Companion planting is one of those gardening topics where folklore and science are tangled together so thoroughly that most gardeners cannot tell which advice is evidence based and which is inherited superstition. I have read dozens of university studies on companion planting, tested many combinations in my own garden, and come to a conclusion: some companion planting claims are well supported by research, some are partially true, and some are pure myth that refuses to die.
This guide separates the proven from the speculative. Each combination is rated based on the quality of scientific evidence supporting it, so you can focus on the pairings that actually work and stop wasting time on the ones that do not.
What Companion Planting Actually Means
The term “companion planting” is used loosely to describe several different phenomena, and this causes a lot of confusion. Let’s untangle them.
The Four Mechanisms of Companion Planting
Pest confusion and repulsion. Some plants emit volatile compounds that repel specific pests or mask the scent of vulnerable crops. This is the most scientifically supported mechanism. When a pest cannot smell its target plant because of the overwhelming scent of a companion, it flies past without landing.
Beneficial insect attraction. Some plants produce flowers or nectar that attract predatory insects (ladybugs, lacewings, parasitic wasps) that eat pest insects. This is also well documented. Providing a nectar source near your vegetables keeps predatory insects in the area.
Nutrient facilitation. Legumes (beans, peas, clover) host nitrogen fixing bacteria in their roots, which convert atmospheric nitrogen into a form plants can use. When legumes are grown near nitrogen hungry crops, some of this nitrogen becomes available to neighboring plants. This is scientifically proven, though the amount transferred is less than many gardeners think.
Physical structure and microclimate. Tall plants can provide shade for heat sensitive crops. Plants with deep roots can break up compacted soil for shallow rooted neighbors. This is real but is more about garden design than the mystical “plant friendship” that companion planting is often associated with.
What Companion Planting Is Not
Companion planting is not a magic system where certain plants “like” each other and grow better together for no observable reason. Every effective companion planting combination works through one of the four mechanisms above. If someone tells you two plants are “friends” but cannot explain the mechanism, be skeptical.
Companion planting is also not a replacement for good soil, proper watering, and adequate sun. No companion combination will save plants growing in poor conditions. Companion planting is an optimization, not a foundation.
The Evidence Rating System
Each companion combination in this guide is rated:
Well supported: Multiple university studies or extensive field trials demonstrate the effect. The mechanism is understood and documented.
Partially supported: Some evidence suggests the effect, but results are inconsistent or the mechanism is not fully understood. May work in some conditions but not others.
Folklore: The claim is widely repeated but has little or no scientific support. May be harmless but probably ineffective.
Disproven: Studies have specifically tested the claim and found no effect, or found a negative effect.
Tomatoes
Good Companions
Basil (Well supported). Basil is the most famous tomato companion, and there is real science behind it. Basil repels tomato hornworms and whiteflies through its strong scent. Plant basil around the base of tomatoes, 12 to 18 inches from the tomato stem. Some studies also suggest that basil improves tomato flavor, though this is harder to measure objectively. Even without the flavor claim, the pest repulsion is worth the pairing.
Marigolds (Well supported). Marigolds produce a compound called alpha terthienyl that is toxic to root knot nematodes, microscopic soil pests that damage tomato roots. Planting marigolds as a cover crop or interplanting with tomatoes reduces nematode populations in the soil. This is one of the best documented companion planting effects. Note: the nematode control only works if you have nematode problems. If your soil is nematode free, marigolds will not improve your tomatoes.
Carrots (Partially supported). Carrots and tomatoes are traditionally planted together because carrots aerate the soil with their deep roots and tomatoes provide shade. The evidence for mutual benefit is weak, but they do not compete, and the shade from tomatoes can keep carrots from getting too hot in midsummer. Plant carrots near the edge of the tomato bed where they get some sun but are shaded during the hottest part of the day.
