Companion planting chart: what is proven, what is plausible, what is folklore
An honest companion planting chart: the classic pairings, what to keep apart, and a straight answer on which ones have real evidence and which are folklore.
Companion planting is part science and part inherited folklore, and most charts do not tell you which is which. The parts that reliably work are physical and ecological: shade, structure, trap crops, and flowers that pull in predatory insects. The parts that usually do not are the long “plant this next to that” pairing lists that promise better flavor or vigor with no mechanism behind them.
This chart separates the two. You can use the supported half with confidence and treat the rest as low-cost experiments.
The four mechanisms that genuinely work
Before any chart, understand why a pairing might help. If you cannot name the mechanism, be skeptical.
1. Structure and space. Plants that occupy different layers do not compete. Corn stands tall, beans climb it, squash sprawls beneath. Lettuce tucked under staked tomatoes gets afternoon shade and bolts later in the heat. This is not mysterious, it is geometry, and it is the most dependable form of companion planting there is.
2. Trap cropping. A pest prefers plant A over plant B, so you plant A at the edge and it soaks up the damage. This is a documented agricultural practice. The catch that nobody mentions: a trap crop only helps if you actually manage it. Left alone, it becomes a nursery that breeds pests into your main crop.
3. Insectary planting. Small, open flowers feed the adult stages of hoverflies, parasitic wasps and lacewings, whose larvae eat aphids and caterpillars. Dill, cilantro left to flower, fennel, alyssum and yarrow all do this. You are not repelling pests, you are hosting the things that eat them, which is a slower and far more durable strategy.
4. Soil-borne effects. Some plants change the soil in measurable ways. French marigolds (Tagetes patula) release compounds from their roots that suppress root-knot nematodes. This is real, but the conditions matter enormously and are almost always left out of the chart.
The chart
| Pairing | The usual claim | What is really going on | Verdict |
|---|---|---|---|
| Corn + pole beans + squash (Three Sisters) | They help each other grow | Different layers, different roots. Corn is a trellis, squash shades out weeds, beans fix nitrogen (mostly for later crops) | Supported |
| Lettuce or spinach under tomatoes, corn or trellised beans | Improves growth | Shade lowers leaf temperature and delays bolting in summer | Supported |
| Nasturtium near squash or brassicas | Repels aphids | Trap crop: aphids and some beetles prefer it. Only works if you monitor and remove infested plants | Supported, with work |
| Blue Hubbard squash at the edge of a cucumber or melon patch | Protects the main crop | Trap crop for cucumber beetles, a documented practice | Supported, with work |
| Dill, cilantro, fennel, alyssum allowed to flower | Attracts good bugs | Insectary planting: nectar for hoverflies and parasitic wasps whose larvae eat aphids | Supported |
| French marigolds as a dense cover crop or rotation | Kills nematodes | Root compounds suppress root-knot nematodes. Needs a dense stand over a season, not a few border plants | Supported, conditionally |
| French marigolds dotted among tomatoes | Repels pests generally | The soil nematode effect does not transfer to leaf pests. Broad repellent claims are weak | Mostly folklore |
| Basil next to tomatoes | Repels pests, improves flavor | Some evidence for volatiles affecting certain insects. The flavor claim has no mechanism. Harmless and space-efficient | Plausible / folklore |
| Borage near tomatoes | Deters hornworms | Real pollinator draw. Hornworm deterrence is not established | Pollinator draw yes, deterrence no |
| Carrots + onions | Each repels the other’s fly | Popular for decades and widely repeated. Trial results are inconsistent | Folklore |
| Chamomile near cabbage | Improves growth and flavor | No mechanism | Folklore |
| Alliums near almost anything | Repels pests broadly | Onion smell does not create a protective field. Alliums are simply easy neighbors | Folklore |
What to keep apart, and the real reasons why
Most “never plant these together” lists are as unsupported as the positive ones. There are three legitimate reasons to separate plants, and one genuine chemical case.
