Companion planting: complete crop-by-crop guide
What to plant next to your tomatoes, broccoli, and peppers — and what to keep apart. Real research from Cornell, Penn State, and Texas A&M, not folk wisdom. Pick a crop below for the full friend / foe matrix.
Companion planting is one of the most over-claimed and under-researched topics in home gardening. The folk wisdom (marigolds repel everything, garlic protects roses) is mostly oversold. But there are real, replicable, Extension-research-backed companion effects worth knowing about: trap crops that pull pests away, allelopathy where one plant suppresses another, pollinator attractants that boost yield in nearby beds, and intercropping that maximizes bed productivity.
This hub is the master entry point. Pick your crop below for the full friend / foe matrix specific to it.
How we built these guides
Every crop-specific companion guide on Outdoor Plant Care follows the same structure:
- Friends with evidence — pairings backed by replicated Extension or peer-reviewed studies, with the source cited inline.
- Friends with tradition — pairings that show up everywhere in folk gardening but lack hard research. We list them, label them, and let you decide.
- Foes — combinations that demonstrably hurt yield (allelopathy, disease sharing, root competition, or shared pests).
- Practical layout — bed planning notes for actually putting it together.
Pick your crop
Vegetables — fruits and roots
- Eggplant companion plants — beans for nitrogen, marigolds as trap crop
- Zucchini companion plants — corn, beans, nasturtium; trap crop strategy
- Watermelon companion plants — corn, radishes, oregano; isolate from cucumbers
- Cantaloupe companion plants
- Sweet potato companion plants — bush beans only, not pole
- Carrot companion plants — onions, leeks, rosemary; avoid dill late season
- Radish companion plants — chervil, peas, carrots; trap crop for flea beetles
- Asparagus companion plants — parsley, basil, tomatoes; permanent bed planning
Vegetables — brassicas
- Broccoli companion plants — dill, chamomile, alliums; avoid strawberries
- Cabbage companion plants — celery, onions; avoid tomatoes
- Kale companion plants — beets, celery, herbs; avoid strawberries and beans
Vegetables — legumes and alliums
- Green bean companion plants — corn, cucumbers, radishes; avoid onions and garlic
- Pea companion plants — carrots, radishes, herbs; avoid onions
- Tomatillo companion plants — note: needs two plants for cross-pollination
Vegetables — greens
- Lettuce companion plants — radishes, carrots; benefits from afternoon shade from taller crops
The Three Sisters
The classic Three Sisters intercropping system — corn, beans, squash — is the best-documented companion planting strategy in North America. Per the Cornell University Three Sisters Garden, Indigenous nations across the continent independently developed variations of this system over thousands of years.
How it works:
- Corn provides vertical support for the climbing beans
- Beans fix nitrogen into the soil via root nodules, feeding the corn and squash
- Squash leaves shade the ground, suppressing weeds and conserving moisture
- All three crops are nutritionally complementary when stored together — carbs, protein, vitamins
If you have space for only one companion experiment, do this one.
What the research actually shows
A 2020 review from Penn State Extension lists the companion effects with the strongest evidence:
- Trap cropping — planting a sacrificial preferred host to draw pests away (nasturtiums for aphids, radishes for flea beetles, blue Hubbard squash for cucumber beetles)
- Habitat for beneficials — flowering insectary plants (sweet alyssum, calendula, dill, fennel, cilantro left to flower) that host predatory wasps and lacewings
- Allelopathy — chemical inhibition (black walnut juglone, onion family suppression of beans)
- Physical sheltering — taller crops providing afternoon shade for heat-stressed lettuce or spinach
- Vertical layering — corn-bean-squash, or tomato-basil-lettuce
What the research does NOT support:
- Most "X repels Y" pest claims (marigolds against rabbits, garlic against everything)
- Most flavor-improvement claims (basil makes tomatoes taste better)
- Single-companion claims based on a single mid-20th-century book
When you see a companion claim, ask: is there an Extension publication or peer-reviewed study supporting it? If not, treat it as folklore — fine to try, no reason to expect it to work.