The Rural Voice, 2000-08, Page 56Andrew Grindlay
What grows under black
Ask three gardeners what plants
will grow under black walnut trees
and you will get three answers.
There is a great deal of confusion
about the toxicity of juglone — a
chemical in black walnut trees — but
one thing the experts agree on is that
not all plants will survive when
exposed to it.
Many universities have done
research into the toxicity of juglone
and have published their results. A
scan of their publications shows that
juglone is found in greatest
concentration in black walnuts trees,
with lower concentrations in English
walnuts, butternuts, hickories and
pecans. It is in the leaves, the nuts,
the nut husks and the bark, but
mainly in the roots.
Plant scientists shy away from
making conclusive statements about
what.will and will not grow under
black walnut trees, preferring instead
to use words such as, "Plants that
appear to be resistant to juglone
include ...." The reason for the
indefiniteness is that few plants have
been experimentally tested for
sensitivity to juglone. Most published
results are based on observations of
what does and does not appear to
grow, rather than on controlled
studies. The
lists, therefore,
should be used
for guidance but
not regarded as
the last word.
There is
disagreement
too. Some
researchers say
that the roots of
the walnut tree
must actually be
in contact with
the roots of
another plant to
kill it. Others
say that rain water dripping from the
walnut's branches will finish off any
plants that are susceptible to juglone.
Some say that poorly drained soils
walnuts
will allow the juglone to flow from a
walnut's roots to those of another
plant, whereas others say that that
does not happen.
For about four decades the Ontario
Ministry of Natural Resources has
been recommending the planting of
black walnut trees in a pattern such
that each tree is surrounded by four
white pines. The pines' role is to
make the walnuts grow straight and
to shade their lower limbs, producing
a self -pruning effect. In time, the
walnuts are supposed to kill the
pines, leaving, eventually, a stand of
black walnut trees with the correct
spacing. But a 1984 bulletin
distributed by the Ontario Ministry of
Agriculture and Food, said, ••...
research in white pine and walnut
plantings in Southern Ontario
indicated that black walnuts
suppressed or killed white pines only
in those areas which were poorly
drained. Under dry soil conditions,
the white pines often suppressed the
walnut." In my own plantation, in
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PARTS TO FIT
VERSATILE 400 AND 4400
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52 THE RURAL VOICE
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