Now that you have said that, Reid, I feel I can continue without being accused of being too preachy:
Compost can only help, but is not an answer at all. What little water your able to conserve would be of little use, as the tree will continue to suck the soil dry no matter how much you add. The real solution is to remove the "needy" plants, and replace with compatible materials. Others include Large flowered Bellwort (Uvularia grandiflora), hellebores, Astrantia, Aruncus aethusifolius (similar to astilbe), and both the native and European forms of ginger (Asarum).
