Black walnut trees have a reputation for being hard to get along with, at least to gardeners wishing to grow tomatoes and other plants in their vicinity. The trees produce a chemical called juglone, which damages other plants. The challenge is understanding which other plants are affected by juglone and the situational differences that play a role in juglone’s toxicity.

The production of juglone is also known as allelopathy. There are other species, such as fescue turf, that are allelopathic and produce chemicals that inhibit the growth of other plants or organisms.

Although scientists know that juglone is produced by black walnut trees and can affect other plants, they are still learning about the other factors that affect juglone production and how it affects other plants.

Juglone damages root hairs in certain other plant species. Root hairs are responsible for much of the water and nutrient uptake by plants and damage interferes with their ability to supply the plant. Plants may take on a wilted appearance and die in some situations.

The confounding part is that a tomato plant might wilt in one setting near a walnut tree but could be fine in another.

Proximity of the tree to the plant plays a role, but juglone concentrations can also vary from tree to tree and season to season. This may be genetic, related to the trees’ age and/or a result of growing conditions such as competition, light, water, soil chemistry, etc.

Also, organic matter and soil particles bind with juglone, reducing its ability to move through soil and contact plant roots. Microbes in the soil break juglone down. In theory at least, a healthy soil with a good concentration of organic matter and a lot of microbial activity may keep juglone from having much of an effect on other plants growing in the vicinity.

Many research institutions and reputable gardening resources have published lists of plants that are supposedly resistant to walnut allelopathy. But some of these lists simply reference the others, and some contradict one another, probably because of the environmental and situational factors mentioned above.

If you want to garden around a walnut tree, trial and error may be the best way to go. Tomato plants and related species such as eggplant are generally considered to be the most susceptible to juglone, but some gardeners report success in raised beds with healthy soil. Native woodland species are probably the most resistant since they would likely coexist in the forest with black walnut.

Fresh hulls and leaves are though to be the greatest sources of juglone, so removing them from areas containing desirable plants may be beneficial. Avoid composting fresh walnut hulls and leaves to keep juglone from contaminating your compost pile.

Gardeners often ask about using mulch that contains walnut wood. This would be possible when obtaining mulch from local arborists. There does not appear to be any research about juglone concentrations in walnut wood or the effects of walnut wood mulch on landscape or garden plants. The general belief of horticulture researchers and the gardening community is that walnut wood mulch is not a concern for movement of juglone.

There is one other factor that can affect a plant’s ability to survive around walnut trees: root competition. Black walnut trees have a dense root system that spreads far from the base of the tree. The roots may simply outcompete other plants for water and nutrients, even if the other species are resistant to damage from juglone.

— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.