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mossy roof

Managing moss safely

Tools for living    Healthy home    Repair, remodel and maintain safely    Managing moss

Choose less-toxic ways to handle the moss in your yard or on your roof or patio.

Mossy carpet of green – beauty or bother?

Mosses are tiny plants that thrive in western Oregon's mild, wet winters. Moss can make a deck or patio slick and dangerous and can damage a roof's shingles, but it also can be a beautiful and carefree ground cover in the garden.

Lawn moss

Mosses form a velvety expanse of green in places where grass is not thriving, usually because of excessive shade, poor drainage or overwatering. The simplest solution is to appreciate the moss as a lovely, trouble-free green ground cover that is soft underfoot. But if you’re looking for a solution, read on for ways to prevent moss from growing in your grass.

mossy grassPrevent lawn moss by changing conditions

  • Let the sun shine in. Remove lower limbs from trees and shrubs to limit shady areas.
  • Promote good drainage. Aerate compacted soil with a sod-coring tool or a spading fork. Topdress with weed-free compost (see “Improve the soil” below). In extreme cases, subsurface drainage like French drains or leachfields may be necessary.
  • Overseed lawn. Sprinkle a locally appropriate lawn seed mix over your lawn to help out-compete moss. Apply about 25 percent of the sowing recommendation for a new lawn.
  • Improve the soil in spring or fall. Top-dress by spreading a quarter- to half-inch layer of compost that's free of weed seeds. If a soil test indicates a need, add an organic or slow-release lawn fertilizer. Grasscycling – leaving the clippings on the lawn – can reduce fertilizer needs by 50 percent or more. If a soil test indicates the pH is low (meaning it is acidic), add ground limestone or oystershell lime along with the topdressing of weed-free compost.
  • Don't over-water. Just one inch of water (from all sources, including rain) per week is enough.

Replace lawn with shade-loving ground covers

If you have a shady yard, you may want to try more maintenance-free groundcovers. Native plants that may be useful include Fragaria vesca (woods strawberry), Oxalis oregano (redwood sorrel), or Maianthemum dilatatum (false lily of the valley).

Remove lawn moss with elbow grease

If the moss isn't too extensive, it can be removed manually in early spring.

Rake it out and renovate. Use a dethatching rake or a mechanical dethatcher available from rental agencies. After this procedure, stimulate grass growth and density with organic fertilizer, a topdressing of weed-free compost and locally appropriate grass seed.

Chemical methods are only temporary. Two chemical treatments that will temporarily kill moss in lawns are iron sulfate and potassium soap salts. While both are allowable by national organic standards, they can cause severe skin and eye irritation. The soap salts are also highly toxic to aquatic invertebrates. With either chemical, care is necessary to avoid runoff into storm drains or surface water. These chemicals are short-lived fixes, and still require elbow grease – raking out the dead moss. Unless lawn conditions are improved, the moss will come back.

mossy roofRoof Moss

Problems with roof moss typically arise on north-facing slopes and in the shade. On most standard residential roofing materials – either composition shingles or cedar shingles and shakes – moss can damage the roof.

Prevent roof moss with maintenance or metal roofing

Leaves and needles hold moisture on the roof's surface and nurture the growth of moss. Maintenance or different roofing materials can prevent problems.

  • Clean twice a year. Regularly remove debris to help keep the roof dry and moss-free.
  • Prune branches. Trim and thin overhanging branches that keep the roof in shade to speed drying after rainfall.
  • Switch to metal. Moss can't grow easily on a smooth surface, so if it's time to replace your roof, consider metal. This is a permanent solution and while it is initially more expensive, metal roofing lasts longer and is a comparable value over the roof’s lifetime.

Remove roof moss with wet sweeping

  • Sweep with a wet broom. As long as the growth isn't too well-established, wet sweeping will remove most of the moss and is the most environmentally-friendly choice.
  • Avoid zinc and pesticides. Three-inch strips of metallic zinc nailed near the top of the roof are effective at reducing moss, but the cumulative effect of the released metal can pollute rivers and streams. Pesticide products most commonly sold for moss control contain zinc sulfate, which is highly water soluble and toxic to aquatic animals.

Don't use pressure washing or detergent on a roof

Pressure washing isn't recommended for roofs because the water can get underneath and damage the shingles. In the past, sprinkling detergent to kill the moss was advised. However, roofers now caution against it because the phosphates in detergents were replaced with a degreasing agent that attacks and degrades the asphalt shingles.

Moss on other surfaces

Moss can build up on benches, and pose a safety hazard by making surfaces like decks and walkways slippery.

Prevent moss with regular maintenance

Moss is easiest to remove in the early stages of growth, so the best prevention is regular monitoring and cleaning beginning in the fall.

Remove moss by scrubbing or pressure washing

Scraping and scrubbing removes moss on most surfaces. Pressure washing can make the job easier. While potassium soaps used for lawns can also be used on sidewalks and other structures, runoff can pollute storm drains and surface water.

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