In one yard, we have 2 aging Black Walnut trees. Every August they start going dormant and lose their leaves. Grass, weeds, vines, (all the undesirables) seem to grow just fine around the base of these trees. I have a feeling that these trees are so old and the soil so mounded up, their level of toxicity doesn't reach the surface where most shallow roots grow. Why anyone would want to do it I don't know but apparently some people have tried using black walnut hulls for stable bedding and found that horses absorb the poison through their feet. martha