The Rural Voice, 2005-07, Page 50CENTRAL HURON)
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VANGUARD ST
Woodlot Management
Walnuts' self-defence plan
Steve Bowers
is a forester
and forest
owner, a
member of
the Huron -
Perth
Chapter of
the Ontario
Woodlot
Association
and
Stewardship
Co-ordinator with the Huron
Stewardship Council.
To continue with the discussion of
black walnut trees started in last
month's column. one of its most
interesting and perhaps best known
traits is the allelopathic effect it has
on many other species.
Allelopathy is a characteristic
developed by walnut and many other
plants in order to help ensure their
survival and propagation, where they
produce a chemical substance that
can kill or inhibit the growth of
nearby plants.
Black walnut produces a
substance called juglone that is given
off by its roots, leaves and nut husks
and is toxic to many other species.
Many gardeners have discovered this
impact first hand after years of failed
attempts to grow vegetable crops near
walnut trees.
Juglone is also toxic to a number
of tree and shrub species. For
example, glossy buckthorn is a
particularly invasive shrub in some
parts of Ontario. In some woodlots I
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46 THE RURAL VOICE
have seen it growing so thickly you
could barely walk through it, except
in areas within the drip line of walnut
trees where not a single buckthorn
shrub can be found. Black walnut
trees also cause mortality in many
coniferous trees, including red and
white pine.
In some cases. foresters have
attempted to use this trait in reducing
future management requirements
when establishing walnut plantations.
Walnut has often been planted in
mixture with white pine with a
walnut as every other tree in every
other row. This creates an
arrangement where each walnut tree
is surrounded by white pine. In
theory, competition by the white pine
forces the walnut to grow straight and
tall with smaller side branches. As
the trees get Targe enough that they
require thinning (likely in 15 - 30
years), the concentration of juglone in
the soil reaches the point where it
starts to kill the white pine, thus
releasing the walnut trees from
competition through this "natural
thinning-. This approach has been
successful on some sites but not
others, so it would be best to consult
with a forester before trying it.
Some species seem to grow very
well or even thrive under walnut
trees. Certainly the best black
raspberry patches are often found in
walnut plantations. In southern
forests walnut occurs naturally as
scattered trees in mixture with other
deciduous tree species, so many of
them appear to be tolerant of juglone
as well.
Juglone may also be toxic to some
species other than plants. Horses
pasturing near walnut trees or bedded
with wood shavings containing
walnut may develop a condition
called laminitis that can be
debilitating or fatal.
Black walnut has very specific
site requirements, needing deep,
moist (but not poorly drained), fertile
soils (preferably two feet of a loamy
texture). They have a tap root and
will often persist on poorer sites, but