Controlling Moss in Seattle Lawns

Lawn mosses are common throughout Western Oregon and Western Washington. Moss growth normally starts with fall rains and reaches a peak in early spring. Because grasses grow poorly in winter, mosses are able to invade and often dominate lawns in only a few months. Moss growth declines in summer as conditions become drier and turfgrass growth increases. Under shady irrigated conditions, moss may grow through the summer. Moss can tolerate long periods of drought in a dehydrated condition and rehydrate and grow with the onset of fall rains. The persistent and recurring nature of lawn mosses is largely due to our mild temperatures and the wet-dry nature of our climate. Even though it has been a significant lawn pest, surprisingly little is known about the types of moss found in turf. Worldwide, only about a dozen species have been identified. 

CULTURAL CONTROL

While moss does occur in well-maintained lawns, severe invasion generally occurs in neglected lawns where poor cultural conditions enable mosses to out-compete turf.  Moss encroachment is generally associated with thin turf, low fertility, highly acidic soils, shade, wet soils, and turf injury from insects, diseases, chemicals or cultural practices.  Long term moss control in these situations in impossible unless cultural conditions are corrected.

In many cases, turf is thin due to lack of fertilizer. Properly timed nitrogen fertilizer applications and overseeding will increase turf density, vigor and competitiveness. Late fall and spring are important times to fertilize (April-May and September to overseed) to minimize moss encroachment. Liming soil to raise pH to 6.0 – 6.5 will benefit some grasses in the long run but will have no direct effect on moss.

Thin turf due to injury is a common cause of moss encroachment.  Unirrigated lawns turn brown and thin out during summer.  When fall rains come, these lawns may not recover fast enough to compete with moss.  Severe dethatching in fall may also predispose the lawn to moss because turf is thin when fall rains come.  Proper culture which encourages healthy, dense turf during the moss season will reduce moss encroachment in most situations.

MECHANICAL CONTROL

Moss can be physically removed by dethatching in early spring.  Optimum timing is mid-March through April when moss is healthy and vigorous.  With flail type dethatcher (available at rental agencies), as much as 75% of the moss can be removed physically.  Dethatching should be followed by nitrogen fertilization and overseeding to stimulate turf growth and increase density.  Where moss is severe, chemical sprays applied after the dethatching operation will enhance control further.

CHEMICAL CONTROL

Many chemical materials are effective for killing moss in lawns.  Most commercially available formulations contain metals such as iron (Fe), copper (Cu) or Zinc (Zn) as the active ingredient.  Cryptocidal soaps are also available.  All of these materials can kill moss but some are more effective than others

Copper and zinc are good moss killers on roofs and walks and will not stain structures.  Unfortunately, copper and zinc compounds act slowly as moss killers and in lawns may injure desirable turf grasses.

Iron compounds are highly effective moss killers in turf.  Moss kill is rapid and iron also stimulates a "green-up" of turf.  A complete fertilizer that includes iron will stimulate healthy growth and kill moss at the same time.  Iron stains concrete and surfaces, however, so it must be applied carefully.  Salts and chelated iron products applied as liquids are generally effective on moss at 0.5 - 1.0 lb. Fe/1000 sq. ft.  Dry fertilizer plus iron products are generally effective at 0.9 –1.5 lb. Fe/1000 sq. ft.  The key to effective control with iron compounds is thorough coverage.  Please be careful to follow product instructions.

Cryptocidal soaps, such as Safer's Moss Attack, are relatively new chemicals for moss control.  Soaps act as constant killers and tend to bleach out the moss to a whitish-yellow color.  This is in contrast to the dark brown color of moss treated with iron.  They are slower working than moss killers that contain iron and the results may be affected by rainfall, but soaps are safe on sidewalks and other structures.