The ledger — updated 2026-07
56 entries Companion Planting Records
Every companion planting pairing we could find for 56 crops — 542 in total — graded by the strength of the evidence behind it and linked to its source. 129 are supported by published research. 379 are traditional practice with no controlled test behind them. 34 are widely repeated but were tested and did not hold up.
Research-backed tested in controlled research Traditional long practice, untested Not supported tested and not supported
Vegetables
37 Entry No. 001
Arugula
8 companions · 3 to avoid · 3 research-backed
Entry No. 002Asparagus
7 companions · 3 to avoid
Entry No. 003Beet
8 companions · 3 to avoid · 2 research-backed
Entry No. 004Bok Choy
8 companions · 3 to avoid · 4 research-backed
Entry No. 005Broccoli
9 companions · 3 to avoid · 5 research-backed
Entry No. 006Brussels Sprouts
9 companions · 4 to avoid · 5 research-backed
Entry No. 007Bush bean
8 companions · 3 to avoid · 2 research-backed
Entry No. 008Cabbage
9 companions · 3 to avoid · 5 research-backed
Entry No. 009Carrot
8 companions · 3 to avoid · 1 research-backed
Entry No. 010Cauliflower
8 companions · 4 to avoid · 5 research-backed
Entry No. 011Celery
7 companions · 3 to avoid · 1 research-backed
Entry No. 012Collard Greens
8 companions · 4 to avoid · 5 research-backed
Entry No. 013Cucumber
8 companions · 3 to avoid · 2 research-backed
Entry No. 014Eggplant
8 companions · 3 to avoid · 3 research-backed
Entry No. 015Garlic
8 companions · 3 to avoid · 1 research-backed
Entry No. 016Kale
8 companions · 3 to avoid · 5 research-backed
Entry No. 017Kohlrabi
9 companions · 4 to avoid · 1 research-backed
Entry No. 018Leek
6 companions · 3 to avoid · 2 research-backed
Entry No. 019Lettuce
8 companions · 3 to avoid · 1 research-backed
Entry No. 020Okra
7 companions · 3 to avoid · 1 research-backed
Entry No. 021Onion
7 companions · 3 to avoid · 2 research-backed
Entry No. 022Parsnip
9 companions · 4 to avoid · 2 research-backed
Entry No. 023Pea
8 companions · 3 to avoid · 2 research-backed
Entry No. 024Pepper (sweet/bell)
8 companions · 3 to avoid
Entry No. 025Pole bean
7 companions · 3 to avoid · 2 research-backed
Entry No. 026Potato
7 companions · 4 to avoid
Entry No. 027Pumpkin
8 companions · 3 to avoid · 5 research-backed
Entry No. 028Radish
8 companions · 3 to avoid · 2 research-backed
Entry No. 029Rhubarb
6 companions · 3 to avoid
Entry No. 030Spinach
8 companions · 4 to avoid · 2 research-backed
Entry No. 031Summer squash / zucchini
8 companions · 3 to avoid · 6 research-backed
Entry No. 032Sweet corn
7 companions · 2 to avoid · 5 research-backed
Entry No. 033Sweet Potato
8 companions · 3 to avoid · 4 research-backed
Entry No. 034Swiss Chard
8 companions · 3 to avoid · 2 research-backed
Entry No. 035Tomato
8 companions · 4 to avoid · 2 research-backed
Entry No. 036Turnip
7 companions · 3 to avoid · 2 research-backed
Entry No. 037Winter Squash (Butternut)
8 companions · 3 to avoid · 3 research-backed
Fruit
3Herbs
11 Entry No. 041
Basil
6 companions · 4 to avoid · 1 research-backed
Entry No. 042Borage
5 companions · 0 to avoid · 1 research-backed
Entry No. 043Chives
5 companions · 3 to avoid
Entry No. 044Cilantro / Coriander
6 companions · 2 to avoid · 1 research-backed
Entry No. 045Common Thyme
6 companions · 2 to avoid · 2 research-backed
Entry No. 046Dill
6 companions · 3 to avoid · 2 research-backed
Entry No. 047Garden Sage
5 companions · 2 to avoid · 1 research-backed
Entry No. 048Greek Oregano
5 companions · 2 to avoid
Entry No. 049Parsley
5 companions · 2 to avoid
Entry No. 050Rosemary
5 companions · 2 to avoid
Entry No. 051Spearmint
5 companions · 1 to avoid
Companion flowers
5 Entry No. 052
Calendula / Pot Marigold
3 companions · 1 to avoid
Entry No. 053French Marigold
3 companions · 1 to avoid · 2 research-backed
Entry No. 054Nasturtium
4 companions · 0 to avoid · 1 research-backed
Entry No. 055Sunflower
3 companions · 3 to avoid
Entry No. 056Sweet Alyssum
5 companions · 0 to avoid · 4 research-backed
Every research-backed pairing on the site
113These are the pairings that survived the evidence test — the ones worth actually planning a bed around.
