I’d like to hear your thoughts about the safest way to
remove moss from the north side of my roof. I do
organic gardening, and I don’t want to contaminate the
soil with runoff.
The City of Portland had a guide to safe moss removal (formerly available online) indicating that that the most common product sold for moss removal, zinc sulfate, is a pollutant and is toxic to aquatic life. They do say that zinc strips on the roof are an alternative, but they also release zinc, and are pretreated with pesticide.
Toxic-Free Future (formerly Washington Toxics Coalition) offers several recommendations, including physical removal of the moss, landscape planning to keep the roof clear, and the aforementioned zinc strips
Oregon State University maintains a website devoted to mosses, and one page addresses the use of zinc strips as a control. Here is an excerpt:
“Zinc strips are usually considered the long-term solution to controlling mosses […] Zinc strips and galvanized flashing are apparently relatively safe and inexpensive. They effectively kill or retard the growth of mosses and fungi and appear to have effect up to 15 feet below the zinc flashing along the length of the flashing. To use: apply the rolled zinc or galvanized flashing to each side of ridge caps along the roof peaks. Place a nail down each foot of the zinc strip. With each rain zinc is released from the strip and kills the mosses below the strip. For best results remove the existing mosses prior to treatment. The active ingredient is metallic zinc.
Effectiveness: Zinc strips are considered to be effective for up to one year for most brands. The effect of galvanized flashing (example above) can persist for decades. Success rates vary with the degree of moss development and weather. Zinc strips or flashing are most effective before mosses are well developed. Physical removal of existing moss followed by installation of zinc strips or flashing is an effective long-term strategy for suppressing moss growth.
Negative Side Effects: Direct runoff from the zinc strips or flashing to surrounding vegetation, fish ponds, or water supplies should be avoided, because some contamination by zinc is likely to occur. Zinc strips should not be used with strong acids or bases.”
The PAN Pesticides Database has an entry for one type of rolled zinc strip product.
Washington Department of Ecology has a lengthy document analyzing zinc
concentrations in industrial runoff, and you can imagine that homes with
zinc strips are simply smaller contributors to this problem.
The Environmental Protection Agency lists zinc metal strips (consisting
solely of zinc metal and impurities) as minimum risk pesticides.
I think there is no easy solution, and it is a matter of deciding on a
lesser of two evils: the physical difficulties of cleaning the moss off
the roof, which is most environmentally sound, versus the relatively
small amount of pollution from using zinc strips.