Black Walnut Toxicity

Black walnuts (Juglans nigra) are native to Iowa and found throughout the state. They are prized for their beautiful wood and tasty nuts. However, black walnuts are poor trees for many home landscapes. Not only are the nuts messy, they also create difficult growing conditions for vegetable and flower gardens.

Like most large trees, black walnuts cast considerable shade. They also absorb copious amounts of water and nutrients. These competitive pressures limit what can be successfully grown under large trees. Black walnuts also produce one additional problem for nearby plants. Walnuts produce an allelopathic compound called juglone that may be toxic to some plants.

Generally the highest concentration of juglone occurs in the soil within the dripline of the tree. The roots (which can extend beyond the dripline) produce the highest concentration of juglone in the summer. Emerging leaves can produce large amounts of juglone in spring and maturing fruit can do so in fall. This constant production of juglone prevents some plants from performing well within the vicinity of a black walnut or other members of the same genus. Butternut, hickory, and pecan also produce juglone, but much less than black walnut.

Tomatoes are one of the most sensitive plants to juglone. Tomatoes grown near a black walnut may wilt and die. Other members of the Nightshade Family such as pepper, eggplant, petunia, and flowering tobacco are also susceptible to juglone. Rhubarb, peonies, lilies, asparagus, blueberry, columbine, mums, false indigo, and cabbage may also be affected by juglone. These plants should not be planted near black walnuts, as they will grow poorly and may die.

While the list of susceptible plants is impressive, there are many plants that are considered "resistant" to the compound. Vegetables like corn, beans, onions, beets, and carrots grow well in the vicinity of black walnut as long as they receive ample sunlight, water, and nutrients. Shade tolerant annuals and perennials such as begonia, pansy, bleeding heart, astilbe, sweet woodruff, coral bells, hosta, Jacob's ladder, Solomon seal, lungwort, spiderwort, and violets are also considered resistant to juglone. Even several bulbs like crocus, snowdrop, hyacinth, grape hyacinth, daffodil, squill, and tulip are not normally injured.

Fortunately, juglone is not long lasting in decaying leaves, wood, and roots of black walnut. Leaves and sawdust can be composted if allowed sufficient time to break down the juglone. Decomposition generally takes 2 to 6 months at which time composted material can be safely used around sensitive plants.

The presence of black walnut trees in the yard does not mean giving up gardening. Unfortunately, it does usually mean giving up growing tomatoes nearby.

This article originally appeared in the June 14, 2002 issue, p. 82.

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