| Pairing | Effect | What the evidence shows |
|---|---|---|
| Arugula + Sweet alyssum | Flowering alyssum planted beside brassica greens feeds hoverflies and parasitoid wasps that suppress aphids and caterpillars. | Replicated organic-cabbage trials found insectary alyssum significantly raised parasitism of brassica caterpillars and the abundance of predatory lady beetle eggs and larvae; the measured effect is on natural-enemy numbers and parasitism rate, and trials used flowering strips rather than a few scattered plants. |
| Arugula + White clover (undersown living mulch) | Undersowing arugula and other brassicas with low-growing living clover reduces colonisation by cabbage root fly and aphids. | Cage and field experiments consistently reduced cabbage root fly egg-laying in clover-undersown brassicas, but the same work showed dried brown clover gave no protection while living green clover did — so the mechanism is disrupted host-plant finding over a green background, not repellency. Clover also competes for water and nitrogen and will cut leaf yield if established too early. |
| Arugula + Mustard (border trap crop) | A border row of mustard or rapeseed pulls harlequin bugs away from the salad greens. | Choice tests and field trials confirm harlequin bug strongly prefers mustard over collard, and mustard borders cut feeding damage. Important caveat for this crop: the same study found arugula itself was among the most attractive hosts, so a mustard border protects arugula less reliably than it protects collards. |
| Beet + French Marigold | A dense marigold planting before beets reduces root-knot nematode pressure in the soil. | Genuinely supported, but only in the tested form: Tagetes grown as a solid preceding cover crop for a full season, which produces alpha-terthienyl and acts as a poor host. Scattering a few marigolds among beets is not what was tested and should not be expected to work. |
| Beet + Buckwheat | Flowering buckwheat nearby supports predators and parasitoids of beet aphids and leafminers. | Extension cites trials showing buckwheat raises natural-enemy numbers; the specific effect on beet leafminer has not been measured. |
| Bok Choy + Sweet alyssum | Alyssum flowers alongside the crop recruit hoverflies, lady beetles and parasitoid wasps that hit brassica aphids and caterpillars. | In replicated organic cabbage plots, insectary alyssum significantly increased parasitism of a brassica caterpillar and the abundance of predatory coccinellid eggs and larvae. Separate laboratory work confirms alyssum volatiles attract the diamondback moth parasitoid and that its flowers extend parasitoid lifespan and egg-laying — but that part is cage work, not field yield data. |
| Bok Choy + White clover (undersown living mulch) | A low living clover mulch between bok choy plants reduces cabbage root fly egg-laying and aphid colonisation. | Reduced cabbage root fly egg counts in clover-undersown brassicas is a repeated result. The critical detail: dead brown clover gave no benefit while live green clover did, so the effect is host-finding disruption by green background, not chemical repellency. Clover competes strongly for water and nitrogen in a crop this shallow-rooted, so sow it only after the choy is well established. |
| Bok Choy + Mustard (border trap crop) | A perimeter row of mustard draws harlequin bugs and other brassica bugs off the choy. | Field trials with mustard as a perimeter trap crop for collard held leaf damage under 1% in spring, and a 2.3 m gap between trap and cash crop cut egg-laying on the cash crop about fourfold versus an adjacent trap row. Trap cropping only works with scouting — pests must be killed on the trap plants or they spill back into the crop. |
| Bok Choy + Yellow rocket (dead-end trap crop) | Yellow rocket planted at the edge attracts diamondback moth to lay eggs where the larvae cannot survive. | Screenhouse trials found diamondback moth laid 19-300x more eggs on yellow rocket than on cabbage in no-choice tests, with zero larval survival on it versus 22-24% on cabbage — a genuine dead-end trap. Caveats: the work was in screenhouses rather than open fields, and yellow rocket is a weedy biennial that can become a management problem itself. |
| Broccoli + Glossy Collard | Two perimeter rows of glossy collards intercept diamondback moth before it reaches the broccoli. | One of the best-documented companion tactics anywhere: in nine Florida commercial fields the collard perimeter kept diamondback moth below action thresholds while 60% of unprotected control fields exceeded them, and perimeter-trapped fields used 56% fewer insecticide sprays. Glossy collards work because moths lay 3x more eggs on them yet larval survival is only 6.7% versus 22.2% on cabbage. |
