Plant Warfare - February 11, 2009
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Plants may not be able to outrun predators, but they are by no means passive participants in the battle for survival. Plants actively compete with other plants for growing space. Many plants produce toxic compounds that discourage herbivory. Some plants alter soil chemistry and others even poison their neighbor's offspring to gain advantage. Many plants form alliances with neighboring plants and other life forms to facilitate processes like nutrient uptake and reproduction. It’s a jungle out there and scientists continue to discover new battle strategies and debunk myths.

Poisonous plants use a simple and direct strategy. Animals and insects that feed on poisonous plants are often repelled or even killed when they eat poisonous plants. This allows the poisonous plant to grow and reproduce more aggressively than surrounding non-toxic plants. Of course, over time, some herbivores can also develop a tolerance to the toxic compound(s) allowing them to have a reliable food source which is not favored by other species.

Another competitive strategy is called allelopathy. This involves a plant's secretion of biochemical materials into the environment to inhibit germination or growth of surrounding vegetation. The Romans observed the toxic effect of black walnut on neighboring plants 2,000 years ago. This decline occurs because the walnut tree produces a non-toxic, colorless, chemical called hydrojuglone. Hydrojuglone is found in leaves, stems, fruit hulls, inner bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic. This same process occurs in many other species and involves many other chemical compounds.

Allelochemicals cause growth inhibition by affecting physiological processes such as respiration, cell division, and water and nutrient uptake. Symptoms of "allelopathic effects" include leaf wilting and yellowing, or death of part or all of a plant. A local “weed tree”, commonly called the tree of heaven (Ailanthus altissima), is also allelopathic. Now, researchers have begun to isolate compounds found in tree of heaven that could be used as a botanical herbicide. Researchers are also working to breed crops and landscape plants that are allelopathic to weeds.

Strategic plant alliances also exist and these are sometimes used to the gardener’s advantage. Trees often root graft with other of the same species. This allows weaker individuals to gain resources from healthier neighbors.

Mycorrhizal fungi form symbiotic associations with plant roots of many species. These associations are characterized by two directional movement of nutrients where carbon (sugar) flows to the fungus and essential nutrients move to the plant, thereby providing a critical linkage between the plant root and soil. Mycorrhizal fungi are critically important in areas of infertile soils. Many plants in north central Arizona have mycorrhizal associations.

Vegetable and fruit growers are also looking to mix in beneficial plant species to promote soil fertility, reduce weeds, confuse insect pests, and attract beneficial and predatory insects. Converting from a monoculture to a mixed “polyculture” has many advantages (see the April 23, 2008 column called “Farmscaping”). One of these is a more interesting and diverse garden or orchard. While many of the concepts of companion planting and cover cropping have been scientifically validated, others have not.

Some of the popular literature regarding companion planting is based on flawed research, in particular, the "sensitive crystallization method" which was originated by Dr. Ehrenfried E. Pfeiffer in the 1930's. The sensitive crystallization method utilizes paper chromatography to discover why plants make good or bad companions. He concluded that mixtures of plants which formed clear and bright chromatograms were mutually beneficial, while mixtures that formed cloudy or dull chromatograms were antagonistic. They did not base their recommendations on field observations and few if any scientists believe that this method can determine compatibility among plant species.

The best way to test these recommendations is to use them in your own garden and document the results in your garden notebook. What, you don’t have one? Then get one right away. Any time you plant, harvest, or initiate a new practice you should take notes and photos, draw maps, and document what you did. How else will you know if it was success or a failure?

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: February 5, 2009
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