The raised garden beds in Portland, which were there when we bought the house, are planted fairly close to a black walnut tree.
That sounds nice…… but, the issue is juglone.
Juglone is an organic compound….
….that is made by plants in the Juglandaceae family, of which, the black walnut tree (Juglans nigra) is the most notorious. Juglone is allelopathic, meaning that this chemical, secreted by one plant, inhibits the growth of other plants. Juglone gives black walnut trees a survival benefit. Black walnuts can outcompete other plants for sunlight and nutrients by secreting juglone into the local environment, thereby inhibiting the growth of nearby plants.
The juglandaceae family has the nickname “the walnut family,” but it also includes pecans, hickory, and some other nuts like wingnuts (never heard of them). Studies from the late 1800s/early 1900s and anecdotal evidence say that juglone is toxic to many different crops, and you should, therefore, never plant a garden near a black walnut tree.
Whoops. Time to rip everything out and start again?
Wait just a minute… I’m hoping it’s more nuanced than some websites make it seem. The best article I came across was this review published in the journal Agronomy out of Purdue University. They acknowledged that juglone has demonstrated allelopathic qualities, but also say, “toxicity of juglone varies depending on several factors, such as the donor plant species, quantity released and amount accumulated, soil pH, texture, and organic matter content.” Later in the article they mention that juglone toxicity is higher in poor quality soils and in moist soils than in dry soils.
I can’t find any credible source that flat-out says “Nah! Don’t worry about juglone!” Most State Extension sites say something along the lines of “Consider an alternate site for you garden” and they give you lists of plants that are more susceptible and less susceptible to juglone.
Fortunately, we brought in fresh, good quality soil when we were building the garden, so there shouldn’t be much juglone in the garden yet. I’ll add compost at the end of the season to keep the soil high in organic matter throughout the years, and I’ll be diligent to remove the falling nuts later in the season. And….I’ll just hope for the best.
As for the list of plants that are more tolerant of juglone. Those include:
Plants that are more susceptible to juglone are:
I’m growing onions and squash – good; but also cabbage and eventually tomatoes – not good. Hmmm…. my cabbage seems to be doing fine so far. I don’t see any yellowing or wilting leaves (signs of juglone toxicity). The highest concentration of juglone is around the dripline of the tree, or approximately 50 feet from the trunk. I haven’t measured exactly how far my garden is from the trunk of that tree, but I think it’s probably within 50 feet.It’s also interesting that in neighboring beds, we have rhubarb growing, which is supposedly juglone sensitive. It’s been growing there for years. It’s not the most robust rhubarb you’ve ever seen, but it’s certainly not been killed off due to juglone yet! There’s hope!