Parsley (Partially supported). Parsley flowers attract parasitic wasps and other beneficial insects that prey on tomato pests. Let some parsley go to flower (bolt) rather than harvesting it all, and the flowers will draw beneficial insects to the tomato patch.
Bad Companions
Brassicas (cabbage, broccoli, kale) (Well supported). Tomatoes and brassicas compete for the same nutrients and are both susceptible to some of the same diseases. Planting them together can stunt both crops. Keep them in separate beds.
Corn (Partially supported). The myth is that tomatoes and corn should not be planted together because they attract the same pest (the corn earworm, which also attacks tomatoes). This is true, but the practical impact is small unless you are growing large quantities of both. In a home garden, the pest overlap is manageable.
Fennel (Well supported). Fennel produces compounds that inhibit the growth of many plants, including tomatoes. Fennel should be grown in its own area, away from most vegetables. This is one of the few genuinely antagonistic plant relationships that is well documented.
Peppers
Good Companions
Basil (Well supported). Same mechanism as with tomatoes. Basil repels aphids and spider mites, which are common pepper pests.
Onions and garlic (Partially supported). Alliums repel aphids and other pests through their sulfur compounds. Plant onions or garlic around the perimeter of the pepper bed. The evidence is mixed, with some studies showing pest reduction and others showing no effect.
Spinach and lettuce (Well supported as a structural pairing). Leafy greens grow in the cooler part of the season and can be harvested before peppers need the full space. Plant greens around peppers in spring, harvest the greens as the peppers grow, and the peppers fill in the space by summer. This is more about timing and space efficiency than plant chemistry.
Bad Companions
Beans (Partially supported). Beans and peppers are sometimes said to be incompatible, but the evidence is weak. They do not actively harm each other. If they are not growing well together, it is more likely due to competition for water or nutrients than to any chemical antagonism.
Brassicas (Well supported). Same as with tomatoes, peppers and brassicas compete and should be separated.
Beans (Bush and Pole)
Good Companions
Corn and squash (Well supported). The Three Sisters combination (corn, beans, and squash) is the most famous companion planting system in the world, developed by indigenous peoples of the Americas. The science is sound: – Corn provides a living trellis for pole beans to climb. – Beans fix nitrogen that feeds the nitrogen hungry corn. – Squash spreads along the ground, shading the soil to retain moisture and suppress weeds, and its prickly leaves deter raccoons and other pests from the corn.
This combination works, but it requires specific spacing and timing. Plant corn first, wait until it is 6 inches tall, then plant beans at the base of each corn stalk. Plant squash between the corn rows at the same time as the beans.
Carrots (Partially supported). Beans and carrots are a traditional pairing. Beans fix nitrogen that may benefit carrots, and carrots’ deep roots aerate the soil. The benefit is modest but they do not compete.
Cucumbers (Well supported as a structural pairing). Pole beans and cucumbers can share a trellis. They have similar growing requirements and do not compete if given enough space. Plant beans on one side of the trellis and cucumbers on the other.
Bad Companions
Onions and garlic (Well supported). Alliums inhibit the growth of beans, particularly the nitrogen fixing bacteria in bean roots. This is a well documented antagonism. Keep beans and alliums at least 4 feet apart.
Peppers (Partially supported). See peppers above. The incompatibility is weak.
Squash and Zucchini
Good Companions
Nasturtiums (Well supported). Nasturtiums are a trap crop for squash bugs and aphids. The pests prefer nasturtiums to squash, so they congregate on the nasturtiums where you can easily find and destroy them. Plant nasturtiums around the perimeter of the squash bed.
Radishes (Partially supported). Radishes planted among squash are said to repel squash borers. The evidence is limited, but radishes are fast growing and do not compete with squash, so there is no downside to trying.
Beans and corn (Well supported). See the Three Sisters combination above.
Bad Companions
Potatoes (Partially supported). Squash and potatoes are both susceptible to some of the same diseases and may compete for water. Keep them separate.
