Are zinc roof strips toxic for my garden?
I am considering putting zinc oxide strips on my roof ridge to control moss growth. I collect roof runoff in a rain barrel for watering our vegetable garden and fruit trees. Will the amount of zinc exuded by this kind of roof ridge strip be enough to hamper plant growth/fruiting, etc.? I think, from what I've read, that the amount of zinc leached out during rainfall is quite small, only enough to prevent emergent moss colonies from forming (i.e., not enough to kill established moss plants, although in higher concentrations, zinc is mossicidal, like when it's applied in granules or sprays as zinc sulfate). Any ideas?
Research into roof runoff from different building materials has shown that zinc does leach from roofing materials, and is released during rainfall events. However, in all the studies I have looked at, zinc levels from roof runoff are considerably below (less than 10% of the standards in most studies) both EPA drinking water standards and storm water effluent standards for zinc. The good news is that it doesn't take much zinc to kill mosses. The bad news is that even though concentrations of zinc from roof runoff are low, they are elevated above background runoff levels, and can be taken up by plants which receive the roof runoff. Different plants have widely varying susceptibilities and responses to zinc concentrations in soils. Factors affecting plant susceptibility to zinc are numerous, and include such things as length of exposure, biological availability, interactions with other metals, and the nutritional status, age, and micorrhizal associations of the plant. Research has shown that Zn levels as low as 100-200 mg/kg can have negative growth effects in plants, but in general, symptoms of zinc toxicity are not observed until zinc levels in soil reach the 1,000 mg/kg level. The highest concentration of zinc I have ever seen in a study of roof storm water runoff is 114 mg/l, well below any level toxic to plants, but at a level that could cause sub-lethal growth effects.
So, it is very difficult to predict whether and how zinc from roof runoff will potentially influence plant growth, but there are several other options that will remove or significantly reduce moss on your roof without the potential long-term negative effects of zinc. See the following publication from Clean Water Services in the Portland/Metro area for more information on alternative treatments for reducing moss on roofs:
The most important advice I can give you on alternative treatments is that if you choose to use one-time treatments, you:
- use them in the summer when mosses are most stressed from lack of water
- disconnect your downspouts from rain barrels, rain gardens, or bioswales during the treatment
- leave your downspouts disconnected from rain water infrastructure through the first significant rainfall of the season to reduce the possibility of contaminating plants with leachate from moss treatments