| Broccoli + White Clover | Undersowing broccoli with white clover as a living mulch sharply reduces pest colonization. | Repeatedly demonstrated at Warwick Crop Centre, and in trials undersown cabbage was harvested to commercial standard without insecticide. The mechanism is visual interference with pest landing, not chemistry — green paper models worked as well as living clover. The catch is competition: clover can reduce head size and must be mown or sown late. |
| Broccoli + Radish | Radish or daikon sown one to two weeks ahead pulls flea beetles off broccoli seedlings. | Supported by multiple replicated trials and extension IPM guidance. Efficacy depends on sowing the trap early, giving it roughly 10% of the planting area, and destroying it before it becomes a pest reservoir or a clubroot host. |
| Broccoli + Mustard | Mustard, arugula and rapeseed act as effective trap crops for brassica pests. | University of Minnesota Extension lists arugula, mustard, rapeseed and napa cabbage as effective brassica trap crops; like radish they must be removed or treated so they do not simply breed more pests. |
| Broccoli + Buckwheat | A buckwheat strip feeds parasitoid wasps and predators that attack cabbageworms and aphids. | Extension cites trials in which flowering buckwheat and cowpea raised predator and parasitoid numbers; the measured pest was corn earworm, so the broccoli benefit follows the mechanism rather than a dedicated trial. |
| Brussels Sprouts + Glossy Collard | A collard perimeter absorbs diamondback moth pressure across the long Brussels sprouts season. | The perimeter trap crop is proven in cole crops and is especially valuable here because Brussels sprouts occupy the ground for 90-110 days, accumulating far more pest pressure than a quick cabbage crop. |
| Brussels Sprouts + White Clover | A clover living mulch between the widely spaced stalks reduces pest colonization. | Demonstrated in brassicas at Warwick Crop Centre; the wide spacing and long season of Brussels sprouts suit an undersown mulch better than most cole crops, though clover competition still costs some yield. |
| Brussels Sprouts + Radish | Radish sown ahead of the transplants diverts flea beetles from young Brussels sprouts. | Supported by replicated flea beetle trap-crop trials; sow 1-2 weeks early and destroy the trap, since radish is a brassica and can host clubroot. |
| Brussels Sprouts + Indian Mustard | Indian mustard attracts diamondback moth egg-laying far more strongly than the cash crop. | Host-choice trials recorded 18 times more eggs on Indian mustard than on cabbage, and yellow rocket produced zero larval survival; the trap must be managed or it becomes a nursery. |
| Brussels Sprouts + Buckwheat | Buckwheat borders raise populations of parasitoids and predators over the long growing season. | Extension cites trials showing flowering buckwheat and cowpea increase natural enemies; the long Brussels sprouts season gives beneficials time to build up, which is the main argument for it here. |
| Bush bean + Sweet corn | Corn and beans grown together out-produce the same area grown as separate monocultures. | The Three Sisters yield advantage is real and measured as a land equivalent ratio above 1; the driver is below-ground niche complementarity in root foraging, not the popular idea that beans feed nitrogen to the corn within the season. |
| Bush bean + Summer squash | Squash completes the Three Sisters guild, shading out weeds beneath corn and beans. | The three-crop polyculture is documented to out-yield component monocultures on a land-equivalent basis, though the practical cost is that harvest access is awkward. |
| Cabbage + Glossy Collard | A two-row collard perimeter keeps diamondback moth out of the cabbage crop. | The single strongest brassica companion result. Across nine commercial cabbage fields the collard perimeter held diamondback moth below action thresholds while 60% of control fields exceeded them; growers cut insecticide sprays 56% and saved $117-156 per hectare. Leave about 60 cm between trap and cash crop, because heads within 30 cm of the trap weighed less. |
| Cabbage + White Clover | Cabbage undersown with white clover suffers far less pest colonization. | Directly demonstrated on cabbage: undersown plots produced commercially acceptable heads without insecticide, fungicide or herbicide, and Dutch trials found subclover and white clover suppressed egg-laying and larval populations enough to improve profit. Expect somewhat smaller heads from clover competition — the pesticide savings offset it. |
