Along with Microbe, what about the shade? With "100%" shade, really, you would have no grass growing under it. So assuming you have like 80%+ shade, that is a huge part of the battle there. I have one customer in particular who has a very shady backyard, and the grass there is always thin and stringy. He simply doesn't want to thin out the trees, so I told him long ago in that case his lawns present condition is about the best he can expect. I explained the situation to him, and he understands. He is actually a very good (and rare!) customer!