Here’s your garden chemistry lesson for the month:

There are many Black Walnut trees (Juglans nigra) growing throughout our Northside neighborhoods. The Urban Gardener had one growing on the hillside, right over our office shed, which produced a tympanic symphony when the large, hard-hulled, hand-staining nuts would fall onto our roof. Many people are aware – some through the hard lessons of personal experience – that most things planted under or near a black walnut tree will die. This is because the black walnut is one of the oldest examples of something called alleopathy, when a plant produces chemicals that interfere with the growth of other plants.

Black walnuts produce a colorless, water-soluble, non-toxic compound called hydrojuglone, which upon exposure to air gets oxidized to become juglone, a highly toxic compound that inhibits plant growth and the ability of a plant to take in nutrients. Highest concentrations of the toxin are located in the tree’s buds, nut hulls and roots. It is also leached from leaves, branches, fallen leaves and nuts. 

If you have a black walnut in or near your yard that you just can’t bear to part with, however, you can minimize the accumulation of juglone by being diligent about raking up leaves, nuts and other debris, and by incorporating organic matter which will encourage microbes that help to speed up the breakdown of the chemical. You can also choose plants that have been shown to tolerate proximity to juglone better than others.

Noted horticulturalist, Frank Robinson, tested a number of cultivars beneath two black walnut trees in his yard and found the following plants did just fine: bugleweed (Ajuga reptans), lamb’s ear (Stachys byzantina), Rose-of-Sharon (Hibiscus syriacus), summer phlox (Phlox paniculata), weeping forsythia (Forsythia suspense), Canadian hemlock (Tsuga canadensis) and Japanese maple (Acer palmatum).

Tomatoes are especially susceptible to the toxin, quickly developing something called ‘walnut wilt,’ which is fatal to the plants. Different cultivars of the same plant can display different susceptibility levels. Small-flowered daffodil cultivars can manage to survive, while the trumpet and large-flowered forms succumb.

Ohio State University Extension has also done research on this topic and a more extensive list of both plants that can be planted, and ones that should never be planted, near black walnut trees can be found at: http://ohioline.osu.edu//hyg-fact/1000/1148.html

And, remember, if you don’t want to walk around with purple-stained hands for several days, wear gloves when collecting or picking up the walnuts!