On the shaded, north side of our six-year-old asphalt shingle roof, there are flat patches of what appears to be lichens growing. In the past, i had used a product form Gardens Alive!, an organic gardening supply company, to eliminate this problem. Now, the company no longer carries the product (I have forgatten the name) and I get blank looks whenever I inquire about such a product and a lawn and garden store or at a lumber yard.
Bleach and some of these products can be hard on vegetation and shingles. A strip of zinc at the ridge will let minute amts of zinc to wash down over the roof and discourage growth of moss and lichens.
piffins has got it right absolutely the zinc strip is miracle worker for growth, just install it under the ridge shingles at the top of your roof (be sure to let some of it stick out of course) and you are good to go, you should be able to find these products at like a home depot ***** or even a roofing wholesale supply shop. be careful with using chemicals on your roof it can severly discolor and ruin the shingles and it may even void your warranty