Plant Partners

Science-Based Companion Planting Strategies for the Vegetable Garden


By Jessica Walliser

Foreword by Jeff Gillman, PhD

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Companion planting has a long history of use by gardeners, but the explanation of why it works has been filled with folklore and conjecture. Plant Partners delivers a research-based rationale for this ever-popular growing technique, offering dozens of ways you can use scientifically tested plant partnerships to benefit your whole garden. Through an enhanced understanding of how plants interact with and influence each other, this guide suggests specific plant combinations that improve soil health and weed control, decrease pest damage, and increase biodiversity, resulting in real and measurable impacts in the garden.

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To Niki Jabbour and Tara Nolan, my amazing
partners and soil sisters.




Chapter 1 The Power of Plant Partnerships How Does Modern Companion Planting Work?

Chapter 2 Soil Preparation & Conditioning From Cover Crops to Living Rototillers

Chapter 3 Weed Management Using Living Mulches and Alleopathy to Combat Weeds

Chapter 4 Support & Structure Plants That Act as Living Trellises

Chapter 5 Pest Management Luring, Trapping, Tricking, and Deterring Pest Insects

Chapter 6 Disease Management Suppressing Disease through Plant Partnerships and Interplanting

Chapter 7 Biological Control Plant Partners That Attract and Support Pest-Eating Beneficial Insects

Chapter 8 Pollination Bringing More Pollinators to the Garden through the Perfect Plant/Pollinator Matches

Epilogue The Companion Planting Journey






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For years gardening "experts" told gardeners to place particular plants next to each other to control insects or disease, or just because those plants "work well" together. Unfortunately, there really wasn't much scientific data to support their recommendations, so, despite good intentions, these combinations were mostly just someone's best guess. Today, scientists still don't have all the answers, but recent studies have produced greater insight into the interactions between plants, providing clarity about how they might benefit one another.

With this book, Jessica Walliser has written something for the thoughtful gardener: the one who wants to understand exactly what they're doing and why, and who is willing to observe, think through, and experiment with their garden choices. The bibliography at the back of this book lets us know that it's the product of real research. This research, along with the experience and careful observations of a committed horticulturist, makes Plant Partners a valuable gardening reference.

If you want garden dogma, then go somewhere else. This is a reference for the open-minded gardener who is willing to think through their gardening choices. It gives us the science-based information we need to make informed decisions when choosing which plants to place next to one another. Plant Partners offers more than just specific plant pairings; it encourages us to think about the relationships between plants, so that we can grow our best garden ever.

— Jeff Gillman, PhD

Director of the UNC Charlotte Botanical Gardens and best-selling author of The Truth about Garden Remedies and Decoding Gardening Advice


As a horticulturist, garden educator, radio host, and self-described science nerd, I've received hundreds of questions and comments from people about the merits of companion planting. They wonder if their beets will grow better if they plant them next to their beans. Or if planting onions next to broccoli yields a better crop of both. I've also had people swear that when they plant dill next to tomatoes, they don't get hornworms. Or that when they plant carrots next to basil, both of the plants thrive because they "like" each other. I've often struggled to provide responses to those inquiries because I haven't had the science to back them up.

You see, while companion planting has a long history among gardeners, it's a history filled with folklore and conjecture, often at the expense of sound science. But times are changing. New and existing research from universities and agricultural facilities around the world isn't necessarily validating the long-held companion planting techniques and beliefs that have been around for generations. Instead, it's pointing us toward a whole new way to companion plant. A way that approaches the garden as an ecosystem comprising many different complex layers of plants, fungi, and animals, all of which are connected in a massive web of life.

Modern companion planting isn't about what plant "loves" growing next to what other plant. It's about using plant partnerships to improve the overall ecosystem of the garden and create a well-balanced environment in which all organisms thrive, from the tiniest soil microbe to the tallest corn plant. It's about pairing plants such that one plant provides a benefit to the other in terms of an ecosystem service. For example, maybe plant A provides food for plant B. Or maybe plant A controls weeds around plant B. Or maybe plant A attracts beneficial insects that control pests on plant B. As you'll soon learn, plants can benefit each other in these and many other ways.

For generations, companion planting has been defined as the close pairing of two or more plant species for the purpose of enhancing growth and production, or trapping or deterring pests. It's a definition that still rings true, even as companion planting undergoes a much-needed science-­based reboot. Farmers and gardeners now have myriad credible studies, controlled experiments, and fact-based research to rely on. Some of these studies provide a pretty jaw-dropping look at mutualism within plant communities, between plants and animals, and even between plants and fungi for the betterment of one or more of the organisms involved. It's all a big web of connections out there in the garden, and it's time we started paying attention to those connections and how they can make us better gardeners.

