Most people think of Black Walnut trees as an asset, even if they don’t take the trouble to gather and shell the plentiful nut harvest under them. Walnut is a prized hardwood traditionally used for gun stocks and fine furniture, so walnut trees have a special status even if they’re not particularly attractive as landscape trees.
Black Walnut trees present a special challenge in the landscape, because they secrete a substance that is toxic to many plants. Known as “juglone” this poison is secreted by walnut roots, which can extend 50 feet from the trunk of a mature walnut tree. This means that homeowners need to pay special attention when planting or landscaping anywhere near an existing walnut tree.
You may already have situations in your landscape where the residues from existing walnut trees, or trees that were there in the past, are affecting the health of your landscape. This may be an explanation you hadn’t considered, for why some of your plants simply fail to thrive in certain parts of your yard.
Even when the tree is removed, juglone lingers in the soil for years and can stunt or kill many types of plants. When planting near Black Walnut trees or where they have been removed, gardeners need to pay close attention to what types of plants tolerate juglone in the soil. Trees sensitive to juglone include Silver Maple, Birch, Apples and Crabapples, Norway Spruce, and most species of Pine. Affected shrubs include Chokeberry, Hydrangea, Privet, Potentilla, Rhododendrons and Azaleas, Lilacs, Taxus (yew), some Viburnums, Blueberry and Blackberry.
Some perennials are also sensitive to Black Walnut trees, including Columbine, Asparagus, Baptisia, some Chrysanthemums, some Narcissus, Lilies, Peonies, and Rhubarb.
Black Walnut toxin can affect annuals and vegetables too. Cabbage, Peppers, Tomatoes, Tobacco, Petunias, Potatoes, and Eggplant are affected.
There is an extensive list of plants that will work near Black Walnut, so if you have mature Walnut in your yard your nursery can help you find plants that aren’t affected. If you are considering planting Black Walnut trees (or other nut trees grafted to Black Walnut rootstock) you should look into the compatibility before you plant.
One final comment about walnut trees. Walnut wood can be valuable (depending on market conditions) if the tree can yield a quality “saw log.” Since walnut trees tend to fork, they rarely make long, straight saw logs on their own. If you have a young walnut tree in your yard you should pay close attention to it when it’s young, to make sure it develops a “clear” straight trunk. You need to cut off low-hanging limbs (a practice called “limbing up”) and make sure the trunk doesn’t fork until the straight section is at least ten feet. The higher you limb up, and the longer the straight section before the first limbs, the more valuable your tree could be in the future.
Steve Boehme is a landscape designer/installer specializing in landscape “makeovers.” “Let’s Grow” is published weekly; column archives are online at www.goodseedfarm.com. For more information call GoodSeed Farm Landscapes at (937) 587-7021.