Black walnut trees poison many of the plants that grow beneath them.
Gardeners have long noticed that tomatoes and apples struggle near black walnuts. The reason is juglone, a chemical the tree makes in its roots, leaves and husks that stunts many other plants. It is a classic case of allelopathy: plants waging chemical warfare on their neighbours.
The harmful effect of walnut trees on nearby plants was noticed as far back as the 1st century CE. The compound responsible, juglone, was first isolated from black walnut in 1856 and identified as the culprit in 1881. It occurs in the leaves, roots, husks and bark of the walnut family, especially the black walnut, Juglans nigra, and the butternut.
Evidence built up through the early 20th century. A 1921 study noticed tomato plants wilting near black walnuts. In 1926 damage to apple orchards in northern Virginia was blamed on walnut and butternut trees, though some disputed whether the offending trees were walnuts at all, and walnut trees in alfalfa fields were seen to kill the crop while grass survived. In 1942 experiments showed that pieces of walnut root inhibited the germination and seedling growth of tomatoes and alfalfa.
Inside the tree, juglone exists in a colourless form that turns into the active compound on contact with air; you can see this in the way a freshly cut walnut husk darkens from yellow to black. In susceptible plants it inhibits enzymes needed for respiration and photosynthesis, even at concentrations common in nature, and disrupts the pores that control water loss. It harms root and shoot growth more than germination.
There is a silver lining for gardeners. Not all plants are susceptible: some maples, birches and beeches are resistant, and grass often copes. Certain soil bacteria can break juglone down and live on it, so it is less active in well-aerated soil. In the American Midwest, where valuable black walnuts are grown alongside maize, pruning and root barriers greatly reduce the damage. Juglone's dark staining power has also made it a traditional dye for wool, inks and hair colourants.
Source: Wikipedia — Juglone · Text summarised from Wikipedia (CC BY-SA 4.0)
