(BroJack) wrote:

The horticultural books say that the roots of juglans (walnuts)
release a poison that prevents other plants from growing near them.
Nonsense. In 30 years of observing them in my "wild" areas, I haven't
found this to be true.
Does anyone know the origin of this misinformation and why the books
keep repeating it?

Black walnuts are allelopathic--they produce substances toxic to other
plants. Walnut roots, leaves, and other plant parts contain the
herbicide, juglone. It is toxic to many plants, especially my
rhododendrons. The squirrels keep planting black walnuts and I keep
pulling them out. The walnuts that do exist have killed several of my
rhododendron plants. I can recover the area by trenching around the
rhododendrons and severing any roots that are entering the area. Then
after a couple years it is safe to plant rhododendrons.

"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. The juglone
toxin occurs in the leaves, bark and wood of walnut, but these contain
lower concentrations than in the roots. Juglone is poorly soluble in
water and does not move very far in the soil." [from Ohio State
University Extension Fact Sheet HYG-1148-93 by Richard C. Funt and Jane
Martin]

The Ohio State University Extension and the American Horticultural
Society [http://www.efn.org/~bsharvy/bwtol.html] have reported that R.
nudiflorum, Pinxterbloom Azalea, and Exbury Azaleas Gibraltar and Balzac
will grow near Black Walnut and Butternut trees. They also list many
other plants that will grow in the root zone of these trees.

The information is correct. People have misquoted it.

Another natural herbicide is juniper needles. They contain a natural
preemergence agent. That is the reason the forest floor under junipers
is barren.

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