YOU’VE HEARD the expression “companion planting,” as in: What plants supposedly “love” growing alongside what other plants? But how many such pairings are folklore, and how many stand up to research?

In her new book, “Plant Partners,” Jessica Walliser looks at the scientific evidence and shares pairings that can help us minimize weeds or improve soil or attract needed pollinators or other beneficial insects.

Jessica is a horticulturist and self-described devoted bug lover who gardens near Pittsburgh. She’s the author of the earlier books “Attracting Beneficial Bugs to your Garden,” and “Good Bug, Bad Bug” (affiliate links), and co-founder of the popular website savvygardening.com.

She shared her botanical tool bag for outsmarting flea beetle and squash pests and more, all by making your vegetable garden deliberately more diverse.

Plus: Enter to win her new book by commenting in the box at the bottom of the page.

Read along as you listen to the March 8, 2021 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).

And for those listening to the audio version: Sorry for a little quiet background classical music occasionally occasionally in the last third of the recording. Gremlins (or a crossed “wire” somewhere in deep space)!

science-based companion plants, with jessica walliser

Margaret Roach: Hi, Jessica. How is it in Pittsburgh?

Jessica Walliser: Well, surprisingly, it’s actually sunny today. We have a little snow left on the ground, but I think within the next few days it’s going to melt.

Margaret: O.K., spring is coming [laughter].

Jessica: Hear, hear.

Margaret: Congratulations on the book. It’s science-based companion planting, and it seems like the logical subject for you, after your previous books, both with “bug” in the titles. So I take it you’re a bug and science nerd?

Jessica: Total science nerd. And I would say my original geeking-out was over plants, of course, having a degree in horticulture. My career initially was focused on plants, but what really ended up happening was, I think I missed my calling.

And I think if I had known way back, long ago, that entomology was a career choice, I might have opted for that one. Because I truly find bugs now to just be so fascinating. They’re so dynamic, their actions in the world, and obviously in our gardens, are just … It’s such an intimate world, and I think that we as gardeners often do the insect world disservice because we only think of them as pests or pollinators, when, in fact, they have so many roles to play in the landscape. So they excite me. I love learning about them and I love reading research and that’s where all of this comes together to make this book.

Margaret: And I completely, wholeheartedly agree. And they’re my fascination, and probably the last decade or so of my gardening career shifted from the plant focus to the bug focus [laughter]. You sound this diversity message at the start of this new book, and then throughout the book, and at the end you remind us about it. And you say it’s an ecosystems-based approach to the landscape, so tell us a little bit about that.

Jessica: Yeah. It’s interesting, because when I first started to do the research on companion planting, we always think of companion planting as being plant A with plant B—it’s two plants put together. When the reality of it is, the benefits that we’re going to see from partnering plants together aren’t necessarily always from plant A with plant B. It’s more about increasing the overall diversity of the garden.

And people speak to that a lot in their perennial bed planting, maybe in a naturalistic garden, but it also is a really big key to successfully growing a vegetable garden. That you’re not just having monocultures of a big long row of tomatoes, or a big long row of peppers.

But instead you’re mimicking more of a wild ecosystem, in that you’re mixing a diversity of plants together. And when you do that, you end up with a whole host of benefits. So building that ecosystem, that biodiversity in the garden, leads to a more stable environment, that’s more resistant to pest outbreaks. You have reduced weed pressure, reduced disease pressure, and as vegetable gardeners, those are all the things that we’re looking for.

Margaret: Yes, indeed. So obviously this book is not the folklore version of companion planting, as you say: modern companion planting isn’t about what plant “loves” growing next to what other plant, it’s drawing conclusions based on research and on science. And is a lot of that research, then, being done in supportive agriculture, in organic agriculture, especially? Is that where it’s coming from?

Jessica: It is, yeah. And when I really started to dive into it, because I knew there had to be research out there. I was really starting to dive into it, and what I discovered was that, yeah, it’s out there, but they don’t call it companion planting. And I think part of that is because of the bad reputation the term has. It’s got a reputation of being based in folklore and conjecture, and so the scientists don’t want to use that term.

