Black Walnut Trees in the Home Landscape and Garden

The other day, I was asked a great question in the comments section on my post about what wood to use in hugelkulture about planting black walnut trees. Specifically, about the ability of black walnut trees to “poison” other nearby plants with “chemical warfare.” I figured I would write up a post about some things to consider before planting these magnificent trees at your home or homestead.

First off, why do we even need to be concerned that a black walnut tree might kill our plants? Well, black walnut, along with hickory and pecan trees, produce a chemical called juglone. This chemical is toxic to many, but not all, species of plants and even some animals if consumed in large quantities. This gives black walnut trees an advantage in the wild, helping it to compete among other trees, vines and shrubs for sunlight, nutrients, and water. But not all plants are sensitive to juglone. In fact, many plants that co-evolved in the same areas with black walnut are not affected by the alleopathic effects of the juglone.

Furthermore, there are many factors that can influence a plant’s ability to live near a walnut tree and not succumb to the juglone poisoning. A few of the factors are related to soil and site conditions (pH, moisture, soil life), while others have to do with proximity to the tree. Because juglone does not travel very far in the soil, only plants immediately around black walnut trees are susceptible to poisoning. Also, by planting buffer trees, or trees that are not affected by juglone such as black locust, mullberry, elderberry, and black cherry, you can essentially contain the juglone to the area immediately under the tree.This area is particularly high in juglone because of the accumulation of leaves, nuts, nut hulls and roots.In this area, it’s best to only plant plants that are tolerant of juglone (check out a great resource here).

Plants that are not juglone tolerant include:

• apple
• azalea
• birch, white
• blackberry
• blueberry
• chrysanthemum
• crocus, autumn
• forget-me-not
• grape, domestic
• lily-of-the-valley
• linden
• mountain laurel
• peony
• pine
• potato
• rhododendron
• thyme
• tomato

Keep these far away from your black walnut tree, and make sure that when it drops its leaves in the fall, that they don’t settle down and decompose around these plants.

Personally, I think that there are a lot of options when it comes to using  black walnut in the landscape, particularly  in a food forest setting. With a little research, and some planning, anyone can take advantage of its majestic shape, delicious nuts, and extremely valuable timber. The nuts are easily planted in the fall, 3-4 deep and covered with a layer of either straw or leaf mulch. After freezing and thawing all winter, they should germinate in the spring. Just make sure to mark them off so you don’t mow them down next year!