Companion planting isn’t garden folklore. Well, some of it is — but there’s real science behind the partnerships that work. The trick is separating the evidence-based pairings from the ones your grandmother swore by but nobody’s actually tested.

This guide covers the pairings that have research behind them, plus the combinations to genuinely avoid.

How Companion Planting Actually Works

There are four mechanisms that make companion planting effective:

Pest confusion. Many pests find their targets by smell. Interplanting aromatic herbs and alliums with target crops disrupts the scent trail. This is well-documented with carrot flies and onions.

Trap cropping. Some plants attract pests away from your main crop. Nasturtiums pull aphids away from brassicas. Blue Hubbard squash lures squash vine borers away from zucchini.

Beneficial insect habitat. Flowers like alyssum, dill, and yarrow attract predatory insects (lacewings, parasitic wasps, hoverflies) that eat your pest insects.

Space and resource optimization. The classic “Three Sisters” (corn, beans, squash) works because corn provides a trellis for beans, beans fix nitrogen for corn, and squash shades the soil to retain moisture and suppress weeds.

The Companion Planting Chart

Tomatoes

Good companions:

  • Basil — Repels whiteflies and aphids. May improve tomato flavor (anecdotal but widely reported). Plant within 18 inches.
  • Carrots — Loosen soil around tomato roots. Plant between tomato cages.
  • Marigolds — French marigolds suppress root-knot nematodes (proven in university studies). Interplant throughout the bed.
  • Borage — Attracts pollinators, repels tomato hornworm. One plant per 4–5 tomatoes.

Bad companions:

  • Brassicas (cabbage, broccoli, cauliflower) — Both are heavy feeders; they’ll compete for nutrients.
  • Fennel — Inhibits growth of most vegetables. Keep fennel isolated.
  • Corn — Both attract the same pest: corn earworm/tomato fruitworm.

Peppers

Good companions:

  • Basil — Same benefits as with tomatoes
  • Spinach — Low-growing, shades soil, harvested before peppers need the space
  • Carrots — Same benefits as with tomatoes
  • Onions — Repel aphids

Bad companions:

  • Fennel — Growth inhibitor
  • Kohlrabi — Stunts pepper growth

Cucumbers

Good companions:

  • Radishes — Repel cucumber beetles. Plant radishes around cucumber hills.
  • Dill — Attracts beneficial insects that prey on cucumber pests. But harvest dill before it goes to seed, or it can inhibit cucumber growth.
  • Sunflowers — Provide a trellis for vining cucumbers and attract pollinators.
  • Beans — Fix nitrogen that cucumbers need.

Bad companions:

  • Potatoes — Cucumbers increase potato blight susceptibility.
  • Aromatic herbs (sage, mint) — Can inhibit cucumber growth.

Beans (Bush and Pole)

Good companions:

  • Corn — Classic pairing. Pole beans climb corn stalks. Plant corn first, add beans when corn is 6 inches tall.
  • Squash — The third “Sister.” Large leaves shade soil.
  • Carrots — Both benefit from the association.
  • Cucumbers — Beans fix nitrogen, cucumbers benefit.

Bad companions:

  • Onions/garlic/chives — Alliums stunt bean growth. This is well-documented.
  • Peppers — Compete for the same nutrients.

Brassicas (Cabbage, Broccoli, Cauliflower, Kale)

Good companions:

  • Nasturtiums — Trap crop for aphids and cabbage moths.
  • Dill — Attracts parasitic wasps that prey on cabbage worms.
  • Onions — Repel cabbage maggots.
  • Thyme — Deters cabbage worms.

Bad companions:

  • Strawberries — Both susceptible to similar fungal diseases.
  • Tomatoes — Heavy feeder competition.

Lettuce and Leafy Greens

Good companions:

  • Tall crops (corn, sunflowers, tomatoes) — Provide shade that extends the lettuce season by 2–3 weeks.
  • Chives — Repel aphids.
  • Radishes — Fast-growing markers that break up soil.

Bad companions:

  • Few genuine incompatibilities. Lettuce plays well with almost everything.

The Three Sisters: Getting It Right

The most famous companion planting combination needs specific execution to work:

  1. Build a mound 12 inches high, 18 inches across
  2. Plant 4–6 corn seeds in the center
  3. Wait until corn is 6 inches tall, then plant 4 bean seeds around the corn
  4. Plant 2–3 squash seeds at the edge of the mound

Common mistakes:

  • Planting everything at once (beans outpace corn and have nothing to climb)
  • Using bush beans instead of pole beans
  • Spacing mounds too close (need 4–5 feet between mounds)
  • Using sweet corn (too short) — use a tall dent or flour corn variety

Companion Planting Myths to Ignore

“Marigolds repel all pests.” Only French marigolds (Tagetes patula) suppress nematodes, and only when planted densely and turned into the soil at season’s end. They don’t create a pest-free force field.

“Carrots love tomatoes.” This is the title of a famous book, but the evidence is thin. They don’t harm each other, but “love” is a stretch.

“Garlic keeps everything away.” Garlic repels some soft-bodied insects but stunts beans and peas. It’s not a universal protector.

Planning Your Companion Planted Garden

The most practical approach: think in terms of guilds rather than individual pairings. A guild groups a main crop with 2–3 companions that serve different functions:

Tomato guild: Tomato + basil (pest repellent) + marigold (nematode control) + carrot (soil loosening)

Brassica guild: Cabbage + nasturtium (trap crop) + dill (beneficial insects) + onion (pest repellent)

Cucumber guild: Cucumber + radish (beetle repellent) + sunflower (trellis) + bean (nitrogen)

Plan these guilds on paper before planting. Sketch your beds and assign each guild a section.