Potassium salts are fertilizer. What did you use? I don't understand how it would hurt plants unless you applied more than you could carry up to the roof. I don't understand how it could hurt the moss either.

Zinc is a lot less soluble than potassium. It will be in an oxidized form unless in a highly acidic environment.

Hi-

It's this stuff, and it killed broadleaf leaves and fern leaves under the dripline. Left big brown spots. (In fact since that experience, I got their "weed killer" which is more or less the same thing, and it works sort of like Roundup on my walkway.) Works great on the roof moss too...

http://www.saferbrand.com/store/garden-care/5324

Thanks for advice...

That's soap. The potassium counter ion is kind of incidental. Zinc strips are not going to work fast if you have a lot of growth up there. The moss will soak up the zinc proportional to the mass and the release rate from the strips is very slow. If they were my shingles, I would first kill the moss with bleach and TSP (or TSP substitute if you can not buy TSP in your area). Just wet down anything else, including beneficial plants and the ground, that will be exposed before and after use. After that, install the strips.

There are spray-on zinc products that could be used in conjunction with the strips to jump start that process. I can't comment its effects on plants on the ground. I have never done any damage to plants on the ground with bleach and TSP if well watered before and after.

thanks