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The short answer: the reliable companion pairings are basil and marigolds with tomatoes, alliums with carrots and brassicas, dill and cilantro flowers near anything troubled by aphids, nasturtium as a trap crop, and corn with beans and squash. Keep apart: tomatoes and potatoes, fennel and almost everything, corn and tomatoes, and any two members of the same plant family in the same bed. The full chart is below, along with an honest note about which parts of the tradition hold up.
The Companion Planting Chart
| Crop | Good companions | Keep apart from | Reason |
|---|---|---|---|
| Tomato | Basil, French marigold, borage, lettuce, carrot, parsley | Potato, fennel, corn, brassicas | Shared blight with potato; shared earworm with corn |
| Pepper, eggplant | Basil, onion, carrot, marigold, spinach | Fennel, potato, brassicas | Same family as potato; fennel inhibits growth |
| Potato | Bush beans, horseradish, marigold, cilantro | Tomato, eggplant, pepper, squash, fennel | Blight shared across the nightshade family |
| Corn | Pole beans, squash, cucumber, sunflower | Tomato | Corn earworm and tomato fruitworm are one insect |
| Pole & bush beans | Corn, squash, cucumber, carrot, radish, potato | Onion, garlic, leek, fennel | Alliums traditionally held to check legume growth |
| Peas | Carrot, radish, turnip, cucumber, spinach | Onion, garlic, leek | As beans; also keeps trellis space free |
| Squash, pumpkin | Corn, beans, nasturtium, radish, borage | Potato | Heavy competition and shared pest pressure |
| Cucumber | Radish, nasturtium, dill, sunflower, beans | Potato, sage, strong aromatic herbs | Radish is a proven cucumber beetle trap crop |
| Carrot | Onion, leek, chives, rosemary, lettuce, radish | Dill, parsnip, fennel | Allium foliage masks carrot scent from carrot rust fly |
| Onion, garlic, leek | Carrot, beet, lettuce, brassicas, strawberry | Beans, peas, asparagus | Deters several leaf-feeding pests in brassicas |
| Broccoli, cabbage, kale | Dill, onion, nasturtium, thyme, rosemary, beets | Tomato, strawberry, pole beans | Dill flowers feed parasitic wasps that kill cabbageworm |
| Lettuce | Carrot, radish, onion, strawberry, tall crops for shade | Brassicas (competition only) | Afternoon shade from taller crops delays bolting |
| Spinach, chard | Strawberry, brassicas, onion, peas | Nothing significant | Undemanding neighbors |
| Beet | Onion, brassicas, lettuce, bush beans | Pole beans | Pole beans shade beets out entirely |
| Radish | Cucumber, squash, lettuce, peas, carrot | Hyssop | Fast trap crop for flea beetles and cucumber beetles |
| Strawberry | Borage, lettuce, spinach, thyme | Brassicas | Brassicas outcompete a shallow-rooted perennial |
| Basil | Tomato, pepper, oregano, asparagus | Rue | Flowers attract pollinators; grows happily in the same conditions |
| Fennel | Grow alone, away from the vegetable beds | Almost everything | Allelopathic — inhibits germination and growth nearby |
The Four Mechanisms That Genuinely Work
Companion planting is not one idea; it is four different ideas wearing the same name, and they are not equally well supported. These are the ones with real research behind them.
1. Trap cropping
Plant something a pest prefers, next to the crop you care about, and the pest concentrates there. Nasturtium pulls aphids and cabbage white butterflies away from brassicas. Radish, sown early and left to grow, absorbs flea beetle and cucumber beetle pressure that would otherwise land on cucumbers and squash. Blue hubbard squash is used commercially as a perimeter trap crop for cucumber beetles in cucurbit fields. The mechanism is straightforward and it works — with one condition. A trap crop concentrates pests, it does not remove them, so you have to actually deal with the trap plants: pull and dispose of heavily infested nasturtiums rather than leaving a nursery next to your cabbages.
