Allelopathy: Harnessing its powers
Virginia Ruschhaupt/Victoria County Master Gardener
June 29, 2024
Marigolds, whose chemicals destroy root knot nematodes, are companion plants for cucumbers, peppers, tomatoes, squash, carrots, lettuce, potatoes, eggplant, onions and radish. (USDA)
Allelopathy pathways of plants. (Research Advances in Allelopathy of Volatile Organic Compounds (VOCs) of Plants
Chinaberry trees release chemicals that alter nitrogen and pH of the soil, hindering survival of native species. (Virginia Ruschhaupt)
When I was asked to write about allelopathy, my initial response was, “What’s that?”
After a short explanation, I realized that although unfamiliar with the term, I knew about the concept. So what is it and how is it relevant?
A simplified definition of allelopathy is the beneficial or harmful effects of one plant on another plant by the release of chemicals from the allelopathic plant. A variety of classes of chemicals and their interactions are involved, allowing certain plants to thrive and control their environment.
They have the ability to weaken or totally wipe out all or select species of plants in the vicinity.
Plants with extensive root systems or thick canopies can be aggressive competitors for water, nutrients, or sunlight, but are not allelopathic if there is no contributing chemical activity. However, research is revealing that more plants have allelopathic qualities than originally believed.
One or multiple plant parts, such as flowers, nuts, seeds, leaves, stems, bark, roots, leaf litter, or residue making its way into the soil can have allelopathic activity, triggering responses in targeted plants. Allelopathic effects can alter the biochemistry of the surrounding soil and the atmosphere.
Allelopathy is not a new concept. As early as 300 BC, Theophrastus noted that chickpeas wear out the soil and ravage weeds. The term was coined by Hans Molisch in 1937. Research escalated about 1990.
Let’s look at some allelopathic plants and how they work. Black walnut, the most dreaded allelopathic plant, secretes the chemical juglone. Very few plants survive beneath and beyond the tree’s canopy, due to juglone’s inhibition of germination and growth of many plant species. While peach trees thrive, tomatoes, potatoes, eggplant, alfalfa, Rhododendrons, and azaleas suffer the worst effects.
Juglone, found in nearly all components of the plant, remains active for many years. If your tomatoes look sickly after mulching with an unknown source, juglone may be the culprit. Juglone is found in lesser quantity in pecans.
The invasive Tree-of-Heaven produces a chemical Ailanthone which has been isolated. It exhibits herbicidal qualities similar to glyphosate and paraquat, making it and other allelopathic compounds possible candidates for herbicide.
If you have ever chopped Johnsongrass with a garden hoe, you will not be surprised that it is allelopathic. It is shocking that the allelopathic properties of radish used as a cover crop, can control Johnsongrass in cotton.
Tobacco has a complex mixture of allelopathic effects through its chemical nicotine, which is released into the soil through rain, fog, root exudate, and plant litter. Seed germination of corn, beans and wheat are primarily affected. Nicotine has a toxic effect on microorganisms, further affecting the soil and surrounding plants. After a tobacco crop has been established for several years, allelopathic effects may turn on itself, known as autotoxicity. Growth of tobacco seedlings are suppressed, decreasing yield; however, tobacco plants continue to make structural or ecological changes in attempts to adapt.
Nicotine, once promoted as a safe insecticide, was banned as a pesticide in the U.S. in 2014 due to toxicity to humans.
Autotoxicity is also prevalent in alfalfa. Established alfalfa interferes with germination of its own seeds. Crop rotation is often the solution. Chemicals responsible for autotoxicity have potential for weed and pest management.
Allelopathic plants contribute significantly to weed control in agricultural conditions when used as cover crops or no-till cropping. These include cereal rye, other winter cereal grains, sorghums, sorghum-sudangrass hybrids, brassica family (mustards, radishes), buckwheat, and certain clovers.
Each of these plants targets different weeds, e.g. sorghums suppress Bermuda grass and nutsedge. Utilization of these cover crops offer many benefits to agriculture and the environment. Allelopathy has much potential if its powers are cautiously harnessed.
The Gardeners’ Dirt is written by members of the Victoria County Master Gardener Association, an educational outreach of Texas A&M AgriLife Extension – Victoria County.
Partial list of allelopathic plants
Ailanthus (Tree-of-Heaven)
American elm
Aster
Barley
Bermudagrass
Black walnut
Brassica family of plants
Buckwheat
Chaste tree
Chinaberry tree
Corn (specifically corn gluten)
Cottonwood
Eastern Red Cedar
Elderberry
Eucalyptus
Ferns
Foxtail
Forsythia
Garlic mustard weed
Goldenrod
Johnsongrass
Juniper
Kentucky Bluegrass
Maple
Nutsedge
Oaks
Oats
Pines
Radish
Rice
Rye
Ragweed
Rhododendron (Azaleas)
Sycamore
Sorghum
Southern Magnolia
Southern waxmyrtle
Sugar Maple
Sumac
Sunflower
Sycamore
Tall Fescue
Wheat
More Information
Negative Effects of Allelopathy in the Home Garden
Broccoli can be harmful to newly planted broccoli and other brassicas (cabbage, mustard, kale, radish, etc).
Onions should not be planted with beans because onions emit a gas which kills the bacteria beans need to fix nitrogen. This will inhibit the growth of beans.
Onions inhibit growth of asparagus. Cabbage inhibits growth of peppers.
Tomatoes inhibit seed germination and seedling growth of lettuce. Garlic produces compounds that stunt the growth of peas, asparagus, beans, sage, parsley, and strawberries.
Fennel adversely affects many plants, particularly tomatoes and eggplant. Plant it in a container distant from other vegetables and herbs.
Oregano, rosemary, and thyme adversely affect the growth of many plants. Many other vegetables do not work together due to non-allelopathic reasons, such as heavy feeders, etc. See references.
REFERENCES:
Texas A&M AgriLife Organic; Allelopahy – What is it, what has it, and how do we use it? by Bob Whitney
https://journals.flvc.org/edis/article/download/109148/104309
14 Vegetables You Should Never Plant Together, by Madeline Buiano, Jan 6, 2024
Allelopathy and plant survival
Allelopathic Plants: What Are They, The List, and How To (and Not To) Use Them