Even in the days of the Romans, people knew that walnut trees affected gardens. Black walnut (Juglans nigra) is an important hardwood lumber and shade tree. However, it produces juglone, a natural chemical that leaches into the soil from all parts of the tree. Juglone acts as a respiratory inhibitor, leaving plants unable to breathe. Plants sensitive to juglone may wilt, have stunted growth or even die. If you're planning to grow edibles near a black walnut tree, consider plants that are tolerant of this toxin.
Juglone-tolerant vegetables that grow on vines, stems or bushes above ground include lima beans, snap beans, corn, melons and squash. Make sure the plants get at least six to eight hours of direct sunlight each day to help battle any toxins from the black walnut tree's drip line.
Root crops that tolerate juglone include beets, onions, parsnips and carrots. Remember that juglone can remain in a tree's roots years after the tree has been chopped down. Squirrels often will harvest the nuts and bury them in your garden.
Never mulch your garden or landscape plants with black walnut wood chips, leaves or bark. Remove fallen leaves or nuts from your garden immediately.
If you can't move your garden away from the tree's dripline, raised beds may help. Construct the beds at least 8 inches high to minimize root penetration from the tree.