Do carrots really love tomatoes? We’ve been hearing companion planting advice for years and at last the new book Plant Partners by Jessica Walliser looks at the research to understand which plant combinations can benefit our gardens.
Also see Popular Garden Myths We’ve All Fallen For where we fact-check popular garden advice.
A Fresh Look at Companion Planting
by Jessica Walliser
For generations, gardeners have used companion planting strategies in hopes of growing a more productive vegetable garden. However, most of these plant partnerships didn’t have research to back them up. Instead, they were based on little more than folklore and conjecture.
But things have become clearer in recent years. In my new book, Plant Partners, I dive into the science behind modern companion planting and give the concept a much-needed reboot. Often called interplanting and intercropping in the scientific research, the benefits of companion planting are clear and perhaps more extensive than you might think.
Reasons to Companion Plant
Deterring pests is one reason for companion planting, but it’s not the only one. In Plant Partners, I introduce seven benefits of partnering plants. From building soil health and enhancing biological control, to managing pests, weeds, and diseases, the perks of modern companion planting are very real and very accessible, even in a small vegetable garden.
Each of these seven benefits of companion planting are reviewed carefully in their own chapter, along with dozens of specific plant combinations aimed at achieving those benefits.
Today, I’d like to share five of the many science-backed companion planting partnerships featured throughout the book.
1Plant combination: Green Beans and Potatoes
Benefit: Soil health and fertility
Why it works: Partnering potatoes with green beans showed an increase in potato tuber size. While green beans don’t fix large amounts of nitrogen, they are capable of sharing some of the nitrogen they do fix with nearby plants via nitrogen transfer, even while they’re still alive. They share this nitrogen via root exudates and the frequent shedding of nitrogen fixing nodules from their roots. It’s also shared with neighboring plants through the underground mycorrhizal network.
How to use it: Alternate closely planted rows of beans and potatoes in the garden or mix the two plants together in the same row or block.
2Plant combination: Cucumbers and Tomatoes
Benefit: Weed suppression
Why it works: Cucumbers are allelopathic, which means they produce several growth-inhibiting allelochemicals that negatively impact the growth of neighboring plants, cinnamic acid being among the most studied.
How to use it: Grow cucumbers as a living mulch under or around taller crops such as tomatoes, okra, and eggplants. The allelochemicals that stifle weed growth are exuded through their roots, so it’s best not to partner them with crops grown from seed. Transplants, however, are little impacted by their allelopathic effects.
3Plant combination: Sweet Alyssum and Lettuce
Benefit: Enhanced biological control
Why it works: Sweet alyssum is an exemplary nectar source for both the syrphid flies and the parasitic wasps whose larvae feed on aphids.
How to use it: Interplant your lettuce crop with rows of sweet alyssum or plant it along the edges of raised beds where lettuce is growing.
4Plant combination: Marigolds and Onions
Benefit: Pest deterrent
Why it works: Marigolds are commonly touted as a great companion plant. However, most of the partnerships they are recommended for have zero research to back them up. In the case of onions, though, the presence of marigolds has been shown to reduce the egg-laying behaviors of the onion maggot fly which is a very destructive pest.
How to use it: Interplant your onion crop with marigolds They do not have to be a particular variety but do need to be intermingled directly with the onion plants.
5Plant combination: Crimson Clover and Blueberries
Benefit: Improved pollination
Why it works: In a Michigan study, blueberry pollination was greatly improved when the bushes were interplanted with crimson clover. Cross-pollination is essential for most blueberry varieties, and bumblebees are four times more effective at pollinating blueberries than honeybees.
How to use it: Underplant blueberry bushes with a living mulch of crimson clover. The nectar produced by the crimson clover flowers in the summer is a quality food source for the bumblebees when the blueberries are not in flower.
The book is packed with lots more practical examples.
About the Author
Jessica Walliser is a horticulturist and co-founder of the website SavvyGardening.com. For 15 years, Jessica co-hosted The Organic Gardeners, an award-winning program on KDKA Radio in Pittsburgh, Pennsylvania, and was the garden columnist for the Pittsburgh Tribune-Review. In addition to writing Plant Partners: Science-based Companion Planting Strategies for the Vegetable Garden, Jessica is also the author of Container Gardening Complete, the Amazon best-seller Good Bug Bad Bug, and the winner of the 2014 American Horticultural Society’s book award, Attracting Beneficial Bugs to the Garden: A Natural Approach to Pest Control, which will be released as a revised and updated edition in December of 2021. She is also the Editorial Director of Cool Springs Press, the gardening imprint of The Quarto Group.