Cucumbers
Good Companions
Radishes (Well supported). Radishes repel cucumber beetles, which are one of the most damaging cucumber pests. Plant radishes among cucumber plants and let some go to flower, as the flowers continue to repel beetles.
Nasturtiums (Partially supported). Same trap crop effect as with squash.
Sunflowers (Well supported as a structural pairing). Sunflowers serve as a living trellis for cucumbers, similar to corn for beans. Plant sunflowers 2 to 3 feet apart and plant cucumbers at their base. The cucumbers climb the sunflower stalks. Choose sunflower varieties with thick, sturdy stalks.
Bad Companions
Sage (Partially supported). Sage is said to inhibit cucumber growth. The evidence is limited, but sage has strong aromatic compounds that could affect some plants. Keep them separate to be safe.
Potatoes (Partially supported). Similar to squash, cucumbers and potatoes may compete for water and attract similar pests.
Cabbage and Other Brassicas
Good Companions
Dill (Well supported). Dill attracts parasitic wasps that lay eggs in cabbage worms, one of the most destructive brassica pests. The wasp larvae eat the cabbage worms from the inside. Plant dill near cabbage and let it flower.
Thyme (Well supported). Thyme repels cabbage moth eggs. Plant thyme around the base of brassicas. The strong scent masks the brassica scent that cabbage moths use to find their host plants.
Onions and garlic (Partially supported). Alliums repel cabbage moths and other brassica pests. Plant onions or garlic in rows between cabbage plants.
Nasturtiums (Well supported). Nasturtiums are a trap crop for aphids, which attack brassicas. Plant nasturtiums nearby to draw aphids away from your cabbage.
Bad Companions
Strawberries (Folklore). The claim that cabbage and strawberries inhibit each other is widely repeated but poorly documented. They probably do not actively harm each other, but they have very different growing requirements and are unlikely to thrive in the same bed.
Tomatoes (Well supported). See tomatoes above.
Lettuce and Leafy Greens
Good Companions
Radishes (Well supported as a space pairing). Radishes grow fast and are harvested before lettuce needs the full space. Plant radishes between lettuce rows and harvest the radishes in 30 days, just as the lettuce is filling in.
Carrots (Well supported as a space pairing). Carrots grow deep while lettuce grows shallow. They use different soil layers and do not compete. Plant carrots between rows of lettuce.
Strawberries (Partially supported). Strawberries and lettuce have similar growing conditions and are traditionally planted together. The benefit is mainly spatial, as they occupy different layers (strawberries spread on the ground, lettuce grows upward).
Tall plants (tomatoes, corn, sunflowers) (Well supported for shade). Lettuce prefers cooler temperatures and bolts (goes to seed) in hot weather. Planting lettuce on the north side of tall crops provides afternoon shade that extends the lettuce harvest by 2 to 3 weeks.
Bad Companions
Brassicas (Well supported). Cabbage family plants compete with lettuce for nutrients and space. Keep them in separate beds.
Onions and Garlic
Good Companions
Carrots (Well supported). The onion and carrot pairing is one of the best documented companion planting combinations. Onions repel carrot rust fly, and carrots repel onion fly. The strong scents of each plant mask the other from its specific pest. Plant onions and carrots in alternating rows.
Tomatoes (Well supported). Onions and garlic repel aphids and spider mites from tomatoes. Plant alliums around the perimeter of the tomato bed.
Strawberries (Partially supported). Garlic is said to repel strawberry pests. The evidence is limited but the pairing is harmless.
Roses (Well supported, for flowers). Garlic planted around roses repels aphids. This is one of the most effective companion planting combinations for ornamental gardens.
Bad Companions
Beans and peas (Well supported). Alliums inhibit the nitrogen fixing bacteria in legume roots. This is a significant antagonism that reduces bean and pea yields. Keep alliums at least 4 feet from legumes.
