The companion planting charts circulating on Pinterest mostly are not supported by research. A handful of plant combinations have decent evidence behind them. Most of the rest is folk wisdom — sometimes useful, sometimes wrong, often impossible to test because dozens of garden variables move at once. Here’s what the actual studies show.
What companion planting actually means
Three different mechanisms get bundled under the same term:
- Pest deterrence — one plant’s smell or chemistry repels insects that would attack another
- Beneficial insect attraction — flowers that bring in predatory insects or pollinators
- Resource sharing — plants with different root depths, nutrient needs, or light tolerances coexisting efficiently
The strongest evidence supports the second and third mechanisms. The first — the classic “plant marigolds to keep aphids away” claim — is mostly weak in published studies.
Combinations with research behind them
The Three Sisters: corn, beans, squash
The most studied companion planting system in North America, used by Indigenous farmers for thousands of years. Corn provides structure for beans to climb. Beans fix nitrogen in the soil. Squash leaves shade out weeds and slow soil moisture loss. Each crop yields less per plant than in monoculture, but total food per square foot exceeds the equivalent monocultures.
Tomatoes and basil
The pairing has cooks’ approval. Research evidence: limited but real. Some studies show basil reduces tomato hornworm damage and may improve tomato yield through unknown mechanisms. The effect size is small but the combination costs you nothing.
Carrots and onions
Onion volatile compounds reduce carrot fly damage in some trials. Carrot foliage may reduce onion fly damage in turn. Effect sizes vary, but the literature is consistent enough that it’s worth trying.
Marigolds and root-knot nematodes
This one has solid evidence — but with a catch. Marigolds release thiophenes that suppress root-knot nematodes, but you need to plant marigolds densely across the whole area for a full season as a cover crop. Sprinkling a few marigolds among your tomatoes does almost nothing for nematodes.
Flowering plants near vegetables
Across multiple studies, beds with flowering plants nearby (alyssum, calendula, dill, fennel, yarrow) attract more parasitoid wasps and predatory hoverflies — which then reduce caterpillar and aphid populations. This is the most reliable companion-planting benefit you can get.
Combinations without evidence
- Marigolds as general pest repellent in vegetable beds (other than the nematode case above)
- Most “avoid planting X with Y” claims — almost none have research support
- Garlic as a universal repellent — pleasant idea, weak evidence
- Nasturtiums as trap crops — they do attract aphids, but whether this protects nearby plants is unclear
Resource sharing — where layout matters most
The most practical benefit of mixed planting has nothing to do with pest deterrence. Combining shallow-rooted leafy greens with deeper-rooted root crops uses garden space more efficiently. Tall plants shading short cool-season ones extends harvest into hot weather. Quick-maturing radishes between slow-growing carrots double the yield per square foot.
A practical companion bed
Here is a 4×8 bed layout we use that combines what works:
- 4 indeterminate tomato plants down the center, staked vertically
- Basil interplanted at the tomato bases
- Lettuce and arugula along the cooler edges in spring (replaced with bush beans in summer)
- Alyssum or calendula at the corners as a beneficial-insect magnet
- A border of marigolds — mostly for visual appeal and slight insect benefit
What to skip from the charts
Any claim that two plants must never be planted together. Real evidence for plant-on-plant antagonism in a home garden is very thin. Beans and onions are reputed to be enemies; we plant them next to each other every year without trouble.
Bottom line
Companion planting works best when it improves space efficiency and attracts beneficial insects. The pest-deterrent claims that fill garden charts are mostly folk knowledge. Plant flowering herbs around your vegetable garden, mix root depths and growth speeds, and ignore the lists that tell you not to put tomatoes near corn.
