Author’s Note: Recently I’ve been noticing increased interest in companion planting. This has resulted in yet another re-publishing of half truths about the technique, something that just gets under my skin. So I’ve updated and republished here an article I did a few years ago on the subject. This is a bit long for one of my posts, but I respect my readers enough to believe you will be willing to take a hard look at something that is riddled with myth and emerge with an understanding of what really works. So here goes….
Gardeners are generally a down-to-earth bunch of backyard scientists. They have a working knowledge of meteorology (“Should I water, or will it rain tonight?”), botany (“Which weed is this?”), and bio-chemistry (“Honey, get the BT out…something’s nibbling the cabbages.”). They can read fertilizer bags and seed packets with equal assurance, greeting wild claims for garden products or techniques with a skeptical eye.
So why do some gardeners so easily accept anything labeled “companion planting” as truth? I admit the concept is intriguing: group herbs and vegetables so their close association provides benefits to each other. Using natural interactions to improve harvest appeals to our ecologically conscious minds. Knowing carrots will grow better next to tomatoes but not well near pole beans could be useful information. But what mechanism provides those benefits?
Do carrots really love tomatoes? Healthy plant growth is a complex interweaving of sunlight, time of year, soil conditions, watering, weather, insect assistance or predation, and the effects of nearby plants or structures. If I plant my carrots next to my tomatoes and they both do well, I haven’t necessarily proved the companion planting rule. There may be a dozen reasons why my crops succeeded. But then again, there may be something at work that hasn’t been discovered yet.
Roots of Companion Planting
The concept of grouping plants to enhance beneficial or prevent harmful effects has been around for centuries. Farmers noticed that certain herbs and vegetables seemed to grow well together and began to develop companion planting rules. Many of these rules we now know were based on undiscovered fact. Other rules were amazing flights of fancy.
By the late nineteenth century, companion planting folk traditions were eclipsed by advances in bio-chemistry and agricultural science. But some suspected that the old ways still had something to teach us. Among them was Richard Gregg, a social philosopher who advocated non-violent protests, something he learned during his sojourn in India with Gandhi. While there he also became interested in the agrarian, low-tech lifestyle often associated with Gandhi and his teaching. After he returned to the UK Gregg spent several years living on a farm. Not long after that he compiled written and oral gardening traditions from the seventeenth century into a list of plants that were reputed to interact beneficially or harmfully with their neighbors. This was published in 1943 by Bio-Dynamics, https://www.biodynamics.com/biodynamics.html a journal focusing on the farm as an organic unit.
Gregg’s work was a foreshadowing of the modern ecological movement. He emphasized the interaction of the whole rather than analysis of one isolated element. In this he made a very valid point. Up to that time much of biological science was the result of studies made by isolating the test subjects, changing one thing and observing the results. This method can and does provide valuable insights into biological processes, but as we now know it isn’t the whole story. Gregg felt that the study of agriculture was too removed from the natural world. He believed that “the observation of these relationships stimulates imagination and sensitivities of observation to other living relationships and thereby opens new doors to further understanding of the world of nature.”
The body of Gregg’s article in Bio-Dynamics was a combination of fact, supposition, and mysticism. In discussing plants that aid others by enriching the soil he mentions the nitrogen-fixing benefits of legumes but also says that caraway “puts the ground in good heart,” without explaining what that means. In the kitchen garden he says corn and pumpkins (two of the “three sisters” of Native-American agriculture) aid each other, no surprise there. But further on he says “strawberries dislike cabbage” and “chervil planted near radishes makes them taste hot.” How or why these pairings have these effects he does not explain. Here are a few more quotes from Gregg’s article:
• Apple trees infested with scab have been helped by chives growing near the roots.
• Sweet basil does not like rue. Perhaps this is a disharmony…from the inherent nature of each plant.
• Beets like to grow near dwarf beans…[but] grow wretchedly near runner beans.
• Strawberries and borage are mutually helpful, if the borage is limited to a small proportion…
• Caraway dislikes fennel.
• In an experimental planting, tomatoes near cabbages kept the white cabbage butterfly away.
• Cauliflower is known to grow better if there is celery in its neighborhood.
• Horseradish aids potatoes if it is restricted to the corners only of the potato plot.
• Hyssop planted near grapevines increases the yield of grapes. Radishes are not happy if hyssop is too close by.
• Stinging nettle growing near any herb plant will increase the pungency and aroma of the herb itself.
Gregg’s emphasis on “imagination and sensitivities” conflicted sharply with mainstream agriculture’s perspective. The problems of global conflict and food shortages served only to further focus the agricultural community on surviving today’s crisis rather than preserving yesterday’s traditions. The ideas in “Companion Planting” barely produced a ripple in the gardening community.
In the post-war peace, agri-business continued to focus on chemicals to solve gardening problems. Although the term “ecology” had been coined nearly a century before, studying nature as an inter-related whole had not yet caught the public’s imagination. Then in 1962, Rachel Carson published Silent Spring, bringing the wisdom of agriculture’s chemical dependence into serious question. Popular thought began to look for alternatives.
