On the positive side, black walnut trees have been prized by furniture makers, carpenters, and wood carvers for their beautiful dark grained wood.  Their tree nuts make it an excellent variety to plant for wildlife.  Squirrels like to bury the tree nuts for later eating.  Humans also like to pick the nutmeat out of the green covering and walnuts are a popular not for use in baking muffins, cookies, and bars.

The tree is found across the northern United States among other forest trees.  Single stands of black walnut trees are usually not found, instead a single tree will tower above other deciduous trees in the forest.  The black walnut can grow to 100 feet tall with deep furrowed dark bark and thick branches, which makes it a great shade tree. Leaves have 11-13 leaflets along a toothed spear. The tree likes moist well drained soils along creeks and rivers.

But black walnut trees carry a major negative characteristic. The entire tree is toxic.  Roots, buds, leaves and nut hulls contain a chemical called juglone toxin.   The soil under the tree will have the highest level of juglone due to the accumulation of roots, fallen leaves, hulls and nuts lying on the ground.  Collecting this debris is a good sanitation habit but, nonetheless, the highest amount of juglone is in the roots, which run underground.   

So, what does that mean for the homeowner who has black walnut trees on their property or may want to grow this tree?  You must be aware that most grass, shrubs, herbaceous flowers, some trees, and vegetables will not grow near black walnut trees.  Vegetables such as asparagus, cabbage, peppers potatoes, rhubarb and tomatoes are sensitive to black walnut tree juglone.  Apple trees, blueberry and pear trees are also affected.  Azalea, chokeberry, hackberry, hydrangea, lilac, red pine, white pine, Norway spruce, and yew, do not tolerate juglone.  

However, there are some landscape plants that do tolerate juglone.  They include, arborvitae, clematis, crabapple, honeysuckle, most maples, oak, and most viburnum.  Flowers that will tolerate the toxic soil are zinnia, Siberian squill, lungwort, bee balm, coral bells and others.  Some sources recommend planting 50-80 feet from the tree. The University of Minnesota Extension has a complete list of plants that can be planted near a black walnut tree.

The black walnut tree is beautiful in the landscape.  But the chemical juglone, that is present in all parts of the black walnut tree, provides a cautionary tale for the suburban gardener. 

Credits

https://extension.umn.edu/managing-woodlands/managing-black-walnut-forests

Photo Credit: www.flickr.com (All Creative Commons) (1), Autarmota.blogspot.com (All Creative Commons) (2), www.Marylandbiodiversity.com (All Creative Commons) (3)