So instead they’ll call it things like inter-planting and inter-cropping. Or in some cases it’s creating a polyculture or improving the diversity of the garden. But all of those things are … they’re all companion planting, in a way, because they’re intentionally putting certain plants together to get, or achieve, a particular benefit.

So when you look at the research, that’s what it’s talking about. And there are many, many studies out there that speak to all of this, and it was exciting to see. Now you’re right, they are done at agricultural research facilities and farms and things like that. They’re not—most of them—are not done in a home environment. So we do have to do a certain amount of assumption that, O.K., how is this going to translate to a home environment? But right now, this is the best science that we have available to us to use.

Margaret: O.K. So let’s run through the range of support services, so to speak, that well-chosen plants can provide for each other. And you outline these early on in the book and you say, “Plant A might provide food for Plant B,” and so on and so forth. So help us with the top level of the types of services that plants can provide.

Jessica: Sure. So I basically have seven main goals of partnering plants together. So there’s seven benefits that we can get out of this. And the first one would be soil preparation and conditioning, and that things like choosing cover crops or plants that fix nitrogen and share it with other plants through the mycorrhizal network underground. And they can do that in a living state, as well, while the two plants are living in the same space together.

So we’ve got soil preparation and conditioning, we’ve got weed management, which is using certain plants, whether as a living mulch or, again, as a cover crop, to really help manage and control weeds.

We’ve got pest management, which is the one that most people are familiar with, with plant partnerships, and there’s all different levels of that. You’ve got disease management, enhancing biological control, improving pollination, and then one that’s a little more related to aesthetics than all the others, which is support—which is using one plant as a living trellis for another plant to climb.

Margaret: Oh, so mechanical [laughter].

Jessica: Yeah, exactly. And I kind of break my own rule in that chapter, because I promised at the beginning of the book that every partnership that I discuss in the book is going to be backed by some research. There’s a big bibliography in the back of the book that shows you all those studies, but that is the one chapter where I don’t have the research to back up, because it is an aesthetic. It’s really, how does it look and how does it save space in the garden? There’s not a lot of research on that one chapter.

Margaret: Well, we’ll let you off the hook for that, Jessica.

Jessica: Thank you [laughter].

Margaret: We’ll give you a pass. So with soil building and weed control, let’s talk about some examples. And so, again, we understand that the studies are being done in agricultural settings and so forth. But you’re a home gardener, and so as you extrapolate and tell us … I mean, you tell us what the big studies say, but also how you’re using them at home.

Because I saw, for instance, a picture, and I think it might’ve been at your garden, of your potatoes interplanted with bush beans, I think. And other such pairings. So tell us a little bit about some of these for soil building and weed control.

Jessica: Sure, yeah, so there’s a couple of different techniques that we can use. One is something called bio drilling, which is basically where you are using a plant to open up channels within the soil, which really allows the movement of nutrients and water and air down through the soil profile.

I am a no-till gardener, so I don’t ever turn over my vegetable garden at all, because I do talk in the beginning of the book about how important that underground network of organisms is, and how, when we till and disturb the soil, we break that up and we’re, again, doing ourselves a disservice as vegetable gardeners to do that.

So bio-drilling, using crops with big, long, thick taproots, like forage radishes, they open up those tunnels in the soil and really allow us to break up heavy clay soils. They changed the way the soil aggregates, in a very positive way, for the plants that get planted in that area, then, the following growing season. So that’s one way.

The other way that we can do it is by using cover crops. So we’re planting the cover crops in that garden area when the soil is otherwise empty, when that space is not being used. So it might be things like winter rye or oats or crimson clover.

I know a lot of home gardeners shy away from the use of cover crops because maybe they had a bad experience of the cover crop reseeding and taking over the garden, or being a big challenge. But it’s really because they haven’t done it right. I mean, I hate to say that, but cover cropping is an art, and you have to follow rules when you cover crop or otherwise you do end up getting in trouble. So in the book, I detail exactly how to do it the right way. And I always suggest that people start with oats, because oats are winter-killed very easily, so we can grow them as a nice late-season cover crop, let them die in place over the winter, and then we actually plant down through that detritus. And then that makes a nice mulch to help control weeds as well. [Above, winter rye cover crop.]

