Can I Grow Vegetables Near a Black Walnut Tree?

The raised garden beds in Portland, which were there when we bought the house, are planted fairly close to a black walnut tree.

The big tree is a black walnut tree.

That sounds nice…… but, the issue is juglone.

Juglone is an organic compound….

Skeletal formula

….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:

  1. Onions
  2. Beets
  3. Corn
  4. Garlic
  5. Leeks
  6. Carrots
  7. Cauliflower
  8. Parsley
  9. Melons
  10. Squash

Plants that are more susceptible to juglone are:

  1. Cabbage
  2. Eggplant
  3. Peas
  4. Peppers
  5. Potatoes
  6. Tomatoes
  7. Rhubarb

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!

3 thoughts on “Can I Grow Vegetables Near a Black Walnut Tree?”

    1. I saw the Michigan State Extension article. The barriers they suggest are “wood, stone, or concrete.” Maybe I’m not understanding this, but it doesn’t seem very practical. I don’t think I’d want to use wood, as that would either have to be treated lumber to prevent rot (chemicals…not sure I’d want those in a vegetable garden) or it would decay quickly and have to be replaced regularly. Plus we would need a lot of stone/concrete/wood if we were to line the bottoms of all of the raised beds in that area. I also imagine it would impede drainage and I certainly don’t want to have overly moist soils, as that (according to the one article I read) leads to higher concentrations of juglone.

      Washington State Extension has a very interesting article (link), which I found after writing this post: WSU seems to be one of the few juglone skeptics! It gave me hope!

      And P.S.: the WSU article was written by Linda Chalker-Scott, who has a PhD in horticulture and contributes to a blog called The Garden Professors, which I *highly* recommend if you found this juglone post interesting.

  1. Bricks might provide some separation while also allowing for drainage. The deepest roots may penetrate to the underlying juglone contaminated soil, but the exposure would likely be lessened.

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