Walnut trees give off juglone, a toxic chemical to most other trees. Not enough to kill 'em, but does stunt their growth. The hulls, leaves and roots give it off, so walnuts tend to grow in groves. Have some that are a foot accross and only 3-4 yrs older than white oaks that are only 3" or so, but away from the walnuts and their tainted soil, another white oak of the same age is as large as the walnuts. If you bag up the walnuts, the worms will accumulate and eat the hulls. Small critters and don't eat anything else. Wish they'd do that with my acorns.