Ah, moss… that beloved (or loathed) and ubiquitous Pacific Northwestern plant. Unlike ordinary landscape plants, mosses don’t have vascular systems to carry water and nutrients, so they must have a damp environment in which to grow. It thrives in our rainy winters and grows on our roofs, lawns, sidewalks and even our cars. Chances are that you have some happily growing on the roof of your home.

While moss on your roof or driveway is a common problem in our region, some moss removal methods can pollute our streams, lakes and wetlands and harm fish and plants in our waterways. Rainwater can wash moss killers that are left on roof down building downspouts, and off pavement, sending it to stormwater pipes and then to the nearest stream, lake or wetland.

Here are some useful tips for environmentally-friendly moss prevention and treatment:

How Much Moss is a Problem?

Growth of moss on most standard residential roofing materials (composite shingles or cedar shingles and shakes) can damage the roof. Growth of moss on pavement can create a dangerous, slippery surface.

Concentrate your moss control efforts on areas that are most prone to moss problems. These will probably be north-facing slopes and areas in the shade of trees or other buildings. Areas that get direct sunlight are unlikely to develop moss problems.

Prevent Moss from Growing

Instead of relying on chemicals to kill the moss, focus on preventative methods that will keep the moss from growing, or at least significantly reduce it:

• Prune back branches to reduce shade and falling leaves and needles. Moss may accumulate heavily on shaded areas. Pruning will let in more light and slow the buildup of moss. You certainly don’t need to remove entire trees for moss control. Just thin out the branches that are causing a lot of shade on your roof or pavement.
• Keep surfaces clean. Clean your roof once or twice a year to remove any leaves or debris that can hold moisture. Sweep pavement regularly to remove debris.
• Look for early signs of moss growth, indicated by green or black discoloration. This can be spot-treated with a less-toxic moss control product.

Physical Removal

Remove as much moss as possible physically. It is easier to clean areas in the summer when the moss is dried out. Don’t wait for a thick coat of moss to build up.

• Roof: If it is safe to walk on your roof, carefully use hand tools, not power washing, to remove moss, leaves and needles from roofs. Power washing can damage shingles or get water under them. A leaf blower or gentle sweeping can be helpful. You may want to hire someone to clean your roof for you.
• Pavement: Use a wire brush or stiff broom to remove moss and debris from pavement. You can also pressure wash pavement with cold water and no chemicals or soap. Divert water onto your lawn or filter debris out the water before it enters a storm drain.

Protect Fish – Only Use Chemicals as a Last Resort

While the safer moss-control products are less toxic, all moss killers are toxic to aquatic organisms. If you decide to use a chemical, pick a safer product for people, pollinators and the environment by using the product rankings in the Grow Smart, Grow Safe product tables. Always read and follow label directions when using chemical pesticides.

Avoid products containing zinc sulfates or copper sulfates because these chemicals are not biodegradable, and the products are often corrosive to skin and eyes.

Zinc is potentially toxic to fish and other aquatic life. If you install zinc strips or zinc-impregnated composition roofing, ensure that roof runoff doesn’t flow directly into storm drains, streams, or other bodies of water. If you collect water from your roof in rain barrels, avoid using the water for your vegetable garden. Copper strips and copper-impregnated shingles should be avoided due to copper’s toxicity: copper is roughly 10-25 times more toxic to aquatic life than zinc.

Any chemical that is used needs to be kept out of the stormwater system. Be sure that rinse water from cleaning does not run off down the street, into a storm drain, or directly into any body of water. Disconnect downspouts before cleaning your roof. Divert water containing chemicals from pavement onto your lawn.

Help protect our local lakes, creek and wetlands! It is important that nothing but rain washes into the storm drains in your street. Everything that washes into a storm drain flows directly to the nearest wetland, stream or lake without any treatment.