As we creep toward spring, many gardeners begin to get the urge to get out in the garden. Seed starting and garden planning activities are well underway. One gardening technique that many gardeners incorporate into their gardens is companion planting. 

What is companion planting?

There isn’t really an agreed-upon definition of what companion planting is. Generally speaking, companion planting is the planting of at least two different types of plants together that will in some way benefit at least one of them. 

Another way to think of companion planting is as intercropping, growing two or more crops together, or polyculture, which not only includes the crops being grown but also non-crop plants. Essentially, trying to increase diversity to create a healthy, well-balanced garden. 

Some potential benefits of companion planting

There are a variety of different ways in which plants can potentially benefit, including improving the soil and pollination and reducing pest and weed pressure. So, what are some ways companion planting can be incorporated into our gardens?

Improving soil

While some may not consider them companion plants, cover crops are a great way to help improve soils, which will benefit the crops that will be following them. Cover crops can help reduce soil erosion, add organic matter to the soil, improve soil structure, and increase soil fertility.

Some plants, particularly those in the pea/bean family, will form symbiotic relationships with nitrogen-fixing bacteria. These bacteria form nodules on the plant's roots and can take atmospheric nitrogen and turn it into a plant-usable form. This nitrogen can become available to other plants as the nitrogen-fixing plants die, or it can potentially be shared via mycorrhizal fungi.

Reducing weeds

In addition to improving soil, cover crops can also be used to suppress weeds. Cover crops like winter/cereal rye can be used as mulch after they have been terminated. Cover crops can also be used as a living mulch to provide continuous ground cover to help suppress weeds. For example, white clover can be grown on walking paths to keep weeds down. The living mulch will need to be maintained, though, to prevent it from becoming a weed itself.

Managing pests

There are several ways companion plants can be used to help manage pests. Trap cropping can be used, where a ‘sacrificial’ companion plant is planted. These trap crops should be planted before the crop you’re trying to protect (main crop). Pests will be attracted to them and can then be killed or disposed of. One example is using blue Hubbard squash, which is highly attractive to squash bugs and vine borer, to protect other pumpkin and squash cultivars. 

Plant-eating insects will find their host plants using a variety of cues such as smell (plant volatiles), visual (colors), and taste. Having diverse plantings and little bare ground can make it difficult for plant-eating insects to find their preferred host plants and potentially decrease the amount of damage done to plants.

Support beneficial insects

Many vegetable crops, such as cucurbits (pumpkin, squash, cucumber, etc.), rely on pollination to produce fruit, while others, like beans, peppers, and tomatoes, may have increased production and quality if they are visited by pollinators. Incorporating flowering plants into vegetable gardens can help attract pollinators and improve pollination. 

In addition to prey, many natural enemies (predators and parasitoids) need sources of pollen and nectar. These sources are especially important for parasitoid wasps as well as the adult stages of insects with predaceous larvae, like syrphid flies. By planting plants that are attractive to these natural enemies, you can help attract them into your garden and they can help manage pest populations.

The Three Sisters 

Probably the best-known companion planting practice is the Three Sisters, which utilizes corn, beans, and squash. In this system, corn acts as a support for the vining bean plants, the beans act as a nitrogen-fixing crop, and the squash plants act as a living mulch to suppress weed growth and to help maintain soil moisture. While this combination has been used for thousands of years, according to Washington State University, “Though there may be historic or cultural reasons for planting a Three Sisters polyculture vegetable garden, there is no compelling, published evidence that it benefits plant productivity or soil quality.”

Some caveats to companion planting

Unfortunately, there hasn’t been a lot of research done, especially on a garden scale, on companion planting. Many recommendations are based on questionable methods (ex. sensitive crystallization) and anecdotal evidence. Others are based on large studies or lab results that may (or may not) apply to a garden setting. 

The interactions between the different plants, pests, diseases, weeds, soil, etc., are complex, making it difficult to determine if growing particular plants together is beneficial. This is perhaps best summarized by Dr. Michael Bomford in his PhD dissertation: “Garden-scale intercropping [companion planting] can offer advantages over monoculture, but these are not achieved simply by combining certain compatible companion species. Crop density, ratio, and relative planting times all affect the way that companion species interact with one another and their environment.”

Good Growing Fact of the Week:  Increasing plant diversity attracts more types of herbivores (what we would typically consider pests in a garden), which will attract more predators and parasites, creating a more biodiverse, complex, and resilient food web. 

Bonus Good Growing Tip of the Week: Want more about companion planting? Check out the recording of the Companion Planting: Bountiful and Beautiful Together Four Seasons Gardening Webinar. 

Resources and for more information:

Bomford, Michael K. 2004 "Yield, pest density, and tomato flavor effects of companion planting in garden-scale studies incorporating tomato, basil, and Brussels sprout.” Graduate Theses, Dissertations, and Problem Reports. 2105. https://researchrepository.wvu.edu/etd/2105.

Chalker-Scott, Linda. 2023. “Gardening with Companion Plants (Home Garden Series).” Washington State University Extension Publications. December 2023. https://pubs.extension.wsu.edu/gardening-with-companion-plants-home-garden-series.

Pinero, Jaime. 2017. “Trap Cropping: A Simple, Effective, and Affordable Integrated Pest Management Strategy to Control Squash Bugs and Squash Vine Borers.” ipm.missouri.edu. March 16, 2017. https://ipm.missouri.edu/MEG/2017/3/Trap_cropping/.

Walliser, Jessica. 2020. Plant Partners. Storey Publishing, LLC.

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MEET THE AUTHOR
Ken Johnson is a Horticulture Educator with University of Illinois Extension, serving Calhoun, Cass, Greene, Morgan, and Scott counties since 2013. Ken provides horticulture programming with an emphasis on fruit and vegetable production, pest management, and beneficial insects. Through his programming, he aims to increase backyard food production and foster a greater appreciation of insects