Toxicity towards other plants
The black walnut (Juglans nigra), an American walnut species, produces a toxin called juglone, which is capable of stunting or even killing susceptible plants, including grasses, alfalfa and members of the apple and tomato families. Juglans regia does not produce juglone but can become problematic when grafted on Juglans nigra (Heimann et al., 1997).