The Effect of Black Walnut Trees on Plants


Black walnuts trees are a well known large shade tree that produce edible walnuts. The scientific name Juglans nigra comes from the black colored toxic substance contained in the tree.


Allelopathy is the production of chemicals by a living organism that inhibits the growth of potential competitors. Black walnuts do this by creating a chemical called juglone.


Not every plant is sensitive to juglone. Those that are susceptible, such lilies and lilacs, suffer from respiration problems which makes normal photosynthesis slow down or stop.


Plants suffering from juglone poisoning will develop yellowing leaves, show poor growth and eventually die. Very sensitive plants, such as members of the nightshade family, will die quickly.

Toxicity Zone

The toxicity zone can extend from 50 to 80 feet away from the trunk on mature trees. On smaller trees it is usually about twice the diameter of the tree canopy.


All parts of the walnut tree contains juglone. Do not mix any part of the tree with compost or the juglone could leech and kill other plants as the compost is used around the garden.


  • Black Walnut Toxicity
  • Black Walnut Toxicity to Plants, Humans and Horses

Who Can Help

  • Black Walnut--Juglans nigra--Companion Plants
Keywords: juglone, juglans nigra, juglone sensitivity

About this Author

Brian Albert has been in the publishing industry since 1999. He is an expert in horticulture, with a focus on aquatics and tropical plants like orchids. He has successfully run an aquatic plant business for the last five years. Albert's writing experience includes the Greater Portland Aquarium Society newsletter and politics coverage for a variety of online journals.