Walnut Trees, Friend or Foe
Black walnut trees (Juglans nigra) and butternut trees, also called white walnut trees (Juglans cinerea) are commonly recognized by gardeners as having a toxic trait. They contain juglone, concentrated primarily in their buds, leaves, stems, nut hulls, and roots. This chemical spells trouble for many surrounding plants, causing wilting and ultimately death. Juglone is the walnut tree’s ultimate survival technique as it reduces any competition for nutrients and water in the surrounding area.
Walnut trees are majestically beautiful trees with great value not only for people, but also for wildlife. If managed properly, they can be incorporated into many landscapes with pleasing results.
Black walnut are big trees, growing 50-60 feet tall, occasionally reaching 100 feet, with a diameter around 36 feet. They are proud trees, straight and clear of branches for half of the height. Their thick very dark bark is divided by thick fissures. They produce large round nuts, maturing in the fall, singly or in pairs that are enclosed in a green husk. The nut shell is dark black and finely ridged. The kernel is highly nutritious to man and animals.
Harvesting walnuts can be a challenge as removing the husks is not a task for the faint of heart. Stains from this process can leave permanent marks on clothing and tools, so be prepared. Husks can be removed by soaking them in water and them peeling them by hand. Alternatively, driving over them with a vehicle is easier and faster. Once the husks are off, soak the nuts in a pail of water. The good ones will sink to the bottom and the poor-quality ones will float. Cure the good ones by spreading in a thin layer in a well-ventilated area, away from any squirrels, for several weeks. This will ensure long term storage before shelling. Once shelled, the nut kernels can be stored in the freezer.
Black walnut wood is heavy, hard, strong, and relatively free from warping. It is highly prized for a wide range of uses including furniture, gun stocks, and even airplane propellors.
Black walnut trees grow naturally in rich bottom lands in the southern part of Wisconsin. They are not shade tolerant, are easily propagated from nuts, and are a valuable part of the ecosystem.
If a walnut tree is already in the landscape, there are ways to garden around it. Consider using a raised bed so that the roots of plants to not come into contact with the walnut roots. Be sure to remove walnut leaves and nuts from the bed as the season progresses. There are plants that are juglone tolerant. The bulletin Black Walnut Toxicity from UW Extension (https://pddc.wisc.edu/wp-content/blogs.dir/39/files/Fact_Sheets/FC_PDF/Black_Walnut_Toxicity.pdf) has a list of the plants that can be planted in the vicinity of these trees. The reality is that there are more plants that are juglone tolerant than there are that are juglone sensitive.
While it can be frustrating to work around the toxic trait of these trees, they are well worth the effort. They provide ample shade, stunning beauty, are host trees for multiple species of moths, provide habitat for wildlife, including nesting spots for the Eastern screech owl, provide food for many small mammals, and provide a very nutritious nut for our consumption.
Carol Shirk
Certified Master Gardener