And then the third way that we can use it for soil preparation is a nitrogen transfer in a living state, which I touched on in the last section that we discussed, where your planting a leguminous plant next to another plant, and that leguminous plant will fix the nitrogen. And then actually, through the mycorrhizal network, and through roots, and the nodules naturally dying off, will help feed the plants that are living nearby.

Margaret: You’ve mentioned that you’re a no-till vegetable gardener, and then we just were mentioning legumes and their ability to fix the nitrogen and so forth, those nodules on their roots. And I’ve always been surprised, whether people are till or no-till, that they plant peas and beans, and then they pull them out at the end of the season. [Laughter.] And so it’s like, well, wait a minute. Well, isn’t all that good stuff still on the roots? You know what I mean, to leave it underground?

Jessica: It is. Yeah, I mean, some of it does get shared while the plants are in a living state and it’s not a huge amount. I think there was one study that I looked at that it talked about … that it was a 70 percent increase in nitrogen being transferred when there was a leguminous plant interplanted with the regular crop. So I thought that was pretty cool.

If you’re growing a leguminous crop, you’re better off just cutting that plant off at its base at the end of the season, and leaving the roots and the nodules intact within the soil, where they can naturally decompose, break down, and release that nitrogen then into the soil and let the microbes process it.

Margaret: Right. One old-school supposition, folklore-y thing, this misunderstanding … Many quote “companion plants” were things that had a strong smell and they were perceived or it was assumed that they were repelling the unwanted insects, for instance. But there was this fascinating “aha” in the book about. That that’s not really what they’re doing after all, is it?

Jessica: Well, the science hasn’t come out and said absolutely, but it’s sure looking like the plants … Actually, when we plant a companion plant, it’s not the smell that’s repelling the pests. What that odor is doing, that volatile chemical is doing, is actually masking the host plants. So it’s making it harder for that pest to home in on its host plant.

So pest insects, they find their hosts with a couple of different cues. One can be a visual cue, where they actually see it and they recognize it as their host plant. And then another big way is through the volatile chemicals that plants produce, or odors. Our noses are not sensitive enough to find them, but certainly an insect’s antennae are, and so they will home in on their host plant based on its odor. Well, if we plant certain companion plants that mask that odor, the pest is much less likely to be able to find their host. [Above, a hoverfly or syrphid on fennel flowers.]

Margaret: Huh, interesting. So masking versus repelling may be one of the things that’s going on there. Interesting.

Jessica: Exactly. Yeah, yep. And also, just the fact that you’re creating a diverse environment. I mean, I don’t know if you remember reading in the book, there’s a sidebar page that’s a different color, that talks about the appropriate and inappropriate landing theory. I don’t know if you remember that. But it was like the bugs, with the receptors on their feet, they kind of quote/unquote “taste” the plant when they land on it, and especially ones that lay eggs on plants. And so they’re looking for a particular taste through their feet in order to lay the egg.

So this theory speaks to the fact that if we have a mixed garden environment, when a pest lands on a plant to taste it, if it lands maybe three or four times on the same host plant, it’s triggered—the egg-laying behavior is triggered, the cue goes off and they’re like, okay, this is the right place for me to be. And that’s when they lay their eggs. They don’t just land once and lay an egg.

If you have a mixed garden environment, where there are all these plants growing together, there’s a greater chance that that pest is going to land on the cabbage leaf one time. And then the next time they land on a dill leaf, and then the next time they’ll land on a basil leaf, and then the next time it’ll be sweet alyssum. And so the cue isn’t triggered for that egg-laying behavior. So there’s an interesting bunch of research going on in that as well.

