The Myth of Allelopathy
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| It’s not hard to find a Black Walnut tree surrounded by a great variety of native and nonnative plants, which should give an observant person reason to doubt the excessive talk about the negative effects of allelopathy. |
ALL TEXT AND PHOTOS © Marlene A. Condon
Allelopathy—the biological
phenomenon by which a species of plant supposedly inhibits, via biochemicals,
the germination, growth, survival, and/or reproduction of a different species
of plant—is much touted in invasive-plant circles. The original scientific
interest in this subdiscipline of chemical ecology arose from the idea of
making use of allelopathy in agriculture. However, the very vocal support of
plant nativists (folks who prefer that alien plant species be replaced by
native ones) has undoubtedly assisted this field of study to flourish. But is
allelopathy just myth?
The beauty of the natural
world is that it is open to anyone with eyes to see. You can prove for yourself
that many of the “facts” repeated ad infinitum about nature don’t bear out in
the real world.
For example, I live in a
forested area where many Black Walnut trees (Juglans nigra) grow. This
tree is the most well known example of allelopathy among gardeners because of
its reputed toxicity to many species of garden plants.
It’s a given that you’ll
be exposed to this information if you plan to grow such plants as tomatoes or
green peppers. You’ll certainly be told to keep your garden as far away as
possible from a Black Walnut because a plant sensitive to the effects of
juglone (the chemical contained within the tree) can be poisoned by contact
with any part of the tree (including roots, leaves, nuts, and even branches
that have fallen to the ground).
It may indeed be true that
solanaceous (nightshade family) plants are susceptible to the effects of
juglone, but why would anyone plant a vegetable requiring full sun under a shade
tree? Thus, to me, all the talk about the Black Walnut tree in regards to
growing such plants is pointless.
Nevertheless, it’s
reasonable to ask if the science is right about juglone causing problems for many
other kinds of plants. To judge whether conclusions from a scientific paper in
the biological sciences is accurate, you really need to read the details of how
the study was done. Unfortunately, many scientific papers are most easily
available only to other scientists, and the public is essentially locked out
from access.
Luckily, I did locate some
original research online regarding this situation, and found that the effects
of Black Walnut on other plants have been contradictory from one study to the
next. I also came across a January 2000 paper that listed plants affected and
plants not affected by growing near Juglans nigra trees, and the number
of plants not affected by walnuts is far longer than the list of plants
affected—which didn’t surprise me. I’ve found quite a variety of plants growing
under Black Walnut trees in Virginia.
http://www.allelopathyjournal.org/Journal_Articles/AJ%207%20(1)%20January,%202000%20(1-55).pdf
I’ve documented many native
and nonnative herbaceous plants, and woody vines, shrubs, and trees surrounding
this tree species. I’ve seen the same for supposedly allelopathic alien herbaceous
plants. To me, the more plausible explanation for why some plants perform
poorly near “allelopathic” plants are environmental conditions (e.g., soil
fertility, soil compactness, etc.).
NATURE NOTE:
Garlic Mustard, Ailanthus, Canada Thistle, and
Foxtail are nonnative plants deemed—without question—to be allelopathic. I can
tell you for a fact it isn’t true. I’ve grown every one of these plants in my
yard, and not a one inhibited the growth of other plants around it. If you
want to know the truth about alien plants, grow them to learn for yourself if
alien-plant lore is true or mythical.
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