Reader B.R. Olsen of Staunton writes, “My garden space cannot be located farther than 50 feet from a grove of black walnut trees. Is there any type of tomato that grows well in this environment? This space gets full sun from mid-morning on.”

An old gardening axiom says we should never plant a vegetable garden in the shade of a black walnut tree (Juglans nigra). This reader question caused me to do some research to find the facts.

Black walnut trees have a curious effect on plants surrounding them. Some of the effects are positive, some negative. The black walnut has a beneficial effect on certain kinds of grasses, especially fescue and bluegrass. It is a good tree to grow in livestock pastures.

If, however, you are a vegetable grower with black walnut trees adjacent to your garden, you have a problem. The most interesting thing about the black walnut tree, from a botanical perspective, is the way it prevents competition.

Unlike the maple or the beech tree, both of which shade out competitors, the black walnut adopts a different technique: It poisons the competition by releasing a substance called juglone from leaves and roots. Juglone is a natural herbicide that kills many plants, especially tomatoes and potatoes. The chemical is released from black walnut leaves when it rains and is carried to the ground below, reducing the competitive growth of certain plants under the tree.

Awareness of black walnut toxicity dates back to Roman times, when Pliny noted the poisoning effect of walnut trees on vegetable plants. Modern research from Purdue University shows large concentrations of juglone occur in walnut buds, nut hulls and roots. Leaves and stems contain a smaller quantity.

Vegetable gardens should always be located away from black walnut trees to prevent damage to susceptible plants. If proximity to a tree is unavoidable, I suggest the use of a raised bed to provide limited protection.

The bed must be constructed in such a way as to minimize tree root penetration into the raised portion. Also, keep the raised bed free of black walnut leaves and nuts. Leaves and bark of black walnut should obviously not be used as mulch. Even after composting, the residue still releases small amounts of juglone.

source : www.newsleader.com