Moss Control in Lawns

Mosses, like most lawn weeds, primarily take advantage of bare areas but do not kill out existing grass as some people believe. Bare areas in lawn can be result from several causes. Unsuitable grass varieties, poor drainage, drought, soil compaction, excessive shade, thatch accumulation, poor fertility, diseases, improper mowing and other cultural practices are common reasons for turf failure leading to moss or other weed problems. Improving these factors to encourage thick, healthy grass is the key to long term prevention.

Moss can be killed with products containing ferrous sulfate, ferrous ammonium sulfate, including Moss-Out, Moss-Kil, Rid-Moss, and a variety of Lawn Fertilizers with Moss Control; or moss & algae killing soaps such as Safer's. None of these materials pose serious threats to the environment; in fact iron and sulfur are essential nutrients for grasses and tend to improve their color. Although these products will kill existing moss, unless the underlying conditions are changed, moss or other weeds are likely to reappear.

Poorly adapted varieties of grass may fail due to cool temperatures, inadequate drainage, or prevalent diseases. Poor drainage and soil compaction can cause roots to suffocate or be subject to disease. Thatch and soil compaction can impede water movement to grass roots, leading to drought stress.

Raising the soil pH with agricultural or dolomitic lime can improve availability of nutrients and supply calcium and thus helping the turf grow better, but will not kill moss directly. The following steps will generally control existing moss and discourage it in the future:

1. Use a moss control product to kill existing moss in late winter or early spring.
2. Dethatch or rake out dead moss, or skip step (1) and dethatch or rake with more vigor.
3. Overseed bare areas with a mixture turftype ryegrasses or ryegrass/fescue blend.
4. Topdress seeds with about l/4 inch of loose weed-free soil, potting mix, or sand.
5. Keep moist until seedlings are established.
6. Maintain the lawn according to recommendations in the WSU EB 0482 Home Lawns.

The following practices may help specific situations:
If the soil is compacted, aeration, use of a wetting agent, and careful watering may help. Apply 20 to 30 lbs per thousand square feet if lime has not been used for several years. Thinning tree limbs may allow more light and reduce competition to the lawn.

For more information, have a look at the "Moss" fact sheet in the "Lawn and Turf" section of "WSU Hort Sense" ( http://pep.wsu.edu/hortsense/ ) or contact your local WSU Extension Office.

By Steve Whitcher, Master Gardener Program Coordinator, Pierce County. Reviewed 1996. Link added 05/06