The Problem With Companion Plant Lists

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Most companion planting advice found in “friend or foe” lists is based on guesswork, not science. Instead of trying to accommodate these made-up lists, learn what to prioritize instead.

If you’re new to gardening, use these tips to better understand your growing conditions and make smart planting decisions.

Examples of common companion planting recommendations including tomato and basil.

What Is Companion Planting?

Companion planting is one of those ideas that sounds great in theory: place certain plants together and they’ll boost each other’s success—fewer pests, better flavor, more vigorous plants. But when you dig into the evidence behind all those “plant this with that” or “don’t plant this near that” lists, things start to fall apart.

At its core, companion planting just means putting two or more kinds of plants together in the hope that one will benefit the other—or avoiding combinations where one might negatively affect the other.

That advantage might come from shading, wind protection, attracting pollinators, repelling pests, or acting as a physical support.

None of that is controversial—and many gardeners already do it, whether they call it companion planting or not. Putting a shade-loving plant next to a taller one, for example, is just common sense.

The Trouble with Traditional Pairings

Things get murky when we start talking about specific plant pairings that supposedly help or hinder each other in mysterious ways—like planting basil with tomatoes or avoiding onions near beans.

Books and websites are packed with these companion planting lists, but most lack any scientific backing. They’re often just collections of plant names, with no details on how many to plant, how far apart, or when to sow them for any supposed effect. And when someone says, “it worked for me,” we don’t really know what “it” was—what they did, what conditions were involved, or what outcome they’re referring to.

The popular book Carrots Love Tomatoes helped popularize many of these ideas, but even it doesn’t offer research or solid explanations—just long lists and anecdotal claims. (And fun fact: the book never actually says carrots love tomatoes. It just calls them “compatible.”)

Some of these cautions, like the idea of allelopathy—where one plant releases chemicals that can suppress the growth of another—sound dramatic.

And yes, there are known examples of allelopathic behavior (the juglone in walnut trees is commonly cited), but they tend to be very specific and often exaggerated or misrepresented on popular companion planting lists. In reality, if your plants have adequate space, the right soil conditions, light, and moisture, it’s unlikely to be a problem. Solid growing practices are always your best bet.

Even trusted sources like university extensions sometimes fall into the same trap. One companion planting sheet from Cornell University¹ includes several pages of suggested pairings—right after admitting there’s no research-proven, reproducible recommendations. In other words, they’re sharing a list they don’t even endorse.

What the Research Really Says

Much of the research related to plant interactions comes from agricultural or controlled laboratory settings—not home gardens. These environments allow for specific variables to be measured, but the results don’t always translate directly to backyard plots with varying soil, sunlight, and watering conditions.

The journal Ecological Entomology published a study² looking at growing strawberries next to borage—a plant known to attract pollinators. The strawberries did produce more fruit and had better quality, but the number of pollinators didn’t actually increase. The researchers guessed it might be due to changes in the mix of pollinators or other unknown factors. So while the results were interesting, they weren’t conclusive—and they were based on container plants, not open garden beds.

Other studies have shown that plants can influence each other chemically or physically—changing root growth based on who’s nearby or sending out compounds into the soil. These findings support the idea that plant interactions are real and complex. But turning that into simple, reliable advice for backyard gardeners? We’re not there yet.

Should You Try Companion Planting?

So, where does that leave us? If you enjoy experimenting, there’s no harm in trying some companion planting combinations. Just keep in mind that these ideas are mostly untested, and should never replace the gardening fundamentals we know work—things like proper soil conditions, adequate light, consistent watering, and giving plants enough space to grow.

If something seems to work well in your garden, that’s great—but it’s still anecdotal, not scientific proof. Feel free to go with what works for you, just be cautious about presenting it as a tried-and-true method of companion planting.