Companion planting has been practiced for centuries, but it’s not always clear why some plants perform better when paired with others. To separate conjecture from facts, my guest this week, horticulturist and author Jessica Walliser, has penned a new book on companion planting strategies supported by science.
Jessica holds a degree in ornamental horticulture from The Pennsylvania State University and is the author of several books on gardening and the insects that reside in gardens. Her latest book, set to be released on December 22, is “Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden.” In eight chapters, the book explains the many benefits of “plant partnering,” such as reducing disease, improving fertility, reducing pests and supporting pollinators.
Jessica lives northwest of Pittsburgh, Pennsylvania, and is currently the acquisitions editor for the gardening imprint of Quarto Publishing Group, Cool Springs Press, and a co-owner of the blog Savvy Gardening. She’s also a contributor to Fine Gardening, Urban Farm, and Hobby Farms magazines, as well as a former contributing editor for Organic Gardening magazine and a former co-host of “The Organic Gardeners” on KDKA in Pittsburgh.
Having been a gardening columnist and radio host, Jessica has received many questions about companion planting. She says she wanted to confidently give answers, but much of the conventional wisdom regarding companion planting is based on folklore rather than science. Having a science background herself, she looked for accurate information coming from agricultural research by trusted sources. She wanted to know what gardeners can glean from that research to grow a better garden.
What Is Companion Planting And Why Should We Practice It?
Companion planting is the practice of growing plants together to the benefit of one or more of the plants.
Jessica says the idea is to plant crops in a way that is reflective of wild environments, where many varieties of plants share the same space. While many farms practice monoculture — rows upon rows of identical plants — crops perform better in a polyculture, in which different types of plants are grown together.
Monocultures tend to be more susceptible to plant diseases and to pest problems, Jessica says. But through interplanting (mixing two or more plant species together) common problems are reduced, and there are even more benefits to be realized.
In the research for “Plant Partners,” Jessica sought an increased understanding of the key connections between plants and how they interact with each other. She looked at both how two plants in close proximity benefit each other and how more diversity in general benefits the entire garden.
Plants don’t grow in rows in nature, Jessica says. They grow in harmony with other members of the plant world.
How Plants Communicate and Share Resources
Jessica says people tend to think of plants as passive, but they actually interact with each other. There is resource competition among plants, and there’s also resource sharing. For instance, a deep-rooted tree may bring water and nutrients closer to the soil surface, where shallow-rooted plants can benefit. Through their roots exudates — the compounds that plants release from their roots to interact with microbes in the soil — plants can distribute nutrients, Jessica explains. That contributes to resource sharing, or even resource stealing.
Plants use semiochemicals (pheromones or similar chemicals) to send signals to other plants and to insects. For instance, a plant under attack by insects may release what’s known as herbivore induced plant volatiles, or HIPVs, into the air to indicate to other plants that it is time to put their chemical defenses up. HIPVs can also lure in beneficial insects that prey on those pests, such as attracting ladybugs to come and eat aphids.
Semiochemicals may also contribute to “canopy shyness,” which is how some plants refrain from touching other plants in close proximity.
Plants can also communicate via mycorrhiza, the fungal organisms that colonize almost every root on the planet, Jessica says. Using the mycorrhizal network underground, a tree can send signals to other trees that can influence things like the kind of pollinators that show up.
Plant communication is complex, Jessica notes, and she says we probably only understand the very tip of the iceberg on how these organisms interact with each other.
Allelopathy is how a plant detrimentally affects the germination, growth or survival of another plant.
A few examples of plants with allelopathic properties are garlic mustard, winter ryegrass, black walnut trees, and Canada goldenrod. Each of these produce allelochemicals that they release into the soil.
Garlic mustard has been found to inhibit the mycorrhizal fungi that support the diversity of our trees. Jessica says that means invasive garlic mustard, a now prevalent weed in the eastern United States, is affecting the diversity of our forests.
But there is beneficial application of allelopathy: weed control.
Winter rye planted with asparagus will reduce the weed pressure in an asparagus patch. Winter rye also makes for a great cover crop because its allelopathy only affects seeds. So after a crop is turned in, weed seeds won’t be able to sprout, but vegetable seedlings can be planted into the garden and grown successfully.
