The Science Behind Companion Planting and Intercropping

Companion planting and intercropping are two different concepts which are often used interchangeably, but should not be.

  • Intercropping is the practice of growing plants together which do not compete in root space, sunlight, nutrients or time of key growth, thereby increasing the net yield of a given growing space.
  • Companion planting is a set of gardening lore which divides plant combinations into “allies” and “enemies.”

Companion planting tables of various sources are all over the internet and in gardening books, often containing conflicting information, or specifying desired companions which do not thrive at the same time, for example, English peas and corn.  You can certainly plant corn on top of peas, but by the time the corn is a foot high, it’ll be too hot for peas in our region.  Companion planting lists have grown as people attempt to apply known science like the effect of a particular chemical in high concentration to wide open gardens and ecosystems.  This kind of application rarely works.

Unfortunately for fans of companion planting, scientific trials have over and over shown absolutely none of the claimed benefits for any of the combinations tried.  Beans or corn and marigolds?  Worse results than monocropping.  Garlic and potatoes?  No change or negative benefits.  Numerous plant masking odors used in concentrated form to protect roses from Japanese beetles?  Beetles undeterred and rates of damage slightly higher.

I could go on, but the science so far is clear: none of the traditional companion plantings have shown any significant promise.  One study in particular is often cited by companion plant enthusiasts as proof.  In the study tomatoes, basil and brussels sprouts were evaluated in the possible combinations.  However, the author concluded:

Double blind taste tests over three years showed no consistent preference for tomatoes grown with companions over those grown in monoculture. An apparent inhibitory effect of companion planting on some pests of Brussels sprout (e.g. imported cabbageworm, Pieris rapae L.; striped flea beetle, Phylollotreta striolata Fab.) in the first study was reversed in the second study when earlier planting of Brussels sprout allowed it to compete more effectively with its companions. Relative yield indices calculated for a range of densities (1.1 – 47.2 plants/square meter) and crop ratios indicated advantages (mean = 20%) to planting either tomato or Brussels sprout with basil companions, but no advantage to planting tomato and Brussels sprout together. The highest yields in tomato, basil, and Brussels sprout monocultures occurred at inter-plant spacings of 25, 25 and 40 cm respectively, suggesting advantages to high-density planting. Yield advantages to diculture were most pronounced at the highest densities tested, and in dicultures incorporating the highest proportions of basil. Canopy light absorption and soil moisture content were inversely correlated, and the use of light and water resources was correlated with plant density and biomass production. I conclude that garden-scale intercropping can offer advantages over monoculture, but these are not achieved simply by combining certain compatible companion species. [emphasis mine]  Crop densty [sic], ratio, and relative planting times all affect the way that companion species interact with one another and their environment.

This brings us to the next topic: intercropping.  Unlike the lore of companion planting, intercropping has been shown to work on a home garden scale.  Originating in tropical climates like Africa, understanding the niches plants grow in and how you can combine them together in limited space and resources has a long history.  Intercropping has three major advantages:

Let’s return to the earlier example of English peas and corn.  Peas are a cool season crop and corn a summer crop in our region, so they will never grow and harvest together.  However, peas planted in mid-February will be producing by mid-April when corn is planted.  By the time the corn is high enough to interfere with the sunlight the peas receive, the peas will benefit from getting some shady relief from the hot sun and slightly extend their season.  By the time corn is tall enough to interfere with harvesting the few remaining peas, the peas will begin to die back and contribute nitrogen to the soil to fertilize the next crop.

For the home gardener with limited space, being able to combine crops in time, like the example above, can be a huge space saver and a beneficial relationship.  The companion planting lore that the peas will fix nitrogen for the corn is incorrect: the nitrogen is not available until the peas die, which in our region is beyond the fertilization window for corn, and in regions where they grow together in the summer the nitrogen will not be released until the corn is also dead.

Most of the science regarding intercropping has been conducted for large cereal grains and livestock pasture, which leaves home gardeners somewhat in the lurch when deciding what to plant where.  When you understand the life cycle and needs of the individual plants, potential matches become clear, but most intercropping happens on the fly, when you realize you have a plant that needs to go in and don’t have space for it.  Happily, this usually works out quite well, so drop the complexity of ineffective lists of companions, are start intercropping by spacing in time.  Meanwhile, pay close attention to the growth habits and needs of your plants, and other forms of successful polyculture plantings will start to become clear.

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