Jennifer Schultz Nelson
Extension Educator, Horticulture
The cold winter winds seem to have found us a bit earlier than usual this year, as has the snow. This is the perfect opportunity to bake some cookies and enjoy a warm fireplace. In my opinion this is a recipe for a great holiday season.
If you are a baker, you've probably at least seen, or may have used black walnuts in your recipes. If you are a gardener with a black walnut tree in your landscape, you may have cursed its existence at one time or another, or maybe you've been one of the many calls our office receives each year seeking advice on co-existing with black walnut trees.
Eastern black walnut, Juglans nigra, is a deciduous tree native to eastern North America. It is exceedingly large at maturity, typically reaching heights of 100 to 130 feet tall. It is a tree highly prized for its lumber, which is used in construction of items such as rifle stocks, furniture and flooring. Trees growing in woodland settings in competition with fellow trees develop a large, straight trunk valuable for lumber. Trees growing in open settings tend to have a more open, spread out branch structure and typically produce less usable lumber.
The other major product harvested from the black walnut is the nuts themselves. The state of Missouri accounts for 65% of the wild harvest of black walnuts, which are processed in Stockton, Missouri. In this case the forest grown tree is less valuable; the black walnut growing on open land produces many more nuts. The flavor of black walnuts is said to be superior to the more common English walnut, Juglans regia. However, extracting black walnuts from their hard shell is extremely difficult. This is part of the reason black walnuts command a premium price at the store.
Part of processing black walnuts to harvest the nuts involves removing the thick green husk from the nuts. These husks contain large amounts of chemicals called juglone, plumbagin, and tannins. These chemicals will stain. Early settlers used this to their advantage, using black walnut extracts as a dye to color everything from woodwork to their own hair!
Once the tough husk is removed from the nuts, the shell must be cracked. The shell is thick and deeply furrowed, making it tough to remove the nuts intact. The shells themselves are used as an abrasive, in anything from cosmetics to industrial metal cleaning and polishing. The shells are also used as an inert material for oil drilling and explosives such as dynamite.
The chemical juglone, present particularly in the nut husks, leaves, and roots plays a major role in helping the black walnut tree secure a place in the forest via a phenomenon known as "allelopathy". The juglone produced by the black walnut tree inhibits the growth of most plants in its vicinity by interfering with these plants' ability to utilize energy. The plants tyupically turn yellow, wilt and eventually die. Plants that are sensitive to juglone eventually die because they are not able to metabolize energy, leaving more space for the black walnut tree to grow without competition. Extensive lists of juglone sensitive plants are readily available online; there are too many to list here!
I have had many a frustrated gardener call me, determined to plant a plant that is sensitive to juglone near a black walnut tree. It's not impossible, but it requires somehow separating the juglone sensitive plant from the soil containing black walnut roots, which secrete juglone. Unfortunately, even after a black walnut tree is cut down, the juglone present in the roots will persist in the soil potentially for years. Using raised beds with a barrier to prevent walnut tree roots from entering is a possible solution.
Keep in mind that juglone is also present in high levels in leaves and nut husks. Cleaning up in the fall and removing this plant material will reduce the levels of juglone present in the soil somewhat over time. Also avoid using black walnut clippings as mulch on juglone sensitive plants.
It's not impossible to have a garden around a black walnut tree, it just requires some planning. With a little extra effort you may even harvest enough walnuts for some holiday baking.