More Reasons Not to Till
The “no-till” gardening method has been growing in popularity — and the science supports it.
Tilling the garden disturbs beneficial mycorrhizal networks, Jessica explains. Keeping that network intact really goes a long way toward helping the health of your plants and helping them acquire nutrients.
Fungi attached to roots reach farther into the soil than the roots themselves do. In exchange for some carbohydrates from the roots, fungi bring plants nutrients.
Jessica recommends “living rototillers,” such as forage radishes or elongated varieties of turnips, which will break up dense soil and then decay in place over winter, creating cavities that allow for water, air and nutrient exchange.
Weed Management Using Living Mulches
Living mulch is a short plant grown under a taller crop. Think of it as a typical cover crop like one you might grow in the off-season, but in this case, the living mulch is interplanted with an actively growing vegetable crop.
Legumes, like white Dutch clover, are an example of living mulch that also provides nitrogen transfer.
Many living mulches bloom, which attracts more pollinators to your garden. Those blooms also supply carbohydrate-rich nectar for pest-eating predatory beneficial insects.
The roots of living mulch penetrate the soil and supply root exudates to the benefit of your edible crops.
Another upside of growing living mulch is that it can outcompete weeds — but be aware of the possibility of resource competition between your vegetable crop and your living mulch. Living mulch or cover crops, done wrongly, can become weedy. Jessica says using a cover crop in a home vegetable garden really is an art, and it requires exact timing, and thoughtful care in the species that you pick and how you plant them. (In her book, she details how to manage living mulch properly.)
Jessica’s book identifies the pros and cons of different varieties of clover — crimson clover, subterranean clover, medium red clover, white Dutch clover, etc. — and points to studies on how they benefit specific types of crops through nitrogen transfer and the increased presence of beneficial insects.
Trap cropping is a method of attracting pests away from one crop by planting something nearby that the pests will prefer. The sacrificial plant “traps” the pests and may suffer or die in the process, but the more desirable plant is more likely to be pest-free.
For example, Jessica’s young tomato plants were often attacked by flea beetles. Now, Jessica always underplants tomatoes with radishes, because flea beetles much prefer the radishes.
Another example is planting the mustard green known as Southern giant mustard, which harlequin bugs love, to protect collards. The mustard greens are sacrificed but the collards remain undamaged.
For the dreaded squash vine borer and squash bug, Jessica suggests a trap crop of blue Hubbard squash to lure them in. A few plants should be planted a little bit away from winter squash and summer squash, she says, and the pests will go for the blue Hubbards first.
More complicated “push-pull” systems found in farms systems involve planting both a companion plant that will deter the pest from coming near a desirable crop while also planting a trap crop elsewhere to draw the pest in. However, Jessica says this is more complicated than a small garden requires.
Appropriate/Inappropriate Landings Theory
More evidence against monoculture comes from the appropriate/inappropriate landings theory. The idea is that an insect must land on its host plant a certain number of times before it recognizes that plant as an appropriate place to lay its eggs.
Insects have receptors on their feet and other ways of receiving contact cues and olfactory cues from plants.
In a field of cabbage, a cabbage moth will always land on cabbage and will know to lay its eggs there. But in a field of diverse plants, a cabbage moth may land on inappropriate plants and decide to move on.
While the appropriate/inappropriate landings theory is still unproven, studies do point to reduced pest pressure in a mixed polyculture.
Masking Hosts Plants
Does planting dill or marigolds with tomatoes repel tomato pests? The conventional wisdom says yes, but Jessica says that may not be the case.
Researchers have found that the scent of dill and marigolds doesn’t actually repel pests. Rather, the scent of these popular companion plants may mask the scent of the tomato plants, so pests can’t locate them.
Impeding the Movement of Pests
Companion plants can become physical barriers to pests.
For instance, farmers know that hedgerows can block pests from moving between a wild, uncultivated area into a farm field. At the same time, the trees and shrubs also bloom, attracting pollinators.
