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Lawn invasion: What to do when moss threatens

Once again this spring I am noticing that moss is encroaching on my lawn in those shady areas where my grass is a little weak. I saw it last year, but didn’t do anything about it, and it’s back with a vengeance this year and is forcing me to act.

The only redeeming quality of moss is the fact that it is green, so from a distance it doesn’t look too bad. At least that’s what I told myself last year, and why I failed to act.

Unfortunately, moss competes with healthy lawn grasses and, if not controlled, eventually will choke grass out almost completely. Moss also consumes soil of all nutrients grasses need to survive.

I first noticed the moss growing in my lawn last fall, while the weather was mild and rainy. These are ideal conditions for its growth. Because grasses grow poorly in the winter, mosses often 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, but under shady, irrigated conditions, moss may grow through the summer.

Although I hate to admit it, moss is most likely to appear in poorly maintained lawns. It spreads quickly in lawns that are weak due to any number of reasons. Moss does well in areas where shade predominates, where there is low fertility, where the soil is kept too wet, where insects or diseases have weakened the lawn or where mowing has been infrequent.

Properly timed nitrogen fertilizer applications make lawn grasses healthier and better able to compete with moss. Fertilize in late fall and spring to minimize moss growth.

Grasses grow poorly in dense shade because of lack of light, which leads to increased plant disease and moss activity in the lawn. Thinning out trees by selective pruning or removing trees completely may reduce or eliminate the problem.

If planting new lawns in shady sites, be careful to select shade-tolerant grass species. In relatively dry shade, the fine fescues will perform well.

In wet, shady sites, roughstalk bluegrass or bentgrass varieties might work best.

Wet soils, caused by poor drainage or excessive irrigation, provide a perfect environment for the germination and growth of moss.

Thatch removal or lawn aerating can improve drainage. If the problem is severe, lower the water table by changing the grade of the flooded area or installing subsurface drain tubing.

Thin turf is an ideal environment for 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. Well maintained lawns have the vigor needed to reduce moss encroachment.

Remove moss by lawn dethatching in early spring. The best time is mid-March through April when the moss is still healthy and vigorous. You can easily remove up to 75 percent of the moss with a flail-type dethatcher available at rental stores.

After thatching, apply a nitrogen fertilization to stimulate grass growth and increase turf density. Where moss is heavily established, apply chemical sprays after dethatching to increase control.

Most moss control chemicals contain iron, copper or zinc as the active ingredient. Cryptocidal (moss-killing) soaps are also available. Iron compounds are effective and work quickly, and the iron in them stimulates a “green-up” of turf. Complete fertilizers containing iron efficiently remove moss and stimulate grass growth.

However, keep in mind that iron stains concrete and many other surfaces, so be sure to apply it carefully. Salts and iron products applied as liquids work well on moss when used at rates of ½ to 1 pound of iron per 1,000 square feet.

The key to effective control with iron compounds is thorough coverage of moss foliage. Liquid materials are extremely effective and give almost instant results. Dusty fertilizer-plus-iron products are more effective than clean, granular products because they provide better coverage of the moss.

Cryptocidal soaps act as contact killers and tend to bleach the moss to a whitish yellow, in contrast to the dark brown of moss treated with iron.

Soaps have the advantage that they are safe on sidewalks and other structures however, they don’t contain the nitrogen that fertilizer plus iron products have that immediately give the grass a much needed boost.

Typical application rates for Cryptocidal soaps are 2 ½ quarts of product per 1,000 square feet. Testing by Oregon State University indicates that these rates are effective.

Copper and zinc will remove moss on roofs and walks and will not stain structures. Unfortunately, compounds containing these elements act slowly as moss killers and may injure desirable turf grasses in lawns.

It is obvious that good lawn maintenance is the best solution to lawn moss problems.

For me, this means that I will need to review and adjust my fall fertilizer program so that my lawn stays strong through the long dormant season.

Chemical treatments, although effective, rid the lawn of moss only temporarily.

Lynn E. Long is an extension horticulturist with Oregon State University.


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