And, ah, yes: ‘companion planting’; a topic laced with more folklore, hopefulness, bad information and just plain hype than just about any other in the gardening world. It’s said to have begun just as the world was going from BC to AD, when the oft-quoted Pliny the Elder wrote that the (highly toxic) plant rue was a “very friendly” companion to figs. — Mike McGrath, “The Truth About Companion Planting”
McGrath’s is not an uncommon position. Companion gardening is a messy topic than needn’t be messy. It’s messy first because a lot of pseudo-scientific nonsense has been published on the topic, and second because even within the area of what makes sense, there’s a wide variety of approaches and techniques.
The basic idea behind companion garden planting is both simple and sensible: many plants grow better near some companions than they do near others or when alone. The devil is, as usual, in the details, in this case in the precise definition.
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While many experts reject what’s come to be called the “traditional” version of companion planting, no one challenges that small shade-loving plants can be successfully grown near tall bulky ones, or that some plants can attract beneficial insects for others.
The hype, the trouble, comes from what’s unfortunately the most familiar version of companion gardening, which is the idea that somehow, mysteriously, companion plants just “like” each other. In this version, that “liking” is determined not by observation of plants in the field, nor by scientific analysis of toxins, excretions, nutrient use, or other biologic characteristics, but by an interpretive “reading” of the crystals* deposited by a solution of the test plants (see Crystal Chromatography). This is the basis for Louise Riott’s Carrots Love Tomatoes: Secrets of Companion Planting for Successful Gardening — book which nevertheless contains a fund of useful information.
Not surprisingly, this language of love, mystery, magic and secrets sets scientific teeth on edge, and the very term “companion gardening” will make some agricultural nostrils flare with disgust. The result has been that some gardeners and university extension sources reject companion gardening wholesale. A Colorado Extension master gardener devotes a single paragraph to the topic, titled “The Myth of Companion Gardening,” and Tennessee, Ohio, and others deliver blanket rejections; this one from Ohio State U. (1998) is typical:
Some gardeners believe that certain plants perform better when grown together. However, no proven beneficial relationship of this type of inter-planting, called companion planting, has been demonstrated under research conditions.
Several of these experts may find themselves backtracking because an increasing number of studies, conducted by reputable institutions in accepted scientific procedures and published in typically dry scientific parlance, bear out the viability of companion plants.
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A source for companion planting methods backed by studies can be found at The National Center For Appropriate Technology Center’s Sustainable Agriculture (ATTRA) website (scroll down). The types of effective companion planting include:
Trap cropping or using neighboring plants to draw pests away from a main crop. A study showing the effectiveness of Perimeter Trap Cropping (PTC) strategies can be found here. Some common plants used in trap cropping include buckwheat, dill and leafy cabbages. In the study cited above, Blue Hubbard squash were used to attract pests away from summer squash and cucumbers. A study of how dill, coriander and buckwheat helped protect peppers from corn borer can be found here (PDF). Generally, planting strong-smelling herbs may camouflage the smell of your main crops. Planting herbs around a garden is always a good idea.
Similar to this is the practice of planting beneficial habitats — also known as “refugia” that will harbor beneficial insects that will feed on pests. Examples of this include the planting of morning glory, Queen Anne’s lace and yarrow to attract ladybugs or growing nearby grasses that will harbor praying mantis.
A well-known companion planting technique is “symbiotic nitrogen fixation,” planting legumes including peas, beans and clover near crops, like corn and tomatoes, that can use the nitrogen boost. Though a well-established principle when done as crop rotation, the simultaneous, nearby planting of heavy nitrogen-feeding plants and nitrogen fixing plants has been less studied. It is known that planting forage legumes, such as clover, in lawns with grasses will reduce the need for nitrogen fertilizers.
The use of marigolds to repel pests is one of the most-accepted principles of companion planting. The marigold’s distinctive smell is reputed to repel a host of pests. Some of these claims have been tested. The African marigold releases thiophene, which repels harmful nematodes. The French Brocade marigold is also known to repel nematodes because of an oil secreted by its roots. While not all of the marigold’s powers have been studied, its use by so many gardeners over a number of seasons (and with multiple claims of results) suggests more studies should be done.
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Many companion planting practices fall under the heading of complementary plants, those that contribute to the growth of their neighbors without chemical magic or the emanation of mystical vibrations. Examples include taller crops, such as corn or tomatoes, shading smaller crops like spinach and lettuce with a tendency to go to seed. Of course the classic example is the complementary planting of corn and beans, the beans using the corn as a trellis as it grows to maturity. No testing is required here.
Purists will always demand scientific proof. Remember that the lack of study doesn’t prove anything. But do beware of untested claims. And don’t be afraid to trust your own observation and experiences. After two years of cabbage moth problems, I planted common marigolds throughout my garden. It was an unusually warm and sunny season and the marigolds did well and I had many fewer problems with moths. That may have been due to factors other than the marigolds — the favorable weather, for example, or changes in the way I watered — but you can bet I’ll be planting marigolds again next year, conducting my own ongoing tests, if you will, just to be safe.