I just found this page on Peaceful Valley Farm Supply’s site that lists http://www.groworganic.com/organic-gardening/articles/companion-plants-that-tolerate-black-walnut-tree-toxicity

This is good, because at the front, or south end of the garden, there are some huge, old walnut trees above the ditch. Who knows how long they’ll last, but I bet their roots will have a longlasting impact. The south end of the garden is pretty dark in winter, but it’ll be nice to have some appropriate stuff growing in that area!