[gardeners] Blackwalnut

bsk (gardeners@globalgarden.com)
Thu, 18 Oct 2001 07:04:30 -0500

This is one of the most complete articles I have seen on Blackwalnut trees
Black Walnut and Juglone Toxin

The black walnut tree emits a toxin that hinders the growth of many plants
in its near proxcimity. Though many plants are effected here are some plants
which tend to do well growing near this tree.

Black Walnut Toxicity to Plants, Humans and Horses

Richard C. Funt
Jane Martin

The roots of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea
L.) produce a substance known as juglone (5-hydroxy-alphanapthaquinone).
Persian (English or Carpathian) walnut trees are sometimes grafted onto
black walnut rootstocks. Many plants such as tomato, potato, blackberry,
blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be
injured or killed within one to two months of growth within the root zone of
these trees. The toxic zone from a mature tree occurs on average in a 50 to
60 foot radius from the trunk, but can be up to 80 feet. The area affected
extends outward each year as a tree enlarges. Young trees two to eight feet
high can have a root diameter twice the height of the top of the tree, with
susceptible plants dead within the root zone and dying at the margins.
Walnut leaves can be composted because the toxin breaks down when exposed to
air, water and bacteria. The toxic effect can be degraded in two to four
weeks. In soil, breakdown may take up to two months. Black walnut leaves may
be composted separately, and the finished compost tested for toxicity by
planting tomato seedlings in it. Sawdust mulch, fresh sawdust or chips from
street tree prunings from black walnut are not suggested for plants
sensitive to juglone, such as blueberry or other plants that are sensitive
to juglone. However, composting of bark for a minimum of six months provides
a safe mulch even for plants sensitive to juglone.

Okie zone 7a