Is Companion Planting Scientific? - January 26, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Companion planting is the practice planting of different crops in close proximity to each other in order to influence nutrient uptake, pest control, pollination, and other factors necessary to crop productivity. Companion plantings seem to be a widely accepted practice, but are all the purported benefits supported by sound science? Some gardeners have determined compatible companion plants through their own observations. Other times, we plant cover crops to improve soil conditions or insect repelling plants to mitigate pest damage. Some companion planting principles make sense intuitively, but others seem to be shrouded in mystery.
North American Indians historically planted corn, beans and squash together in an intercropping system called “Three Sisters”. Beans are nitrogen-fixers and continually supply this macronutrient to the soil. Corn stalks provide structure for beans to climb, and squash vines provide a living mulch with their broad leaves that shade the soil, reducing evaporation and inhibiting weed seed germination. These three species have similar environmental requirements, and when planted with suitable spacing, don’t unduly compete with each other for water and nutrients, thus allowing all three species to survive and grow together in a compatible manner. However, does this mean these plants “like” each other? – not necessarily.
Using a scientific view, plants must either adapt to their environment or alter it to survive. The presence of a single plant modifies environmental variables including temperature, soil moisture content, soil pH, solar radiation availability, and nutrient availability. Plants with a narrow range of tolerance for environmental change will be less adapted to their new environment and may not survive. Other species tolerate or even require environmental changes to become established in their stead – sometimes we call these plants weeds. Some plants also release compounds toxic to other plants (this is called alleopathy). Remember, it’s a jungle out there and not everybody plays nice.
As for companion planting, many vegetable gardeners swear by it. They read lists compiled by “experts” and accept it as the gospel. The problem with using the phrase “companion plants” is that it is broadly used to describe plant interactions in the realms of science, pseudoscience, and the occult. One simply needs to conduct a web search to see that there are several thousands of references on companion planting. Some are credible and supported by research – others are not.
On less credible sites and in some popular books are the pseudoscientific claims that companion plants can be determined by “sensitive crystallization” of their extracts (i.e. to discover which plants “love” each other), or through study of a plant’s “rhythm, its vibration, its music, and its note.” These supernatural interactions often pique gardener’s interest, but scientists and professionals often cringe at the lack of scientific data to support these claims.
Dr. Linda Chalker-Scott, an Extension colleague with Washington State University, has reviewed the subject. Here are her bottom line recommendations on companion planting:
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The Myth of Companion Plantings
(Linda Chalker-Scott, Washington State Extension)
(Robert Beyfuss and Marvin Pritts, Cornell Cooperative Extension)
Companion Planting: Basic Concept and Resource
(George Kuepper & Mardi Dodson, The National Sustainable Agriculture Information Service)
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