Garden Variety: The myths and facts about companion planting
Companion planting is a centuries-old practice of planting one or more crops near each other for singular or mutual benefit. New gardeners hearing the term for the first time might wonder which practices are best; experienced gardeners might wonder if it really works.
Some companion planting suggestions are beneficial, but others persist despite the lack of scientific evidence supporting any benefit from them.
Most people think of companion planting in relation to pest control, with one species of plant repelling insects from another or being more attractive to insect pests than the desired crop. Pest repellency and trap cropping are potential benefits from companion planting. Other benefits include improved pollination, increased natural predators, natural trellising, nutrient provision, shading of soil and others depending on the crops and situation.
The most widely known example of companion planting is the “Three Sisters” planting, which is a Native American system for growing corn, beans and squash. There are variations of the practice from those who used it across the continent. Most systems use corn planted close together in the center of a circle. When the corn is about 6 inches tall, climbing beans and squash are planted around it in an alternating pattern.
Corn, beans, and squash benefit each other in this method in a few different ways. Corn provides a natural trellis on which the beans can grow. Beans “fix” nitrogen, meaning they take nitrogen from the atmosphere and put it into the soil where plants in very close proximity can use it. Squash has large leaves that shade the soil around the other plants, reducing soil temperature and moisture fluctuations. This system also reduces weeds as the shading prevents weed seed germination and makes very efficient use of space.
In some places, cleome flowers were planted as a fourth sister to attract pollinators for the beans and squash.
The Three Sisters companion planting method works, but it is rarely used in modern planting systems.
Another companion planting method that is truly effective is the use of sweet alyssum with lettuce. Alyssum attracts beneficial predatory insects that eat aphids and other plant pests. USDA’s Agricultural Research Service recommends planting one or two sweet alyssum plants for every 50 lettuce plants. Lettuce does not have many insect pest problems in Kansas, but the use of alyssum could be especially beneficial for large-scale production, market gardens and organic production.
Another companion planting practice that is commonly recommended is the use of marigolds to repel all sorts of pests and supposedly make certain vegetables taste better. Most of the claims about marigolds have been disproven in research trials. In some cases, marigolds reduce the number of a certain species of soil nematodes. This only works if the marigolds are planted in an area one year and the nematode-susceptible crop is planted in the same spot the following year. Nematodes are microscopic worms, and the soil species that marigolds are effective on are rare in Kansas.
Marigolds do not repel mosquitoes, cabbage moths, whiteflies or other plant pests in research trials. They do attract spider mites and aphids.
Marigolds are also recommended in some companion planting guides to attract pollinators to the garden. A better option to attract pollinators is to use plants recommended by pollinator researchers and pollinator garden support organization. Examples that grow well in Kansas include the native species purple prairie clover (Dalea purpurea), prairie blazing star (Liatris pycnostachya), and showy goldenrod (Solidago speciosa).
For other companion planting methods, check the research before using them. Look for extension research from land-grant universities across the country or organizations that support research on gardening and agricultural production methods. While most are harmless and might provide enjoyment in other ways (such as the planting of marigolds), they might also be a waste of time and concern. There are countless books, blogs, and other sources that simply repeat information and lack evidence of efficacy or validity.
— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.