Margaret: So another thing I loved, and this is something that friends who are farmers have talked to me about, is trap cropping. So I wanted to talk a little bit about that, about pest control. The idea of luring the pests elsewhere, away from the main crop you want to protect, and other tactics for dealing with, uh-oh, flea beetles [laughter] or squash bugs. So let’s talk about some of the targeted … Well, first of all, what is trap cropping? Or I guess you also mentioned there’s such a thing as a banker plant. Tell us a little bit about that tactic.

Jessica: Trap cropping is actually one of my favorite ways to help manage pests in my garden. And basically, you’re using a sacrificial crop. I’m planting something with the purpose that I want the bugs to go to it. I want them to go over there, and leave the rest of my plants alone. So for example, flea beetles are really problematic in my garden. They drive me bonkers. And they don’t really affect plants as much when the plants are older, but they can really do quite a bit of damage on young seedlings, especially my tomatoes and my eggplants. And they can really set them back in their growth and development, and they produce yields later and they’re just not as healthy. So I always interplant my tomato plants with radish, because flea beetles much prefer radish to the tomato plants.

So when I do that, the radish become the sacrifice. And the cool thing about radishes, they’re so darn tough, that they still produce edible roots, even when they have a significant amount of flea beetle damage. But they leave my young tomato transplants alone when I do that. So there’s lots of examples of trap crops. And another favorite of mine that I always do is, I always plant, if I have the space, I will plant a ‘Blue Hubbard’ squash in my vegetable garden. Because the squash bugs, and squash vine bores, much prefer ‘Blue Hubbard’ squash to any of the other types of squashes that I plant, so they go over there and leave my more desired squash plants alone.

Margaret: It’s funny. And I love, you said “sacrifice crop,” but I love that you outsmarted them with the radishes, because you got radishes, anyway [laughter].

Jessica: Exactly, yeah. So another one you can do for flea beetles, another trap crop, is bok choi. So those little miniature bok choi varieties, they’re really good for it, too, for luring away the flea beetles. But they get pretty holey. And one of my favorite things with bok choi is that it looks so pretty on a plate, but if it’s all torn to shreds from the flea beetles, that one doesn’t look so hot. So that’s one I don’t harvest afterwards.

Margaret: Yeah. Nasturtiums, and you mentioned sweet alyssum before, but nasturtiums show up in the book. Tell us about what they do for you? They seem to be in your tool bag.

Jessica: Yeah. So there’s one particular plant partnership that works wonderfully for nasturtiums, and I know that it’s a pest that many gardeners face. So always interplant your zucchini plants, or summer squash, with nasturtiums. Because the nasturtiums help deter and limit the number of squash bugs that you find on the plants. And that was a really cool study that was out of Iowa State University, where they looked at this and they found that it was a great reduction in the number of squash bugs, and certainly the damage from squash bugs as well.

And nasturtiums are lovely and they’ll help enhance pollination as well—not necessarily on your squash plants, but on other garden plants, so it’s a pretty cool combination.

Margaret: Any other ones? I mean, these are such great examples, and I’m writing myself a note while you’re speaking [laughter]. It’s like, I’ve got to order nasturtium seeds, oh my goodness; I’ve got to get some radishes. So other ones that are, again, in that tool bag of yours? That you like to have to diversify and add ecosystem services to your vegetable garden?

Jessica: Yeah. So I mean, there’s a kajillion of them, but I would say one that I always talk about, because it’s one that also fits in the old-school companion planting, based on what loves what. People always talk about, oh, we have to put tomatoes and basil together. You have to plant these two plants together because they’re great on the plate together, but they’re also great in the garden and the basil will help deter pests and blah blah.

And a lot of that is not backed by science, but there is some that is backed by science, and it’s particularly with the pest, thrips. So if you have a big thrips problem with your tomato plants, and also with your pepper plants—and we know thrips can do a real number, especially on peppers—you do want to interplant them with basil plants, because there is some great research showing that growing those two in tandem with each other will help deter the thrips. So that’s a really good one. And that’s sort of a classic combination that has been proven to actually work.

Margaret: What about the sweet alyssum? Now the funny thing about that is, you wouldn’t think it, but it’s in the cabbage family, isn’t it?