2. Insectary planting
The most reliably useful mechanism in the whole tradition, and the one gardeners most often overlook. Many of the insects that control garden pests — parasitic wasps, hoverflies, lacewings, tachinid flies — are predatory only as larvae, and as adults they live on nectar. They need small, shallow, open flowers to reach it, which is exactly what the umbel-shaped flower heads of dill, cilantro, fennel, parsley and carrot provide, along with sweet alyssum, yarrow and buckwheat.
Letting a few cilantro and dill plants bolt rather than pulling them is one of the highest-return things you can do in a vegetable garden. The tiny wasps that parasitize cabbageworm and aphids are supported by exactly those flowers, and the effect on pest numbers is measurable.
3. Physical and spatial partnerships
Corn provides a living trellis for pole beans. Squash leaves are living mulch that shades soil, holds moisture and blocks weeds. Lettuce grown on the east side of a tomato row gets afternoon shade and bolts weeks later than lettuce in the open. Radishes germinate in four days between slow carrot rows and mark the row, breaking the crust for the carrots and being harvested before the carrots need the room.
None of this involves chemistry between plants. It is spacing, shading and timing — and it is where a chart earns most of its practical value.
4. Odor masking and host-finding disruption
Many pests find host plants by scent. Interplanting a strongly aromatic crop can interfere with that search. The classic example is onions with carrots against the carrot rust fly, and results in trials are real but partial — pressure is reduced, not eliminated, and the effect fades once the alliums are harvested. Undersowing brassicas with clover has shown consistent reductions in cabbage root fly, apparently because the green background makes it harder for the fly to identify the host plant.
This is worth doing. It is not worth relying on as your only defense against a serious pest — a floating row cover over brassicas is far more effective than any planting arrangement.
What Is Folklore
Much of the modern companion planting chart traces to a small number of twentieth-century books that recorded traditional practice without testing it. That does not make all of it wrong, but it does mean several confidently repeated claims have never been demonstrated.
Basil improves tomato flavor. No evidence. Basil is still a good tomato neighbor — same water and heat preferences, useful flowers, harvested together — but the flavor claim is decoration.
Marigolds repel garden pests generally. Overstated. The genuine effect is below ground, against root-knot nematodes, and only from French marigolds grown densely for a season and incorporated into the soil. A few plants at the bed corners will not do it. Above ground they are a fine nectar source, which is reason enough to plant them.
Onions harm beans and peas. This is the most repeated antagonism in every chart, including the one above, and it rests on tradition rather than trials. Included here because the cost of separating them is nothing, but do not rearrange a garden over it.
Plant X hates plant Y. Genuine allelopathy — one plant chemically suppressing another — is documented for black walnut, for sunflower to a degree, and for fennel. The rest of the "enemies" list is mostly competition for light, water and nutrients wearing a more dramatic name.
The Thing That Matters More
Notice how many entries in the chart above come down to "same plant family." Tomatoes with potatoes, peppers with eggplant, the whole nightshade cluster. That is really a rotation problem expressed as a spacing problem.
Crop rotation — leaving three to four years before a plant family returns to the same bed — addresses the soil-borne diseases and pests that companion planting cannot touch: clubroot in brassicas, blight spores overwintering in nightshade debris, nematode populations, allium white rot. If you have limited time to plan, spend it on rotation first and pairings second. Group your beds by family, move the groups on each year, and most of the antagonisms in the chart resolve themselves.
How to Use the Chart
Treat it as a tiebreaker rather than a rulebook. Lay out your beds by family for rotation, size, sun and water needs first. Then, where you have a genuine choice about what goes next to what, use the chart. Put the basil with the tomatoes, keep the potatoes in a different bed, edge the brassicas with nasturtium, let some cilantro bolt, and sow radishes between the carrot rows.
Spacing is the part people compromise on and should not. Crowded plants compete for light and airflow, and poor airflow is what turns a damp week into powdery mildew. Whatever the chart says about a pairing, if the two plants do not both have room, the pairing is a bad one — which is also why planning which crops go into trays and which go straight into the ground pays off, covered in direct sow vs transplant.
Finally, pair the chart with a sowing calendar. Many of the best partnerships are as much about timing as position — radishes finishing as carrots need room, lettuce sown to catch the shade a tomato row will cast in July. That planning sits alongside a succession planting schedule and the sowing dates in when to start seeds by zone.
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