In Defense of Nature

 The Myth of Allelopathy

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.