Better Together: The New Science of “Companion Planting”

Jun 16, 2022


Like many things in life, the best results are achieved by working together.  This is especially true with plants! 

Although you may have heard about plants that are thought to aid in insect control, disease prevention or the benefits of pairing particular plants in certain areas, the latest research on “companion planting” goes far beyond random recommendations or traditional lore. The latest research reveals the need to look at our gardens in a more holistic way: as ecosystems where plants interact with each other to create a healthy, bio-diverse place.  Key to understanding this concept is that plants actively affect each other.  They do this through fungal associations, chemical messaging and allelopathy, which is the ability of one plant's chemistry to affect the growth and development of another.  They also share resources, attract pest predators, and improve each other's nutrient availability and absorption.

When we plant one type of plant in an area, whether it's tomatoes or other vegetables, we create what is known as a “monoculture.” Although it makes large scale farming possible, monoculture leads to increased reliance on pesticides and chemical fertilizers. However, when we choose different plants to grow together, we create a “polyculture,” which leads to biodiversity. Why is this important?  Biodiversity means a more stable environment for veggies and other plants to thrive by enhancing nutrient cycling, water conservation, fewer pests and ultimately, more carbon sequestration.

What is modern research-based companion planting? Scientists prefer terms like inter-cropping or inter-planting to describe creating a polyculture to achieve desired benefits in the garden. Using a scientific approach helps us to understand the why and how of successful planting combinations. 

Jessica Walliser, author of “Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden,” provides plant partnerships that have undergone scientific scrutiny and shares plant pairings that help to reduce pests, minimize disease, support pollination and improve soil fertility. Walliser's findings are the basis of this article. 

Soil Preparation and Conditioning
Plant companions improve soil via cover cropping, nitrogen transfer, and breaking up heavy soils. Plants used as cover crops to condition the soil include oats, buckwheat, winter rye, crimson clover, winter wheat, and cowpeas. Plant partners that improve nitrogen transfer include:  garden beans + potatoes; fava beans + sweet corn; cowpeas + peppers; peas + lettuce; and edamame + fall greens. Plants that help break up heavy soils include:  buckwheat, forage radish, and turnips. 
Weed Management
Suppressing weeds without herbicides is also possible with companion planting. Intercropping of plants can make “living mulch,” which crowds out unwanted plants and blocks light to keep new seeds from sprouting. In allelopathy, some plants can help gardeners combat weeds by their growth inhibiting compounds. Plant companions for living mulches include:  crimson clover + cole crops (broccoli, cauliflower, brussels sprouts, and kale); medium red clover + winter squash; white clover + strawberries or blueberries; white clover + tomatoes, peppers, eggplants; and cowpeas + peppers. 
Pest Management
Plant companions can also help manage unwanted insects. There are several methods which include the following:  “trap cropping,” where one type of plant is used to lure pests away from another, more desirable one (examples:  tomatoes + cowpeas, bell peppers + hot cherry pepper, and cabbage + collard greens); “masking strategies,” ways to “hide” or deflect a pest's attention from a particular plant (example: tomatoes + basil); and creating physical barriers such as hedgerows or low growing plants to impede movement and limit access  to insects that lay eggs, pupate, or live in the ground. 
Disease Management
Plant partnerships and interplanting can also help to suppress soilborne diseases like stem or crown rot, wilt disease, root rot, and bacterial diseases. Examples include: potatoes + oats or winter rye for verticillium wilt  and cauliflower and lettuce + brassicas for verticillium wilt and sclerotin stem rot. 
Biological Control
Using companion partners can be effective in keeping pest populations in check by creating a habitat for pest-eating, beneficial insects and spiders. Combine pest-prone plants with other types that provide nectar and pollen and ensure there are there is enough variety to keep insects interested year-round. Examples include: lettuce and other greens + dill and fennel and cole crops + black-eyed susans and cosmos for aphid control.
We can attract many species of native bees by providing nesting habitats, eliminating pesticides, and companion planting. Improving pollination examples include: Tomatoes, peppers, and eggplants + large or hooded flowers and blueberries + crimson clover to attract bumblebees.  

Using science as your guide, gardening with companion plants will help you learn about your own unique corner of the world. Have fun as you experiment with plant combinations and create a bio-diverse habitat outside your door. You'll look at your garden in a whole new way as you observe the changes in color and texture--all the while appreciating the overall health in your garden. 

Cynthia Nations is a UC Master Gardener who is currently learning and employing companion partnerships in her garden in El Granada. The article was co-authored and edited by Maggie Mah, a UC Master Gardener, who is trying to catch up with Cynthia Nations.


By Cynthia Nations
By Maggie Mah
By Maggie Mah

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