Chances are, if you have been hanging out in horticultural circles for long, you have heard about the dreaded tree that is the bane of forest gardeners, that giant that does not play well with others, the tree that releases a toxin that causes other trees to die .. I am talking about the walnut tree, of course, and the hormone juglone. You might be surprised to know that there are several species of plants that can grow quite well in an ecosystem including a walnut tree - For an ongoing project, I have compiled a list of both the species that are particularly prone to damage from juglone, and another, much larger list of the species resistant to (or completely oblivious of) juglone. Enjoy!
Appendix I: A word on Juglone
The life of the forest is not limited to the above ground portion that we routinely see: just below the surface is an entire world of activity called the rhizosphere, or root zone. The rhizosphere is filled with fungi and bacteria, microscopic protozoa and nematodes, worms, insects, and some larger animals .. all going about their lives surrounded by the roots of the plants and trees above. Just like in a human community, there are symbiotic and cooperative relationships in the rhizosphere, but there is competition, as well .. and sometimes, warfare.
Although many plant species engage in chemical warfare with each other and with potential pests, the most notorious perpetrators of plant-on-plant violence are members of the family Juglandaceae, the walnuts. All members of the walnut family secrete a toxic substance called juglone from their roots, which is capable of killing transplanted trees and inhibiting the germination of seeds. Black walnut trees Juglans Nigra Spp. Is by far the worst offender – so much so that with only a few conditional exceptions, most other members of the family are capable of peaceful coexistence, unless environmental stress causes them to crank up the juglone production.
The symptoms of juglone poisoning mimic dehydration: Wilting, and yellowing of the leaves of affected plants, and potentially death. These symptoms have been observed in conjunction with Black Walnuts for a long time, and the toxin itself has been identified since the middle 1800s. Juglone is subject to decomposition in an aerobic environment, and will be rendered harmless within four months – so, if you are using a horticultural compost or mulch containing members of the Juglandaceae family, allow it to properly age before using it in a seed bed, or around young transplants. Some plants are more susceptible than others, so exercise extra caution with these:
Brassicas (Cabbage, Mustard, Pak Choi)
Pinaceate Trees and Shrubs (Fir, Larch, Pine, Spruce, True Cedars)
Solanaceous plants (Eggplant, Peppers, Potatoes, Tomatoes)
Please note that just because these plants are more susceptible, it does not mean that you cannot successfully inter plant them with members of the Juglandaceae family. I have several examples from this list growing quite happily in the shade of our pecan trees without issue – so far.
At the opposite end of the spectrum, there are many plants that can tolerate growing in close proximity to a black walnut without suffering from juglone poisoning:
Alliums (Garlic, Leeks, Onions, Ramps, Shallots)
Asters (All members of the Asteraceae family)
Bee Balm (Monarda)
Bell Flower (Campanulas)
Corn (Zea Maize)
Hollies (All members of the Ilex genus)
Ipomoea (Sweet Potatoes)
Maples (Except Silver Maple)
Redbud (Cercis genus)
Squash (All Cucurbits)
This list is far from complete – but as you can see, there are enough compatible species that there is no need to exclude Black Walnut trees from consideration in your food forest.