In the pages of this book, you'll find both broad and narrow looks at many of the studies I mention above. My aim, however, is to put a modern twist on the practice of companion planting by offering gardeners ways to use these findings in their own landscapes to improve plant health, yields, and productivity.

Chapter 1 explores many of the possible benefits of modern companion planting and takes a deeper look at why this long-standing practice needs to be approached in a whole new way. We'll examine the role polycultures, plant associations, and interplanting play in the garden and look at why so many scientists dislike the term companion planting. One long-held yet scientifically unproved companion planting theory is that plants whose extracts have a similar crystalline structure grow well together. Other outdated theories are based on the growth patterns or "energies" of a plant. We now know, however, that things like fungal associations, resource competition, chemical messaging, plant diversity, and nutrient absorption have far greater impacts on how well one plant grows alongside another than the myth-based theories could ever tell us. This chapter takes a look at each of these influencing factors and how they translate into a successful companion planting strategy.

Chapters 2 through 8 each tackle a common gardening problem by using appropriate plant partnerships to overcome it. You'll find a chapter on companion planting to boost soil health, one on controlling weeds, another on using plant associations to limit pest damage, and much more. Whether you want to manage plant diseases, improve pollination, or encourage a naturally high population of pest-eating beneficial insects, these chapters offer everything you'll need in the form of one or more methods of companion planting. Each chapter begins with an introduction to the problem, followed by several research-backed plant combos and other strategies that use companion planting as an effective tool to solve that particular problem.

Every chapter in this book speaks to how important an ecosystems-based approach is to the modern landscape. Even as many of our yards and gardens — along with the fields, forests, deserts, and prairies of this Earth — have become less and less diverse over the years, our understanding of the value of environmental diversity has never been greater. When you implement some of these concepts and plant partnerships in your home garden, the impacts of your actions will extend well beyond your garden's gate.

As you'll come to see, today's companion planting methods generate real and measurable benefits. Plant Partners aims to redefine the science of successful plant partnerships by examining the many ways one plant influences another. After closing the final pages of this book, I hope that you, like me, have a whole new appreciation for companion planting in its modern form.

Chapter 1

The Power of Plant Partnerships

How Does Modern Companion Planting Work?

One might assume a garden is made up of independent parts, each managed independently by the gardener. But in truth, the many parts of a garden are in constant interaction, much like in a wild ecosystem. Yes, the gardener plays a role in managing these interactions, but all of the different components — from the plants and soil to the insects, fungi, and microorganisms — have ongoing effects on each other whether managed or unmanaged.

Modern companion planting strategies recognize these interactions and acknowledge gardens as ecological habitats in which plants are capable of influencing each other in a variety of ways, including through the chemical signals they produce, the underground network of fungi in and around their roots, and the toxic compounds some of them exude. Many strategies are aimed at influencing the insect world. Though in many ways we're only just beginning to understand the complexities of these interactions, we can already use our current knowledge to become better gardeners and build healthier, more productive gardens.

A Modern Take on Gardening

Today's ornamental gardens are, in many ways, different from those built by past generations. For decades, plantings of perennials, annuals, trees, and shrubs were almost exclusively focused on the ornamental value of the garden. Now this approach is shifting to one that is much less centered on human aesthetics and more focused on providing wildlife habitat and conserving resources. While a huge chunk of the "gardening public" certainly still thinks of gardens exclusively from a human point of view (What color is that flower? How tall does that shrub grow? How hard is that plant to maintain?), there are now more gardeners than ever who create and cherish gardens for their ability to provide something beyond their appeal to the human eye. These gardeners plant flowers, trees, shrubs, and ground covers to attract and support pollinators, to absorb rainwater runoff, to sequester carbon, to conserve irrigation water, to filter pollutants from the air, to create habitat for a variety of animals and insects, to build healthy soil, and to improve the overall biodiversity of their little part of the world. Yes, it's a bold jump to make, but millions of gardeners are now looking at their favorite hobby as an opportunity to make a positive difference in the health of the planet.

Despite all of these changes in the world of ornamental gardening, times don't seem to have changed much in the vegetable patch. The victory gardens planted a few generations ago mimicked farm-style plantings with their highly organized "soldiers in a row" design of row after row of single crops. Unfortunately, the vast majority of modern vegetable gardeners here in North America still view the garden as a place of order, where crops are planted in straight rows to become a mini monoculture of sorts. Perfect lines of vegetables are flanked on either side by empty rows of bare or mulched soil, and flowering plants that do not produce edible crops are relegated to the garden's edge or are not included at all. Vegetable gardens have been slow to evolve. For some reason, many vegetable gardeners don't view their garden as a working ecosystem, seeing it only as a space for maximizing yields for human consumption.

