Black walnut is one of the best-known examples of allelopathy, when a plant produces a chemical with herbicidal qualities that limit growth of nearby plants. This is a highly effective way to limit competition. All parts of the black walnut contain juglone, the allelopathic chemical. It is secreted by roots into the soil and is also found in the leaves. Therefore, people using leaves in their garden as organic material should avoid walnut leaves. Even black walnut sawdust from woodworking should not be composted or dug into a garden because of the harmful effects of the juglone.
Some plants, in particular, are susceptible to the chemical, including but not limited to tomatoes and most roses. Other plants are not affected at all, such as Kentucky bluegrass and tall fescue used as lawn grasses. A black walnut's roots, like those of most trees, will extend way beyond the branches at maturity, possibly affecting plants well away from the walnut itself.
Because the black walnut becomes a large tree, exhibits allelopathy, and has a persistent problem with walnut aphids, plant black walnut only if you have a large property and can plant it away from other garden areas. This tree is hardy in Zones 4 - 9.
