Black Walnut Trees

Posted: June 18, 2013

Black walnut trees have been a part of our natural landscapes forever. This native tree grows along streams in moist, rich soils, and sunny locations. They do not tolerate dry sites, often dropping leaves at the first sign of drought. They have a nice canopy, potentially reaching 100’ in height. They provide light shade and yellow fall color. These trees are very valuable, not only for the lumber it provides – often coveted by woodworkers – but also for the necessary food source it provides for our wildlife. There are problems that threaten our native black walnut. A disease - thousand canker disease – has been introduced to Pennsylvania in Bucks County (southeast PA, near Philadelphia). As a result, a quarantine was directed that no firewood, lumber, nursery stock, or scions can be transported outside county lines. This has been a disease thought to be limited to the western part of the country, but is now here as well. “The disease poses a significant threat to the state's $25 billion hardwoods industry. Black walnut trees, which make up less than half of one percent of hardwood trees in Pennsylvania, produce high-valued lumber used in woodworking and furniture-making. The nuts of the trees are consumed by humans and wildlife.” Symptoms of this disease include yellowing leaves, reduced leaf cover, and flagging of branches and eventually death.
Black Walnut Tree

Black Walnut Tree

Other issues with the black walnut include tent caterpillar, fall webworm, powdery mildew as well as other diseases that cause black spots and yellowing of leaves.  These problems happen throughout the growing season. However, these issues alone do not typically cause death to the tree – weakening, likely; death, not.

Transportation of lumber and firewood seem to be our biggest problem when it comes to distribution of any invasive problems, which is how Thousand Canker Disease is classified. Purchasing or gathering local firewood is necessary when trying to control invasive diseases and pests  in our landscapes and forests.

However beneficial the walnut tree is, in the home landscape they not only are messy due to the nuts falling and the large leaves that fall, but because of the toxin that the tree produces. This plant does not allow other sensitive plants to the toxin to compete with it.

“The Source of Toxicity”

“Plants adversely affected by being grown near black walnut trees exhibit symptoms such as foliar yellowing, wilting, and eventually death. The causal agent is a chemical called “juglone” (5 hydroxy-1,4napthoquinone), which occurs naturally in all parts of the black walnut. Juglone has experimentally been shown to be a respiration inhibitor which deprives sensitive plants of needed energy for metabolic activity.

Since small amounts of juglone are released by live roots, particularly juglone-sensitive plants may show toxicity symptoms anywhere within the area of root growth of a black walnut tree. However, greater quantities of juglone are generally present in the area immediately under the canopy of a black walnut tree, due to greater root density and the accumulation of juglone from decaying leaves and nut hulls. Because decaying roots still release juglone, toxicity can persist for some years after a tree is removed.”

What does this mean for us?  Typically, homeowners are not searching out black walnut trees to grow in their yard as a shade tree.  Because of their messy habit alone, we avoid this plant.  However, many homes may have been built where an existing hedgerow lives, or near a stream where walnuts are flourishing. If that’s the case, we need to be cautious as to what plants we place near those trees.

As you can imagine, the more sensitive a plant is to juglone, the worse it does closer to the tree.  Very sensitive vegetable plants include tomatoes, peppers, cabbage, asparagus, sweet potatoes and others. Some shrubs that are sensitive include blueberries, chokeberries, cotoneaster, hydrangeas, lilacs, just to name a few. Many herbaceous plants are also considered sensitive.  To see a complete list, go to   

The best choice is to plant things that are tolerant of the toxin. Some of these plants are serviceberry, birch, and hornbeam.  Vegetables include onions, beets, squash, melons, beans and corn.  For a complete list of tolerant plants visit

In addition to the black walnut, other trees have shown to be alleopathic as well.  A tree that is just as much of a problem as the black walnut is Butternut.  Other trees that are not quite as much of a problem, but still have juglone, are English walnut and hickories. Most of these trees are not readily available on the retail market, but could be growing naturally in the landscape.  Research is always the best choice when planting trees, shrubs and herbaceous material around nut trees, so check out the lists from the websites above to make good decisions about what to grow.

In nature, we need black walnut trees.  The benefits they provide are far reaching, from lumber to wildlife. But in our home landscapes, plant with care.  As with all trees, we want to do our homework prior to planting any species.  But when choosing the black walnut, be sure you understand the needs of the tree as well as your purpose for planting it and what the results may be when enhancing the landscape around it.