Gather no moss: Ways to rid lawn, deck, walkways and roof of the hardy pest

We joke about our moss-friendly climate in Western Washington — stand still long enough and you might just end up draped in the stuff — but studying mosses inspires a real sense of awe.

Mosses fascinate by weaving themselves into many forms, colors and textures. They resemble feathers, plush felt and miniature forests. More than 700 types inhabit the Pacific Northwest, lending variety to woods and back yards. Thousands more colonize habitats from the Arctic Circle to the equatorial tropics.

As fall rains descend, mosses rise and revive. I do not consider mosses weeds, but they can be frustrating because they grow so vigorously where they're often not wanted. Mosses don't have roots, and they don't photosynthesize as most familiar plants do. They require moisture and nutrients — they settle into the landscape where both are available.

Their chosen spot may be a shady concrete sidewalk, a cedar shake roof, or the lawn's edge. In those locations, the gardener or homeowner may demand, "Do something about the moss!"

Deciding whether moss-control is needed depends on the circumstances. Consider the lawn. Moss, like other lawn invaders, signifies that the turf isn't thriving. If the grass dies back, mosses move into the spaces created, especially if the lawn is shady and poorly drained.

If you have a shaded area of lawn and can't correct the lack of light, moss may be a constant companion, and you simply need to accept its texture. The best moss-fighting technique is cultivating a thick, healthy turf in a sunny spot.

Raking moss out of turf is a temporary solution but often satisfies the need for action, especially in a lawn with small square footage. If you choose to use chemical moss control, read the pesticide label carefully. Most moss control for lawns contains iron, usually in the form of iron sulfate. When you check the label, look for the concentration of the iron as "active ingredient" — the percentage ranges from about 17 percent to 99 percent.

Some of the higher-concentration products carry the EPA signal word "DANGER," which in this case means "corrosive." Handle these carefully and follow all label instructions, even though they are in fine print and difficult to read. Choosing a product with the less dangerous "CAUTION" label makes sense and will control moss effectively. And, iron sulfate stains concrete, so keep it off sidewalks.

After application, you'll have dead black moss. Rake it out, and be sure to overseed the bare spots with new grass seed in early spring. (Seed put out in November often doesn't germinate fast enough to get roots down for winter.)

Another approach to lawn moss control, one that can also be used for roof and deck cleaning, is a cryptocidal soap that is a contact killer for moss. It's nonstaining, and will turn lawn moss a strange whitish yellow rather than brown-black. Soaps carry the "CAUTION" label and are generally very low in toxicity.

Lime applications do not kill moss (though folk wisdom frequently asserts this.) The lime, along with fall fertilizer, will stimulate grass growth to help the turf fill in and compete with moss.

Walking surfaces absolutely need treatment to remove moss. Wood stairs develop treacherous slickness, as do sloped sidewalks. Pressure washing, even without chemicals, will remove surface growth.

If you use a pesticide like cryptocidal soap or a zinc compound before pressure washing, the moss and algae will return much more slowly. Keep vegetation cut back off sidewalks and roofs to slow down the accumulation of biomass that moss lives on happily.

Cedar shake roofs can be damaged by thick moss growth, which holds moisture against the wood. Regular sweeping helps prevent accumulation.

Check your home-building-supply store for various clever roof moss management tools: zinc impregnated strips that can be tucked into composition roofs, or copper wire that also retards roof moss growth.

Moss, the beautiful pest of the West, defines this region — and even as you read this, it's probably colonizing some part of your garden.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.

She shares gardening tips every Wednesday. Her e-mail is gardeningtips@seattletimes.com.

The Green Gardening Program provides information on environmentally sound garden choices. Green Gardening is sponsored by Seattle Public Utilities and funded by the Local Hazardous Waste Management Program in King County.