However, even a slight shift in thinking can result in big changes in the vegetable plot. Since food gardens are the primary places where companion planting strategies come into play, gardeners who focus on creating mixed plantings that include a diversity of vegetables, fruits, herbs, and flowers all growing together are benefiting not just themselves but also the garden ecosystem as a whole. They're supporting a diversity of pollinators, staving off soil nutrient depletion, naturally deterring pests, and more — all while still maximizing their yields for human consumption, just in a different way. Raised beds, containers, planter boxes, and in-ground plots filled with a combination of plants are far from a monoculture, and as you'll come to learn, when those combinations are made with specific plant partnerships in mind, the benefits are magnified.

Whether vegetable or ornamental, gardens are ecosystems capable of supporting biodiversity, filtering rainwater, sequestering carbon, and providing many other services.

The Benefits of Companion Planting

Let's spend a little time discussing some of the possible benefits of companion planting and look at how these mixed-planting strategies can affect vegetable gardens. Most of the benefits that companion planting strategies provide fall into one of the following seven categories.

One possible benefit of companion planting is improved pollination.

  1. 1. Reduced pest pressure. Minimizing pest damage tends to be the most sought-after benefit of companion planting. The research into it is mind boggling, with countless studies looking at everything from how pests find their host plants to strategies for luring pests away from desired crops before they can cause significant harm. Companion planting to reduce pest pressure utilizes luring, trapping, tricking, and deterring pests to keep vegetable garden damage at a minimum.
  2. 2. Reduced weed pressure. A reduction in weeds without the use of herbicides is another possible benefit of some companion planting strategies. You'll be introduced to the science of allelopathy in chapter 3 and how it can be used to impact weed growth in the garden. Companion plants can also serve as a living mulch to reduce weed pressure through crowding and shading.
  3. 3. Reduced disease pressure. Perhaps surprisingly, companion planting is being studied for its ability to suppress certain plant diseases. Though this branch of companion planting does not appear to be as well studied as some others, the interplay between disease organisms and the plants they affect can indeed be influenced by certain companion planting strategies.
  4. 4. Improved soil fertility or structure. Green manures and cover crops have long been used as companions to vegetable and grain crops, though primarily in larger agricultural operations. Home gardeners, too, can reap the benefits of these soil-building strategies when used properly, even on a small scale. Soil structure can also be improved by using certain plant partnerships, including those aimed at breaking up heavy clay soils or improving the condition of the soil through the presence of root exudates (compounds produced and excreted by the roots of living plants). Other companion planting techniques assist in nitrogen transfer to improve the fertility of the soil.
  5. 5. Improved pollination. Companion plants are capable of improving garden yields by increasing the number and diversity of pollinators in the area. By carefully selecting plant partnerships that encourage and support the specific species of bees known to pollinate target crops, pollination rates may be improved.
  6. 6. Improved biological control. Another benefit of certain companion planting techniques is an increase in the population and diversity of the many beneficial insect species that dine on common garden pests or use them to house and feed their developing young. Partnering plants that attract and support pest-eating insects results in greater biological control and fewer pest outbreaks in the garden. Companion plants can create habitat for these "good" bugs, as well as provide the insects with essential nutrition in the form of pollen and nectar. Plant partners can also be chosen for their ability to serve as "banker" plants that are intentionally grown to attract and support pests so that beneficial insects can use them as a food source when pest populations are low in crop plants. This practice may help improve or equalize the seasonal population of beneficial insects by giving them a reason to stick around. There are many studies under this umbrella that reveal some wonderful ways for gardeners to encourage a healthy balance between pests and the predatory and parasitic insects that help control them.

    These parasitic wasp larvae emerging from a tobacco hornworm are a classic example of biological control in the vegetable garden.

  7. 7. Improved aesthetics. While monocultures do occasionally deserve a place in the garden (think slope-covering ground covers, for example), you'd be hard pressed to find a garden visitor who doesn't find a mixed planting more attractive than a monoculture. Unlike on farms where row planting is necessary to simplify mechanical harvesting, home gardens are the perfect places to feature a mixed-planting design. And since layered gardens with many levels of plant structures and many growth habits, from ground covers to trees and everything in between, are more inviting to a broader diversity of insects and other wildlife, the aesthetics of companion planting come with additional benefits.

Six of the seven benefits just discussed are featured in their own subsequent chapters, each approaching companion planting from a problem-solving point of view. Since garden aesthetics are both personal and opinion based, I've left that topic out of the mix. However, I do introduce one type of companion planting that undoubtedly offers the benefit of improved aesthetics, among others: chapter 4 is all about companion planting for support and structure. It highlights plant partnerships that combine one plant that serves as a living structure or trellis with another that needs something to climb.

How Close Do Companion Plants Need to Be?

