These don't appreciate shade and would be liable to appear even more gloomy and scruffy stuck beneath another tree than they do anyway. Then there is the matter of the visual tension produced by having a conical conifer stabbing up through a deciduous tree (unless the shade is heavy enough to prevent it from growing up through the branches of the walnut much). If you don't require tall plants use broadleaved evergreen shrubs with rounded or spreading growth habits instead of coniferous evergreens. If you require tall plants that will be likely to grow well above the bottom level of walnut branches it would be greatly preferable to have these located behind or beyond the spread of the tree branches.

Whether or not a tree develops a so-called tap root has little bearing on the appearance and location of the rest of the root system. All trees will concentrate the bulk of their roots very close to the soil surface, typically within the top 12-18", with most of the very fine and dense feeder roots directly below the soil surface. That's where the oxygen, soil moisture and nutrients are located and so that's where the roots predominate. Any forestry or arboriculture resource will confirm this. It is one thing to establish smaller growing, very shallowly rooted perennials like hostas under the canopy of any large tree -- it is quite another to plant another tree or large shrub. The root systems will compete and the larger one usually wins unless special care is provided for the smaller species until it is well-established. Often this need for additional attention will persist, especially with drought intolerant species like Japanese maples.

Thanks all for your thoughts! Yes, this is the only place I can plant. It is a typically smallish downtown Toronto yard, with several trees shading the (Eastern) 2/3 of it. That leaves the back (Western) 1/3 open for planting. I've actually decided to do fewer beds because I have become less optimistic about my sunlight situation (and my budget for buying soil). The research I've done suggests that the highest concentration of Juglone is in the roots of the tree, and if I can keep the roots from coming in contact with the soil around my sensitive plants (as well as keeping the beds away from the dripline and clearing them of any Walnut tree debris), I am hoping that even the sensitive plants should be ok. The whole yard is pretty dense with tree roots (maple, cedar...) which could also creep into my beds and, while not poisoning my plants, they could crowd their roots and feed on their nutrients. I've been told that as I water my garden, the tree roots are likely to rise towards the surface of the yard where the moisture is, rather that growing down. I hope I don't sound too paranoid, but I figure I should try to do this right the first time.

here is what grows under my black walnut: mint, sorrel, sage, oregano, chives, parsley, nettles, fennel, various flower bulbs (asian lily, daffodil), various daisy type flowers (echinacea, black eye susan, regular daisy, cosmos), delphinium, iris. what does not grow: tomatoes, potatoes. grew poorly: several types of cucurbita, garlic and onions.