Jessica: It is, yeah. But what’s great about sweet alyssum… And the big lettuce farms out on the West Coast, out in California, if you’ve ever driven through them, you actually see that they have strips, some of them, the organic ones, will have strips of sweet alyssum alternated with their strips of lettuce growing in their fields. And that’s done, in particular, because sweet alyssum enhances the biological control of aphids, which are one of the main pests that feed on the lettuce crop. Especially when we have them planted in big, long rows like that. [Above, sweet alyssum with lettuce.]

The sweet alyssum is attractive to your syrphid flies and to some of the species of parasitic wasps that use aphids to house and feed their developing young. So that’s an excellent plant partnership that’s really easy to employ in a home garden.

Margaret: What are some of the tools that you would suggest for us as gardeners? Like I love BugGuide.net, for instance, for learning to ID bugs, insects. What are some of the other tools that you think all gardeners should be using and know about? Like field guides, besides your books, which are great. I mean, any advice there? Because I think you just said thrips and then you said, I forget what other kinds of insects you just said; syrphids, maybe. You know what I mean? People might be like, “Oh, what’s that? I don’t know what that is. I wouldn’t recognize that.”

Jessica: Yeah. So those sites like What’s That Bug are really good. Obviously your local extension office, that’s a really good one, where you can take a picture or send a picture. So we do oftentimes get some good ID from that as well. I also find, I follow a lot of people on Facebook, there’s Confessions of a Bug Addict, the Caterpillar Lab, I’m always out there looking for folks to follow who will teach me something. Something interesting.

And certainly with the internet being what it is, there’s plenty of opportunity for you to just use your search engine and type in “black and red beetle on kale plants” and come up with some photos, and look at those photos, and what does yours look like? And, “Oh, O.K., well maybe that’s a harlequin bug, so now I know what it is and then what I can do about it from there.” So the internet, for all its curses, also has a lot of blessings as well.

Margaret: So as we wind down here, do planting partnerships have any downsides? Can you have too much going on and there’s … Do you know what I mean—can you overdo it and have too much competition or whatever? Do we have to keep that in mind?

Jessica: You do have to keep that in mind. You absolutely have to keep that in mind. And that is one of the biggest downsides of it, is that there is potential for competition. But when I think about it, I always think about in terms of natural plant communities. Plants are not evenly spaced in a natural plant community. Things are messy, they’re growing together, they’re there intertwining with each other. And eventually, obviously if it’s a perennial planting, you’ll end up with dominant species and ones that are a little more recessive. That’s not going to happen so much in an annual vegetable garden, but don’t be afraid to put things closely together, but do it smartly as well. Keep in mind, O.K., I know this is going to get this big, so I should give it plenty of room for that to happen.

If you’re using cover crops, don’t sow them too thickly, it’s not a case of more is better. You want to make sure that you sow them at the appropriate rate, and you do it at the right time. So there’s lots of advice in “Plant Partners” for doing all of that stuff properly, but there’s also a section that does discuss the potential for competition, and that some of these things, even though you’re trying to create this diverse ecosystem, that some of them could yield a possible negative result. But I mean, that happens in anything we do in the vegetable garden. There’s always going to be some bads that happened with the goods.

Margaret: Right. Well, the new book is “Plant Partners,” and thank you, Jessica Walliser, for being here and opening our eyes to this scientifically based version of an old idea. I’m so glad to speak to you.

Jessica: It was my pleasure, Margaret, thank you so much for the invitation.

enter to win a copy of ‘plant partners’

I’LL BUY A COPY of “Plant Partners” by Jessica Walliser for one lucky reader. All you have to do to enter is answer this question in the comments box at the very bottom of the page:

What plants do you use in your vegetable garden as helpers, or plant partners?

No answer, or feeling shy? Just say something like “count me in” and I will, but a reply is even better. I’ll pick a random winner after entries close at midnight Tuesday, March 16, 2021. Good luck to all.

(Disclosure: As an Amazon Associate I earn from qualifying purchases.)

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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the March 8, 2021 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).