To protect themselves from competition, black walnut trees exude a toxin called juglone. Gardens planted near black walnut trees or roostocks frequently fail to thrive due to this toxin. Many plants can be affected, but tomatoes and potatoes show the greatest susceptibility.
Juglone enters the soil from the roots, bark, leaves, and nut hulls of the walnut tree. It cannot move far, as it is not water-soluble, but it impacts nearby garden plants by interfering with their respiration. This weakens the plants and eventually causes their death.
- Stunted growth.
- Sudden wilting.
- Twisted, yellow leaves.
There is no cure for walnut toxicity. If it becomes a problem, the garden must be relocated or moved into raised beds.
If you can, choose a garden location away from walnut trees. On small properties, this may not be an option, but you still have two other possibilities to consider:
- Choose only plants tolerant of juglone (see Helpful Resources below).
- Grow all plants in raised beds, regularly swept clean of leaves and nuts.
Even if your garden is in an ideal location, never use black walnut leaves, bark, or chips as mulch.
Black Walnut Toxicity
Handy factsheet offering more information, including lists of plants susceptible and resistant to juglone.