Asparagus (Partially supported). Onions and asparagus are said to be incompatible. The evidence is weak, but they have different growing seasons and soil preferences, so they are unlikely to be planted together anyway.
Potatoes
Good Companions
Beans (Well supported). Beans fix nitrogen that potatoes need. Plant bush beans around the edge of the potato bed after the potatoes have emerged.
Corn (Partially supported). Corn and potatoes are said to mutually benefit, with corn repelling potato beetles. The evidence is limited.
Marigolds (Partially supported). Marigolds may repel potato beetles, though the effect is less pronounced than the nematode control marigolds provide for tomatoes.
Bad Companions
Tomatoes (Well supported). Tomatoes and potatoes are both in the nightshade family and are susceptible to the same diseases, especially early and late blight. Planting them together increases disease risk for both. Keep them at least 10 feet apart.
Squash and cucumbers (Partially supported). Potatoes and squash compete for water and nutrients.
Sunflowers (Folklore). Sunflowers are said to inhibit potato growth. There is little evidence for this, but sunflowers do produce compounds that can inhibit some plants (allelopathy), so it is possible.
Carrots
Good Companions
Onions (Well supported). See onions above. The onion carrot pairing is mutually beneficial for pest control.
Tomatoes (Partially supported). Tomatoes provide shade that carrots appreciate in hot weather, and carrots aerate the soil. The benefit is modest.
Radishes (Well supported as a space pairing). Radishes mature fast and are harvested before carrots need the full space. Plant radishes in the same row as carrots, and harvest the radishes in 30 days, leaving space for the slower growing carrots.
Rosemary and sage (Partially supported). These aromatic herbs are said to repel the carrot rust fly. Plant them near carrots as a border.
Bad Companions
Dill (Well supported). Dill and carrots are in the same plant family and can cross pollinate, producing poor quality seeds. Dill also attracts the same pests as carrots. Keep them separate.
Herbs as Companion Plants
Herbs are the most useful companion plants because their strong scents repel a wide range of pests while their flowers attract beneficial insects.
The Most Useful Companion Herbs
Basil: Repels tomato hornworms, aphids, whiteflies. Plant with tomatoes and peppers.
Dill: Attracts parasitic wasps and predatory insects. Plant with brassicas and cucumbers. Let it flower.
Mint: Repels ants, aphids, and cabbage moths. Plant in containers (it is invasive in the ground) and place the containers near vulnerable crops.
Rosemary: Repels cabbage moths, carrot flies, and bean beetles. Plant as a perennial border near the vegetable garden.
Sage: Repels cabbage moths and carrot flies. Plant with brassicas and carrots.
Thyme: Repels cabbage moth eggs. Plant with brassicas.
Oregano: Repels many pests and attracts beneficial insects. Plant near any vegetable.
Chamomile: Said to improve the flavor and health of onions and other vegetables. The evidence is limited, but chamomile is a gentle plant that does not compete with anything.
Borage: Attracts bees for pollination and is said to improve strawberry flavor. It also accumulates trace minerals in its leaves, which can be used as a mulch. Let borage self seed around the garden.
Flowers as Companion Plants
Certain flowers are valuable companion plants not just for pest control but for attracting pollinators.
The Most Useful Companion Flowers
Marigolds: The workhorse of companion planting. Repels nematodes (in soil), deters some flying pests, and attracts beneficial insects. Plant throughout the vegetable garden.
Nasturtiums: Trap crop for aphids and squash bugs. The flowers and leaves are also edible. Plant near squash, cucumbers, and brassicas.
Calendula: Attracts pollinators and predatory insects. The flowers are edible and medicinal. Plant near tomatoes and squash.
Sunflowers: Attract bees for pollination and serve as a trellis for climbing plants. Plant at the north edge of the garden.
Sweet alyssum: Attracts hoverflies, whose larvae eat aphids. Plant as a ground cover around taller vegetables.
Zinnias: Attract butterflies and bees. Plant near vegetables that need pollination (squash, cucumbers).
