I feed mine with a general fruit and vegetable fertiliser (something like Growmore) with a couple of handfuls scattered around the root area (the diameter of the branch spread) in about March, depending on the weather, and then mulch with leafmould. My trees are anything from 2 - 6 years old, grown in a wildflower orchard and we keep the root area well weeded and gently forked over. We would probably need to mulch more heavily than in the UK because our springs are generally warmer. You can also give a feed of rose fertiliser in late winter every few years once they are established. To be honest, once they are really well established, you won't need to feed them much at all, as long as you are on a good and deep soil. I continue to feed mine because we only have a relatively shallow soil over rock.