Q: What is "companion planting"?
A: Companion planting is an old term used for planting different varieties of plants close to each other to benefit one or both plants. A newer term for companion planting is intercropping. You may have heard of a long-used method of intercropping called "The Three Sisters," a Native American practice using corn, beans and squash/pumpkin to help one another. More recently, there has been some scientific evidence to show that intercropping may be beneficial. But the jury is still out as far as extensive research-based conclusions.
Intercropping is often touted as a way to reduce pest damage. It can be effective - though not always in the ways people think. For example, gardeners sometimes plant marigolds hoping their fragrant leaves will repel pests. There's little evidence to support this practice. In fact, marigolds may attract pests such as spider mites. But marigold roots may have a deterrent effect on some species of root nematodes.
Another way of using intercropping to combat pests is to plant a trap crop, a plant or group of plants meant to draw a pest away from the main crop. Planting dill, which tomato hornworms prefer, among tomatoes is an example. Similarly, intercropping with mustard or radishes may help keep flea beetles off your lettuce. An Iowa State study (Companion Planting; A Method for Sustainable Pest Control-Riesselman) done in 2014 showed that intercropping was more advantageous than not using any trap plants. The pests still did appear on the desired plants, but at a later date.
Intercropping can also be used to attract desirable bugs. Flowering plants can be used to attract pollinators. Or you can plant species that will harbor a predator that attacks the pest you're trying to get rid of. For example, research has shown that planting dill, coriander and buckwheat close to green peppers can reduce damage from corn borers, because it increases the number of predatory insects that eat corn borer eggs.
Plant structure can also be used in intercropping. You may use shallow rooted plants by deep rooted plants. One plant that loves sun may provide shade for another that does not appreciate the sun as much. A tall stalwart plant may serve a support to vining plant.
Intercropping is a way to add diversity to our gardens. Additional research is needed to figure out which plants really offer benefits to each other. For further information, see extension.udel.edu/factsheets/adding-diversity-garden/.