Companion Planting: What Actually Works (And What's Garden Folklore)
Some companion planting pairings have a real, explainable mechanism behind them — pest confusion, nitrogen fixing, physical support — while others are folklore repeated from gardener to gardener with little evidence either way.
Companion planting has a slightly awkward reputation — half of it is solid, explainable horticulture, and half of it is folklore that's been copied from gardening book to gardening book for a century with surprisingly little testing behind it. Worth pulling apart, because the good half is genuinely useful.
The three mechanisms that actually hold up
Pest confusion. Strongly scented herbs like basil planted among tomatoes can make it harder for pest insects to locate their target plant by smell — this is the best-supported mechanism in companion planting, even if the effect size varies by garden and pest pressure.
Nitrogen fixing. This one isn't folklore at all — it's basic plant biology. Legumes (beans, peas) host bacteria in their root nodules that convert atmospheric nitrogen into a form other plants can use. Planting a nitrogen-hungry crop like corn near beans, and leaving the bean roots in the ground after harvest, genuinely feeds the following crop.
Physical structure. The classic "Three Sisters" planting (corn, beans, squash) is a real, mechanically sound system: corn gives beans something to climb, beans fix nitrogen for the corn, and squash's broad leaves shade the soil and suppress weeds for both. Three unrelated benefits stacked in one bed.
Where the evidence gets thinner
Marigolds deterring nematodes has some genuine research behind specific marigold species and specific nematode types — but it's often overstated into "marigolds fix all pest problems," which isn't supported. Many of the more specific pairing claims (particular flower next to particular vegetable for particular reasons) trace back to a handful of mid-20th-century gardening books rather than controlled studies, and have simply been repeated since. That doesn't automatically make them wrong — a lot of traditional gardening knowledge holds up under scrutiny — but it does mean the confidence some sources present them with often outpaces the actual evidence.
What we'd suggest doing with this
Lean on the two well-established mechanisms first — legumes near heavy feeders, and scented herbs near susceptible crops — since you're on genuinely solid ground there. Beyond that, companion planting is generally low-risk to experiment with (worst case, no measurable benefit, rarely actual harm) and garden folklore accumulated over generations of paying attention isn't nothing either. We built a Companion Planting Checker tool that pulls the specific pairings noted in each of our 118 grow guides, if you want a quick reference rather than trying to remember which pairing does what.