Common Companion Planting Myths
These claims are widely repeated in gardening books and websites but have little or no scientific support.
Myth: Marigolds Repel All Pests
Marigolds repel specific pests (nematodes, some flying insects) but they do not create a pest free zone. Many pests are unaffected by marigolds, and some pests are actually attracted to them. Marigolds are useful but they are not a general purpose pest repellent.
Myth: Certain Plant Pairs Improve Flavor
Claims that basil improves tomato flavor or that chamomile improves onion flavor are widely repeated but have never been demonstrated in controlled studies. Flavor is determined by variety, soil, sun, water, and harvest timing, not by neighboring plants.
Myth: All Herbs Repel All Pests
Herbs have specific effects on specific pests. Basil repels tomato hornworms but has no effect on, say, potato beetles. Treating all herbs as general pest repellents leads to disappointment.
Myth: Plants Have “Friends” and “Enemies”
This anthropomorphic framing implies that plants have preferences or emotions. Plants do not have relationships. They have competitive and facilitative interactions that can be measured. Some plants compete for the same resources (bad companions) and some provide benefits to neighbors (good companions), but these are ecological interactions, not friendships.
Myth: Companion Planting Replaces Other Pest Control
Companion planting is one tool in an integrated pest management approach. It reduces pest pressure but does not eliminate it. You still need to monitor for pests, use physical barriers when necessary, and maintain healthy soil and plants.
Designing a Companion Planted Garden
The Polyculture Approach
Instead of planting in monoculture blocks (all tomatoes together, all beans together), interplant compatible species throughout the bed. A diverse planting confuses pests, attracts a variety of beneficial insects, and makes more efficient use of space.
Example polyculture bed (4×8 feet): – North row (tall): 2 indeterminate tomatoes with basil at their base – Middle row: 4 pepper plants with onions between them – South row (short): lettuce and radishes, with marigolds at the corners
This bed produces tomatoes, basil, peppers, onions, lettuce, radishes, and marigold flowers, all in 32 square feet. The diversity reduces pest problems, and the different heights ensure all plants get adequate sun.
The Border Approach
If polyculture seems too chaotic, plant your vegetables in blocks as usual but surround each block with a border of companion plants. A border of marigolds, nasturtiums, or aromatic herbs around the vegetable bed provides pest protection without the complexity of interplanting.
The Timing Approach
Some companion planting is about timing rather than chemistry. Plant radishes with slow germinating crops like carrots or parsnips. The radishes mark the row and are harvested before the slower crops need the space. Plant lettuce under tomato seedlings in spring. The lettuce is harvested by the time the tomatoes are large enough to need the full space.
My Companion Planting Strategy
After testing many combinations, I settled on a simple, evidence based approach:
- Basil with every tomato and pepper plant. This is the most reliable pairing I have tested. Pest pressure is noticeably lower, and the basil is useful in the kitchen.
- Marigolds throughout the garden. Not for nematodes (I do not have a problem), but for general beneficial insect attraction and a bit of pest confusion.
- Onions and carrots in alternating rows. The pest control effect is real and measurable in my garden.
- Nasturtiums around squash and cucumbers. The trap crop effect draws aphids and squash bugs away from the vegetables.
- Dill and flowering herbs near brassicas. The parasitic wasps they attract keep cabbage worm populations manageable.
- I avoid planting fennel near anything and keep alliums away from beans.
This is not an elaborate system. It is six evidence based pairings that I have confirmed work in my garden. I do not bother with combinations that lack scientific support, because the effort is better spent on soil improvement, proper watering, and crop rotation, all of which have a larger impact on yield than any companion planting combination.
Companion planting is a useful tool, not a gardening religion. Use the pairings that are proven, skip the folklore, and spend your gardening time on the fundamentals that matter most: good soil, consistent water, and healthy plants. The companion plants are a bonus, not a substitute.
