While black walnut (Juglans nigra) trees are beloved for their rich history, they have accrued a problematic reputation among gardeners for a few reasons. They develop an extensive root system, cast a dense shade, and, most notably, they produce a toxic chemical called juglone. Or, really, they produce the precursor called hydrojuglone. Hydrojuglone is found in high concentrations in the roots and hulls of black walnuts, and as it makes contact with the soil, it is transformed into juglone. Why is this problematic? Long story short: juglone is toxic enough to kill surrounding plants. Researchers aren’t quite sure how it happens, but they believe it interferes with photosynthesis, respiration, and water uptake.

There are two ways juglone spreads to nearby plants. Large, fleshy nuts develop early to mid-fall and drop to the ground. As the outer shells (or hulls) break down, they release high concentrations of juglone. The second is allelopathy – which is practically chemical warfare for plants. Their roots exude chemicals (in this case, juglone) into the soil for nearby plants to uptake.

Is growing near a black walnut tree a death sentence? No, it’s absolutely not. For how much it’s discussed, there surprisingly hasn’t been much research on it. The scientific community can’t conclusively agree on whether allelopathic juglone can be spread in high enough concentrations to kill nearby plants. Plus, many symptoms from juglone poisoning overlap with drought stress, so it may be misdiagnosed in the garden. Gardening near (or even under) walnut trees is still possible, so don’t resent your trees just yet. We’ve compiled a list of plants showing juglone tolerance in the landscape to help.