Expert Advice

Companion Planting

Authored by:

Earth Science

Companion planting is the practice of combining different plant species in a planting to produce beneficial effects for both types of plants. A classic example of companion planting is the Native American practice of planting corn, beans, and squash together. In this type of “three sisters” planting, beans capture nitrogen from the air and make it available, corn stalks provide support for the bean vines, and squash plants cover the soil, helping to suppress weeds.

Anecdote vs. Science

There is a great deal of anecdotal information and recommendations on companion planting. Many web sites and books have companion planting charts that suggest a wide range of benefits. Most of these recommendations and alleged effects have not been validated through scientific trials, so it is best to be skeptical of these claims.

Marigolds are commonly recommended as companion plants and are alleged to have pest-repelling characteristics while they grow. Some types of marigolds do produce compounds that are toxic to soil nematodes (microscopic worm-like organisms). However, these compounds are not produced by living marigolds. They are formed when marigold tissue breaks down in the soil. For this reason, marigolds can be used as a “green manure” cover crop that is tilled into the soil before nematode susceptible crops are planted. Marigold flowers are attractive to pollinators and other beneficial insects, so they do have value in the garden.

There are documented benefits of having a diverse range of species in the garden. Habitat for beneficial insects is created by having plants with a range of growth habits and plants that serve as pollen and nectar sources throughout the season. Predatory insects, like ladybugs and hoverflies feed on pest insects. Other types of beneficial insects, called parasitoids, lay their eggs in pest insects. And much like in the movie Alien, after the egg hatches the larvae feeds on the host before emerging and pupating.

Diverse gardens with a wide range of plant species may also reduce the ability of pest insects to find their host plants, reducing pest problems.

There are also well-studied negative effects that plants can have on neighbors. One example is black walnut (Julglans nigra), which produces a compound (juglone) that inhibits growth of many types of plants. This type of chemical inhibition of growth of other plants is called allelopathy. Not all types of plants are harmed by black walnuts. Grasses, raspberries, carrots, and onions are tolerant of juglone.

A growing body of research is showing that there can be root connections among plants, even different plant species, primarily through mycorrhizal fungi (beneficial fungi that grow around plant roots). This is an interesting field of study that may provide valuable insights into ways to effectively combine plants.

Tips for Plant Combinations

When combining plants in the garden, be sure that they have the same basic requirements for light and moisture. Combining species that require well-drained, dry soil with those that need continually moist soil is not a good mix.

Try intercropping with fast maturing plants. For instance, radishes can be sown between slower growing transplants like tomatoes or peppers. The radishes will be harvested before the transplants fill in, making use of space that would otherwise be unused. This technique can be particularly beneficial in small gardens.

Include flowering plants that are highly attractive to beneficial insects.

Certain plant families are particularly rich in species that are excellent for providing food and habitat for beneficial insects like predators and parasitoids, as well as pollinators.

  • Aster family – examples include cosmos (Cosmos bipinnatus), coreopsis (Coreopsis species), marigold (Tagetes species), and sunflower (Helianthus annuus). These produce showy flowers that are excellent for attracting beneficial insects.
  • Bean family – examples include garden lupine (Lupinus polyphyllus) and clovers (Trifolium species). In addition to capturing nitrogen from the air and converting it into plant available forms, their flowers are attractive to pollinators and beneficial insects.
  • Carrot family – examples include dill (Anethum graveolens), fennel (Foeniculum vulgare), bishop’s weed (Ammi majus). These produce flowers that are very attractive to beneficial predators like syrphid flies and lady bugs.
  • Mustard family – examples include candytuft (Iberis species), wallflower (Erysimum species), mustards (Brassica species), and common garden vegetables like broccoli, collards, cabbage, and radishes. Mustard flowers are attractive to a range of species and many mustard family plants bloom early in the spring when few other flowers are available.

Don’t be afraid to experiment with companion planting and intercropping in your garden. But take anecdotal companion planting recommendations with a grain of salt. For more information on companion planting, contact us today!