Certain plants will block soil-dwelling pests like the root maggot fly, larval cucumber beetles, or larval flea beetles. Low-growing crops can prevent flies from laying their eggs in the soil by physically impeding them.
Disease Management Through Companion Planting
Interplanting can create plant partnerships that suppress diseases, though this is an area of companion planting that has not been studied as much as others, Jessica says.
Certain plants, especially members of the mustard family, produce antifungal compounds that can reduce root-rot infections. And some cover crops, once tilled into the soil, act as a biofumigant that reduces the occurrence of wilt bacteria and other types of pathogens.
Living mulches help in disease management as well, because they reduce the splash-up effect of soil-borne pathogens getting on plant foliage in the rain, such as blight on tomatoes.
Hairy vetch, a legume, is a cover crop that can reduce foliar diseases on tomatoes. Jessica says to cut down the vetch and plant the tomatoes in the plant debris to reduce splash up and to suppress those particular pathogens.
Reducing pest pressure through companion planting also has an impact on plant diseases, because insects can be vectors for disease and their damage can create places for pathogens to enter plants.
Biological Control – Attracting Good Bugs
Jessica says plant partnerships have a huge role to play in supporting the good bugs that make a home in our garden and reduce pests.
This is accomplished by planting crops specifically to attract predatory beneficial insects. The right partner plants will attract the good bugs you need to save your vegetables from pests.
For example, members of the celery, carrot and parsley family — such as dill, fennel, Angelica and cilantro — will attract the adult Cotesia wasp. This is the parasitoid wasp that lays its eggs inside tomato hornworms and tobacco hornworms, both of which can quickly devour tomato plants. More Cotesia wasps in your garden will equal fewer hornworms.
Growing Banker Plants
Banker plants are sacrificial plants that are grown for the purpose of ensuring there are always pests around for good bugs to eat. That way, when a pest problem does emerge, there will be a healthy and happy population of good bugs nearby.
Jessica calls banker plants an all-you-can-eat bug buffet for beneficial insects. In her garden, it’s nasturtiums that serve as banker plants for aphids. She knows that when her nasturtiums have aphids it means her garden will soon have a strong presence of Aphidius wasps — a wasp genus that uses aphids to house and feed their young, killing the aphids in the process.
Nasturtiums are also shown to repel squash bugs, so they can pull double duty in the garden when interplanted with squash.
Companion Plants for Improved Pollination
It takes big, fat bumblebees to open pea and bean flowers and shake the pollen, Jessica says, and companion plantings of flowers provide large landing pads to bring bees into the garden.
Bees also pollinate tomatoes and peppers through “buzz pollination,” so you really can never have enough bees in your garden.
Jessica says to plant ornamentals that will be in flower before your vegetable crops are in flower, so the pollinators will already be there when the time comes.
Companion Plants for Structural Support
The classic “Three Sisters” method of companion planting involved growing corn so that the stalks can act as supports for pole beans to grow on. Jessica spends a chapter in her book on this idea of providing structural support for vining and twining plants.
A few examples are amaranth with Malaba spinach, kiss-me-over-the-garden-gate with Gherkin cucumbers, and sunchokes with cucamelons. Not only are the vining plants supported, but the practice also adds a great deal of aesthetic interest to the garden.
If you haven’t listened yet to my conversation with Jessica Walliser, don’t miss out. Scroll up and click the Play button in the green bar above.
Do you practice companion planting? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See the full disclosure below.
joegardener Online Gardening Academy™: Three popular courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
joegardener Online Gardening Academy Essential Gardening Fundamentals: The basics on healthy soil, planting, watering techniques, composting, raised bed and other gardening methods, fertilizer, the many benefits of mulch, and more.
“Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden” by Jessica Walliser
“Gardener’s Guide to Compact Plants: Edibles and Ornamentals for Small-Space Gardening” by Jessica Walliser
“Attracting Beneficial Bugs to Your Garden” by Jessica Walliser
“A Gardener’s Notebook: Life With My Garden” by Jessica Walliser and Doug Oster
“Good Bug Bad Bug: Who’s Who, What They Do, and How to Manage Them Organically” by Jessica Walliser
*Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Park Seed, and Exmark. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.