Competition. Anything vigorous will bully anything small and slow. Pumpkins and melons will run over a carrot row. Mint will take a bed and never give it back, which is why it belongs in a pot. Sunflowers and corn shade out anything on their north side. This is the reason behind most real spacing failures.
Shared pests and diseases. This one matters. Tomatoes, potatoes, peppers and eggplant are all nightshades and share blights and beetles, so clustering them makes an outbreak easier and rotation harder. All brassicas together is a buffet for cabbage worms. Group crops by family and move the families around the garden each year: crop rotation delivers more measurable benefit than almost any pairing in the chart above.
Fungal pressure from crowding. Two plants packed together with no airflow will trade fungal disease regardless of how compatible the chart says they are. Spacing is a disease control tool.
Juglone. The one classic “keep apart” that is chemically real. Black walnut trees release juglone from roots, hulls and leaves, and it is genuinely toxic to tomatoes, peppers, eggplant and potatoes. Do not put vegetable beds inside the root zone of a black walnut. This is allelopathy, and it is the exception that proves how rare the effect is.
Fennel is often listed as inhibiting neighbors. There is some allelopathic evidence, but it is not the settled case that juglone is, so treat that one as unproven and grow fennel at the edge if it worries you.
How to use a companion chart without fooling yourself
The reason folklore survives in gardening is that gardens are noisy. Weather, soil, variety and pest pressure all swing results far more than a neighbor plant does. Plant basil next to a tomato in a good year and you will credit the basil.
So do this instead:
- Build the plan on the supported mechanisms first. Rotation by plant family, spacing for airflow, vertical structure, shade for cool-season crops in summer, and a strip of flowering herbs left to bloom. That is a genuinely better garden.
- Treat pairing claims as experiments, not rules. They are cheap to test and nothing bad happens if they do nothing.
- Change one thing at a time, and grow a comparison row where you can.
- Write down what you saw, including the weather and the variety, because a single season proves almost nothing.
That last point is where most home experiments collapse. You will not remember in April which bed had the aphid problem or which pairing you tried last July. Keeping the bed map, the crops and the season’s observations together is what turns three years of guessing into something you can actually read a pattern out of, and it is why GardenOS pairs its bed and crop layout with a journal that carries forward season to season. Last year’s notes are what make next year’s plan worth anything.
Do this next: skip the pairing chart for a moment and sketch your beds by plant family instead: nightshades here, brassicas there, alliums over there, legumes in the fourth spot. Rotate that grouping next season, and add one strip of dill or alyssum you let flower. That single change will outperform every pairing on the list. When you are ready to time it all out, our seed starting schedule covers the counting-back math, and the raised bed garden plan covers the spacing.
Frequently asked questions
Does companion planting actually work?
Parts of it do. Mechanisms like shade from a taller crop, trap cropping, drawing in predatory insects with flowers, and using vertical space have real evidence behind them. The long pairing charts that say plant X next to Y for better flavor or vigor are mostly folklore passed between gardening books without testing.
Do marigolds repel garden pests?
French marigolds have a genuine, well documented effect on soil-dwelling root-knot nematodes through compounds released by their roots. That effect requires growing them densely as a cover crop or rotation over a season, not dotting a few around tomatoes. The broader claim that marigolds repel most above-ground pests is not well supported.
What should never be planted together?
The reliable reasons to separate plants are competition, shared pests and shared diseases, not mystical incompatibility. Do not crowd a heavy feeder next to a small slow crop, do not cluster all your tomatoes and potatoes together where blight can jump between them, and keep vegetables well away from a black walnut tree, which releases juglone that genuinely harms tomatoes and other nightshades.
What is the Three Sisters planting?
It is corn, pole beans and squash grown together. Corn provides a living trellis, beans climb it and fix nitrogen, and squash sprawls below to shade the soil and suppress weeds. It works because the three crops use different space and resources rather than competing, though the nitrogen from the beans mostly benefits later crops rather than the corn beside them.
Ecuato builds interactive dashboard planners as single offline HTML apps. Browse all planners or visit the Etsy shop.