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Black Walnut Toxicity

March 1, 2012 Blog No comments

I have recently planted a couple of Tsuga canadensis, hemlock trees on our property in order to screen the view of our barn and other heavy equipment.  Screening generally isn’t a hard thing to do, unless you have mature, Juglans nigra, black walnut tree growing in close proximity.  We learned the hard way when I was growing up and we planted Norway spruce and white pine trees in the same area.  Within 1-3 years, those plants died.  It wasn’t until I began my schooling for Horticulture in the Career and Technology Center that I learned that you can’t grow all kinds of plants under or near black walnut trees!

The black walnut tree, Juglons nigra L. is a valuable hardwood lumber tree that is native to the state of Indiana.  The black walnut tree is commonly grown for is shade value and sometimes for its edible nuts.  Many native plants are capable of growing in close proximity to black walnuts, however, there are quite a few that are not!  The relationship between plants in which one produces a substance which affects the growth of another is known as “Allelopathy.”  Black walnut trees naturally use this process to have dominance over other establishing trees and shrubs.

Mature Julgans nigra L. tree bark

Black walnut trees can be a large issue for homeowners (especially those with limited garden space, as it inhibits growth of many desirable plants common to the garden.  The casual agent is a chemical called “juglone” which is emitted from the black walnut trees.  This chemical is found in all parts of the plant.  The highest concentrations of this chemical is in the leaf buds, nut hulls and the roots.  However, as previously stated, it is present in all areas and is still able to affect plant growth when emitted.  Plants will mostly be affected if they are within the canopy of a black walnut tree, or within the root zone of the tree (which can extend much further beyond the canopy than you think).  Juglone is poorly soluble in the soil, which means it will not move far from where it is absorbed.  It may still be released from dead and dying stumps or roots in the soil.  This means that juglone toxin may be present in the soil long after a tree has been removed from an area.  Black walnut trees are not the only plants that produce the juglone toxin.  *Other plants that produce the same toxin in smaller concentrations include butternut, English walnut, pecan, shagbark hickory, and bitternut hickory.

Common symptoms of this toxicity include; yellowing, wilting and sudden stunting or death within a plant.  The respiration of a plant is inhibited by this chemical, which deprives juglone-sensitive plants from obtaining needed energy through photosynthesis.

Prevention:

  • Locate garden away from the juglone source
  • Clean up leaf and nut debris which falls near susceptible plants
  • Plant the right plants in the right place (see link below to find a list of plants to-grow and not-to-grow)
  • Create a root barrier, preventing roots from extending toward other plants
  • Grow plants in raised beds with protective coverings
  • Remove the tree and replace with other native trees, shrubs or perennials.

View a list of Sensitive and Tolerant plants from Purdue University

On our property, we have roughly 11 acres of woodland habitat.  Within this habitat, many of the trees are Black Walnut.  We desired to create a screen between our driveway and the barn area where most of our large equipment and materials are stored.  Since black walnut tree are present, we had to select evergreen trees that were tolerant to the juglone toxin.

Hemlock trees have been shown to have a resistance to the juglone toxin and have been observed to grow normally underneath the canopy of a walnut tree.  This is why I chose to plant hemlock trees to add an evergreen screen to our property.  Also, hemlocks are great, native trees which will add to the bio-diversity of our landscape.

I purchased two trees, both the Canadian hemlock, Tsuga canadensis.  The one on the right is much denser and is from North Carolina, and the one on the left was grown locally.  This could be from the climate difference, light exposure of where it was grown (the locally grown tree was in more shade), and possibly fertilizer used to feed the plant.

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