Jennifer Schultz Nelson
Extension Educator, Horticulture
Plants have evolved some pretty interesting ways of insuring their own existence in the world. One tactic that fascinates me is allelopathy, which means that one plant harms another with chemicals it produces. A common example, and one that plagues homeowners is the black walnut, Juglans nigra, a tree native to eastern North America, including Illinois.
All parts of black walnut trees produce a molecule called juglone. This chemical inhibits the growth of a wide variety of 'non-walnut' plants. Researchers think the chemical does this by inhibiting other plant's metabolic activity. Other trees in the same family such as English walnut, pecan, and hickory also produce juglone, but in such small quantities it's rarely enough to affect other plants nearby.
Juglone is released in small amounts from roots below ground, and from decaying nuts and leaves on the soil surface. Any plants that are sensitive to juglone and are planted or attempt to grow naturally in areas containing juglone will grow poorly and develop what some call 'Walnut Wilt'–yellowing, wilting, and eventually death.
If you're a walnut tree, this is a pretty convenient way to carve out some space to grow in a crowded forest. However, many homeowners that have a black walnut tree tear their hair out in frustration. To many it seems like each year more of their landscape dies as their black walnut tree grows larger! Short of cutting down their beautiful tree, they are desperate for a solution.
But even the drastic measure of cutting down the tree will not solve this problem. Unfortunately, the roots of black walnut continue to release juglone for years after the tree has been cut down or dies. Juglone is not very soluble in water, so it does not move through the soil very readily. However, juglone is a very reactive chemical, and it is quickly deactivated when it reacts with soil components, so there is no worry that levels will 'build up' in the soil and persist forever after the tree dies or is removed.
Juglone sensitive plants that encounter old black walnut roots will still be affected if the roots are actively releasing juglone. Predicting how long old roots will release juglone in the soil depends on soil conditions and micro-organisms present. Most experts recommend waiting at least two years to plant juglone sensitive plants after a black walnut tree is removed from an area.
Fortunately for homeowners, not every plant is sensitive to juglone, and those that are may be only mildly sensitive. The lists of plants that can tolerate being planted near black walnut is extensive, and readily found on the internet or through our office.
Researchers are not entirely clear on what determines the quantity of juglone produced by a black walnut, but observations correlate good drainage with decreased juglone toxicity in a given area. It may be that juglone is produced more when the tree is under stress from poor drainage.
If you have ever tried to shell a black walnut, you know there is a thick husk covering the nut's shell that is very difficult to remove. This husk also tends to exude a thick juice that leaves a dark brown stain on everything it touches, sometimes permanently.
Juglone is what causes the stains. But what seems like a nuisance to someone trying to eat walnuts is actually a great natural coloring agent for foods and cosmetics such as hair dye. Juglone is also used as a natural dye for fabrics, particularly wool.
If you get past the staining hulls, the black walnut is edible. Its shell is very tough to crack, and grows so tightly around the nutmeats inside that it can be very hard to extract them without breaking them into many pieces. Highly-prized oil can also be pressed from the nutmeats that is used in cooking.
Processing black walnuts takes a lot more work than other types of walnuts, so they typically end up costing much more in stores. Some people gladly pay the increased price, as they find the flavor of black walnuts to be superior to other walnuts.
The shell does make black walnut processing more difficult, but again, people have found a way to use the tough shells to their advantage. The ground shells are tough enough to polish metals, but do not scratch soft metals and plastics in the process. They are also used as an additive in paints with textured surfaces and paints used to fill cracks. Many cosmetic products use finely ground walnut shells to add grit and scrubbing power.
Black walnut is also highly valued for its dark wood. It is very strong, yet easy to work with, making it a favorite of woodworkers. The price of the wood can make it cost-prohibitive to build furniture out of solid walnut, so often veneers are used instead.
Though the black walnut can cause some headaches for homeowners, it clearly has several redeeming qualities. Besides that it's a great example of chemical warfare in the plant kingdom.