(I hope) For the past three summers, I’ve grown a few dahlias in some pretty crummy soil above a boulder retaining wall, where I had ripped out some big old shrubs. The first year, the dahlias did really well, growing bushy and full of colorful flowers. This past year, not so much. Three out of nine, brand new, didn’t even come up. The six that grew — three new, three second-year — didn’t flower very much. I realized I had not given them any fertilizer since late spring. At least I kept them watered. By the end of this summer…