The Truth about Black Walnut Poisoning
Black walnuts are beautiful trees that are prized for their form and their wood in the landscape. Unfortunately, they can also cause problems with other plants growing the same landscape area. The roots of black walnuts and their relatives (butternuts and hickories) all give off a substance called juglone. There are a number of plants that are very sensitive to this toxin. But there are even more plants that don't seem to be affected. In spite of what many gardeners seem to think, when you find a black walnut growing in your landscaping, it is never growing alone. Juglone is obviously not poisonous to everything.
One of the common misconceptions about all trees, not just black walnut, is just how far the root system grows. It used to be taught that the roots extend just beyond the drip-line at the edge of the canopy of leaves. Actually, in average soils, the roots of a shade tree will extend at least the same distance in all directions as the height of the tree. In sandy soils and with some varieties of tree, the roots might extend twice that far.
When plants that are sensitive to juglones are grown close to the roots of a black walnut, the poison will actually shut down their respiration. With woody plants, you will see a yellowing and growth might be stunted sometimes followed by death. With plants such as tomatoes that grow very quickly, often the first noticeable symptom is a sudden wilting that isn't related to water.
Whle the juglone toxin is produced by all parts of the tree including the leaves and the nuts, the strongest source is the root system. Raking and disposing of the leaves will help minimize the amount of juglones in the soil. The leaves should not be composted or used as a mulch, because the toxin will persist.
The nuts seems to have no effect on the people or squirrels that love them. Juglone does, however, seem to keep the earthworms away. There are black walnuts problems not related to the juglone toxin. Skin reactions, sometimes severe, may occur. People can also be allergic to the pollen given off by the catkins in late spring.
Unless it is just a small seedling, removing the tree isn't a practical solution because the decaying roots will continue to give off juglone for years. Is there anything you can do about the problem? First of all, use plants that are resistant to juglone. Some research has been done regarding the effect of black walnuts on other plants, but most of it involved field crops, not landscape plants. One of the most encouraging bits of information is that the effects of the toxin seem to be much less if the soil is heavily amended with organic matter. Apparently the bacteria in the compost help break down the juglone. So try amending the area with lots of compost. If all else fails, try growing your plants in a container above ground.
Here is a list of plants that are known to be susceptible or known to be resistant. Use the lists as guidelines, keeping in mind that many of the plants have made the list based on several gardeners personal experiences. There hasn't been enough research done to test most varieties of plants, but the lists give you a place to start.
Sensitive to Juglone
These plants are known to be very sensitive to juglones. They will usually be severely affected or killed.
Trees and Shrubs
Azaleas and Rhododendrons
Fruits and Vegetables
Resistant to Juglones Perennials
The plants on these lists have been grown successfully under black walnut trees. You may want to try some plants that aren't listed. Be sure to amend the soil with lots of organic matter.
Maidenhair Fern Adiatum
Lady's Mantle Alchemilla
Bleeding Heart Dicentra
Leopard's Bane Doronicum
Wood Fern Dryopteris
Coral Bells Heuchera
Bee Balm Monarda
Cinnamon Fern Osmunda
Garden Phlox Phlox pan.
Solomon's Seal Polygonatum
Autumn Joy Sedum Sedum spec.
Lamb's Ears Stachys
Toad Lily Tricyrtis
Trees and Shrubs
Eastern Red Cedar
Fruits and Vegetables
Vines and Groundcovers