In this atmosphere of increasing environmental concern, Richard Gregg and Helen Philbrick published Companion Plants and How to Use Them in 1966. During the years since his previous article Gregg had developed a method that he said revealed plants’ interactions using “sensitive crystallization.” The extracted juices from two plants were combined with copper chloride on a glass plate. After the solution dried, it left a crystalline pattern. Harmonious patterns revealed beneficial interactions. Erratic or jumbled patterns demonstrated harmful interactions. He gave no explanation as to how he came to this conclusion nor did he describe the bio-chemical mechanism that provided the results.
Gregg’s laboratory results were greeted with considerable skepticism. How do those crystals prove beneficial or harmful interaction? Is there a sound basis for accepting companion planting or is it a bunch of nonsense repeated over the centuries?
How Does Companion Planting Work?
The answer to this question is complex. Some companion planting claims have been substantiated by modern testing methods. Some may be valid but have not yet been tested. Others are just interesting historical curiosities. The benefits of companion planting come from a number of mechanisms. Certain flowering plants attract beneficial insects, such as bees that pollinate or ladybugs that prey on aphids. Others act as a “trap crop,” luring harmful insects away from other plants. Still others emit chemicals that affect their neighbors, inhibiting or encouraging growth. Companion planting has traditionally applied to small-scale farming or backyard gardens, making it often difficult to test. Nevertheless, some have succeeded in taken companion planting rules and subjecting them to controlled testing. Let’s examine a few of these studies.
Gardeners spend a lot of time and money dealing with insect pests. Companion planting urges them to takes advantage of plants that give off insect repelling chemicals. Some of these interactions have been proven. African marigolds exude thiopene, which repels nematodes and makes it an excellent companion for root crops. Rosemary oil has been shown to repel aphids, making a rosemary hedge potentially beneficial to rose gardens.
Using flowering plants that attract beneficial insects to the garden is another aspect of companion planting. This was tested in California vineyards to see if flower corridors in the vine fields would lure desirable insects from nearby forested areas. They planted fleabane, butterfly bush, yarrow, and fennel in rows bisecting the vineyard. Examination of the insect population revealed an increase in beneficial insects plus reduced destruction from leafhoppers and thrips.
Herbs are sometimes used in companion planting to lure harmful insects away from other plants, thus acting as a “trap crop.” The University of California at Davis tested this idea by sponsoring a study of five varieties of mustards as a trap crop for controlling flea beetles on brassicas –broccoli, cabbage, and cauliflower. Every third row in the target field was planted with mustard. During the trial, flea beetles were vacuumed from the mustard trap crop and counted.
Although the study proved that the flea beetles were attracted to the mustard, researchers did not get the expected reduction in damage to the adjacent brassicas. The mustard concentrated the flea beetle population so that neighboring brassicas sustained less damage. However the damage reduction dropped significantly as the distance from the trap crop increased.
Another study by the Ontario Ministry of Agriculture had more encouraging results. They found noticeable reductions in crop damage when Chinese Southern Giant Mustard was used as a trap crop. On possible reason for their success where the California study had failed is that they were dealing with a different species of beetle in a colder climate. They summarized their observations by saying:
“Some keys to success with this trap crop include making sure there is always enough trap crop available to lure the flea beetles away, not letting the trap crop go to seed…there should always be enough healthy, growing trap crop around, so regular sowings [of the trap crop] are necessary. Flea beetles will quickly destroy the trap crop and move to the commercial crop if there isn’t another choice.”
Another possible benefit of companion planting is soil modification through root action. It’s well known that legumes take nitrogen from the air and deposit it in the soil, making the soil more hospitable for other plants. Studies have shown that inter-planting garlic chives with tomatoes helps to suppress bacterial wilt. On the flip side, black walnut trees produce juglone, which suppresses the growth of several plants. The blue gum eucalyptus uses similar chemical tactics to prevent competition under its canopy. Studies have also implicated tansy, wormwood, and French marigolds as herbs that wage chemical war against beans, peas, lettuce, and collards.
Suggestions for Home Gardeners
I’ve been throwing a lot of history and scientific studies at you. Now it’s only fair that I at least give you some useful advice. Many companion planting rules have yet to be proven. However we can apply the results of what has been tested to date. To encourage beneficial insects, include flowering members of the Apiaceae family (fennel, yarrow, dill) in your garden area to provide nectar. Use trap plants such as nasturtiums and mustard with caution. They will attract harmful insects but those same insects will also munch on nearby plants if they run out of food at the trap plant. Try planting trap crops on the opposite side of the garden.
Here’s the bottom line. Ongoing studies of companion planting will prove or disprove more of the rules passed down from our ancestors. But until then, don’t believe everything you hear. Think critically and consider how the benefit is being provided.