An oft-asked question of many gardeners when it comes to companion planting is "How close do the plants have to be for them to be considered companion plants?" The answer is "It depends." Physical proximity as well as planting time varies quite a bit, depending on the desired results and the methodology behind each specific combination. Sometimes the companion plants are planted at the same time, while other times the plant partners are planted in succession. Sometimes the companions are in physical contact, while in other cases the two plants might be several feet apart. Plant partnerships don't necessarily have to occur simultaneously. As you are introduced to the benefits and techniques of the many plant combinations in this book, you will also learn the particulars of timing, physical proximity, and other details.

Companion Planting by Other Names

I'd bet that most of the scientists who study plant partnerships don't call the subject of their work "companion planting," probably due in no small part to the fact that the term has developed some negative connotations over the years. When applied to growing on a larger scale, terms like polyculture and intercropping are more often used. Let's take a closer look at these two terms and what they mean.

Even small vegetable gardens like this one can be a polyculture filled with a diversity of plants.

Polyculture is defined as an agriculture system in which multiple plants are growing in the same space to reflect the diversity of a natural ecosystem and create an environment where pests and diseases don't spread as easily as they do in a monoculture. Companion planting is one way to create a polyculture.

Intercropping is the practice of growing multiple crops in the same field area to promote beneficial results. On the smaller scale of the home garden, it's called interplanting. Intercropping or interplanting creates a polyculture. There are several types of intercropping/interplanting:

  • When crops are mixed such that partner plants are blended together with no distinct rows or other formal arrangement, it's called mixed intercropping/interplanting.
  • When the crops are mixed together in alternating rows, it's called row intercropping/interplanting.
  • On farms, when a second crop is planted right into an existing crop just before harvest of the first crop, it's called relay intercropping.
  • In a closely related practice, intercropping can also take the form of different crops being planted at different times of the year. This is called crop rotation.


  • “[Plant Partners] takes a new look at a popular subject that has long relied on folklore and conjecture rather than research.” — Margaret Roach, The New York Times

    Plant Partners offers more than just specific plant pairings; it encourages us to think about the relationships between plants, so that we can grow our best garden ever."
    — Jeff Gillman, PhD, Director of the UNC Charlotte Botanical Gardens and best-selling author of The Truth about GardenRemedies and Decoding Gardening Advice

    "Finally, a science-based book that addresses the 'companion planting' concept of how and why plants can - and really do - play a tangible role in helping to create healthier, more productive vegetable gardens. Plant Partners is a fun and fascinating read. It is well-researched and Jessica Walliser’s ability to pack so much useful information into this book just adds to the experience. I enthusiastically recommend this book!"
    — Joe Lamp’l, executive producer and host of Growing a Greener World

    "Plant Partners takes the age-old concept of companion planting and adds a modern, scientific rigor. You'll better understand the true reasons to companion plant and why certain combinations work on a scientific basis. I guarantee you'll learn something you never knew within the first 10 pages of the book.”
    — Kevin Espiritu, author and founder of Epic Gardening 

    “Those who know what to expect from Jessica Walliser’s previous work will not be disappointed in this important book. Here, in easy-to-understand language, is the science-based knowledge vegetable gardeners need to make informed choices when combining plants. Who knew I should be planting nasturtiums with my summer squash? This book can turn your favorite hobby into a way to make the planet a better place.” 
    — James Baggett, senior editor, Garden Gate magazine 

    “For over a decade, Jessica Walliser has been my go-to resource for information on beneficial insects, pollinators, and companion planting. Plant Partners is a fresh look at a traditional topic. It approaches the subject of companion planting through the eyes of science and is an approachable, fascinating read. As you learn the many, many benefits to pairing plants, you’ll look at your garden in a whole new way. Plant Partners is set to become a new garden classic.”
    — Niki Jabbour, author of The Year-Round Vegetable Gardener and Growing Under Cover

    "A well-supported and helpful look at the practice of plant partnering... Walliser’s lively guide will aid many a horticulturally minded reader."
    — Publisher's Weekly

On Sale
Dec 22, 2020
Page Count
216 pages

Jessica Walliser

Jessica Walliser

About the Author

Jessica Walliser is a horticulturalist and the author of several gardening books, including Plant Partners and Attracting Beneficial Bugs to Your Garden. In addition to cohosting The Organic Gardeners, an award-winning radio program in Pennsylvania, she is the co-owner of the popular gardening website and a regular contributor to Fine Gardening, Urban Farm, and Hobby Farms, and the Pittsburgh Tribune-Review. Walliser is the recipient of an American Horticultural Society Book Award. 

Jeff Gillman, PhD is director of the UNC Charlotte Botanical Gardens and best-selling author of The Truth about Garden Remedies and Decoding Gardening Advice.

Learn more about this author