Black Walnut Trees & Gardens

The black walnut is a large deciduous tree, reaching up to 75 feet in height, with dark crinkly-looking bark. Coveted for its hardwood, and a popular shade tree, the black walnut also sprouts long green flower clusters in the spring, and produces edible nuts wrapped in green husks that turn dark as the enclosed nuts mature and ripen. The black walnut can be challenging to the home garden due to its production of juglone, a toxic substance released into the soil that adversely affects surrounding plant life.

Naturally produced, juglone is a chemical found throughout the black walnut tree. Surrounding plants affected by this toxin will turn yellow, wilt and eventually die off. The toxin leaches into the ground via tree roots, and the impacted toxic area spreads outward each year as the black walnut tree continues to grow. The largest concentration of juglone occurs in the roots, buds and nut hulls, but the leaves and stems do contain small levels of toxin. While a large percentage of plants will not survive when growing near black walnut, some do resist the effects of the toxin. Annuals such as zinnia, marigold, morning glory and pansy perform well near black walnuts, and hosta, hollyhock, spiderwort, phlox and speedwell are a few perennials not affected by juglone. A number of trees and shrubs are also juglone-resistant, and tall fescue and Kentucky blue grass grow well around black walnut trees.

To help stop the toxin spread in the garden, regularly clean up any nuts or leaves from the ground, and keep any tree debris away from healthy garden areas. If the garden area is located near the tree, plant only juglone-resistant plants, and keep any sensitive plants away from the root area, or away from areas where tree leaves may fall. Keep the soil well nourished. Healthy soil with a high organic content helps to promote an environment that can tolerate toxicity. Do not use the leaves, bark or wood chips from a black walnut as mulch in the garden as the toxins will continue to damage non-resistant plants.

Compost black walnut leaves, as over time the toxin weakens when exposed to air, water and bacteria, typically within four weeks. The same toxin breakdown when composted in soil may take up to two months. If uncertain, compost the leaves separately and test for toxicity by planting a small tomato seedling and observing the results. Since they are sensitive to the toxin, even a small amount of juglone will damage tomato plants. Bark or tree cuttings will require a compost period of at least six months to break down toxins before they can be used again.

The black walnut is hardy to U.S. Department of Agriculture Plant Hardiness Zone 4, prefers moist, well-drained soil, and will struggle in overly wet or drought conditions. Best grown in full sun, the black walnut tolerates very limited shade. The wood of the black walnut, known as heartwood, is used mainly for furniture and veneer, and the pulp and roots are used to make black dye. Squirrels favor the walnuts, and are one of the few animals with teeth strong enough to break through the tough hulls.