and cottonseed are useful around shrub borders, flower beds, and rose gardens. Decorative, with a richly textured appearance, hulls are expensive and blow or wash away easily necessitating frequent replenishment. Cocoa hulls are high in phosphorus and may be toxic to certain plants.

Lawn clippings can be used as mulch, but a mulching mower is generally preferred so that grass clippings are recycled directly back onto the lawn. Applied as a thin mulch layer or mixed with other materials, lawn clippings add some nutrients to the soil without heavy compacting or excessive heat. Do not use grass clippings if herbicides have recently been applied.

Leaves should be partially rotted and dried before using because large wet leaves can form mats that shed water. Leaves may harbor diseases, insects, rodents and weed seeds. Oak leaves are acidic while maple leaves are alkaline. Walnut leaves are toxic and should not be used. Composted leaves are effective as a soil amendment, alone or mixed with grass clippings.

Straw can be used as loose mulch in a 6"-8" layer for temporary protection, such as for grass seed germination. Straw decomposes slowly and is a good source of humus, but there are several problems with straw. Straw has a low nitrogen content, and can harbor insects, diseases and weed seed. If tree beds are mulched with straw, rodents are attracted to the seeds and may feed on the bark when the seed is gone. In addition, straw is a potential fire hazard. Do not confuse straw with hay, which should never be used as a mulch because of its high weed seed content.

Peat moss is expensive, and when used as mulch the surface dries out quickly making it easily wind blown. Once dry, peat moss is very difficult to rewet and tends to shed water. Peat moss also encourages shallow rooting into the mulch layer which reduces drought tolerance. Peat moss is a better choice for amending the soil of large planting areas than for mulching.

Crushed stone and Gravel (types of inorganic mulches) are best used over black plastic or landscape fabric in permanent locations, such as walks or driveways, or in areas where no additional planting is anticipated. Some limestone-based rock mulches may add alkalinity and/or trace elements to the soil, making monitoring for possible pH changes necessary.

Information from "Selection and Use of Mulches and Landscape Fabrics" by Bonnie Appleton, Extension Horticulturist and Kathy Kauffman, Graduate Student, Hampton Roads AREC, Virginia Tech.