| Cabbage + Radish | Radish and daikon sown ahead of cabbage act as flea beetle trap crops. | Multiple trials support radish and daikon as flea beetle traps; sow 1-2 weeks before the cabbage, allow roughly 10% of the area, and place the trap as a perimeter. Destroy it afterwards, since radish is itself a brassica and can host clubroot. |
| Cabbage + Indian Mustard | Indian mustard is far more attractive to diamondback moth than cabbage and diverts egg-laying. | In host-choice trials diamondback moths laid 18 times more eggs on Indian mustard than on cabbage; extension lists mustard, arugula, rapeseed and napa cabbage among effective brassica trap crops. Trap plants must be managed or they amplify the next generation. |
| Cabbage + Buckwheat | Flowering buckwheat nearby increases parasitoids and predators of cabbageworm and aphids. | Extension cites trials showing buckwheat and cowpea raise natural-enemy numbers; the tested target was corn earworm, so the cabbage benefit follows the same mechanism rather than a dedicated trial. |
| Carrot + Onion | Onions intercropped with carrots reduce carrot rust fly damage. | One of the very few companion pairings with real controlled data: mixed cropping cut carrot fly attack by up to 65 percent versus carrot monoculture, and also raised numbers of carabid and staphylinid predators of carrot fly eggs. |
| Cauliflower + Glossy Collard | A collard perimeter intercepts diamondback moth before it reaches the cauliflower. | The perimeter trap crop system is proven on cabbage and cole crops generally — moths lay far more eggs on glossy collards while larval survival there is only 6.7% versus 22.2% on cabbage. Trials used cabbage as the cash crop, so applying it to cauliflower is a reasonable but untested extension. |
| Cauliflower + White Clover | A clover living mulch reduces the number of pests that successfully colonise the crop. | Demonstrated across brassicas at Warwick Crop Centre. Use with caution on cauliflower specifically: it is the least forgiving cole crop and any check to growth from clover competition or water stress causes buttoning and poor curd quality. |
| Cauliflower + Radish | Radish sown early diverts flea beetles from young cauliflower transplants. | Supported by replicated flea beetle trap-cropping trials; sow 1-2 weeks ahead, dedicate roughly 10% of the area, and destroy the trap crop rather than leaving it as a clubroot host. |
| Cauliflower + Mustard | Mustard, arugula and napa cabbage work as trap crops for the brassica pest complex. | Named by extension among effective brassica trap crops; must be managed or destroyed so the trap does not become a nursery. |
| Cauliflower + Buckwheat | Buckwheat borders build up parasitoid and predator populations near the crop. | Trials cited by extension show flowering buckwheat and cowpea increase natural enemies; the effect on cauliflower pests specifically has not been quantified. |
| Celery + Leek | Alternating rows of leek and celery smother weeds and produce more total yield per unit of ground than either crop alone. | Replicated Swiss field trials (Baumann, Kropff & Bastiaans, Weed Research 2000) cut weed soil cover about 41% and weed biomass about 58% versus leek alone and raised relative yield total about 10%, but leek quality was reduced; the demonstrated benefit is weed suppression and land-use efficiency, not pest control. |
| Collard Greens + Mustard (perimeter trap crop) | A perimeter row of mustard pulls harlequin bugs off the collards and concentrates them where they can be killed. | This is one of the best-evidenced pairings in vegetable gardening. Choice tests showed harlequin bug consistently prefers mustard over collard; field trials at two sites showed mustard borders reduced feeding damage, with spring collards holding under 1% leaf damage. A follow-up found spacing matters: a 2.3 m gap between mustard and collards cut egg-laying on collards about fourfold and damage about 2.5-fold versus an adjacent trap row, because females otherwise commute across to lay. Requires scouting and killing the bugs on the trap crop. |
| Collard Greens + Arugula (alternative trap plant) | Arugula, rapini and rapeseed also draw harlequin bugs away from collards. | In the same replicated host-preference study, mustard, rapeseed, rapini and arugula were all significantly more attractive to harlequin bug than collard. Mustard was the most consistently preferred and is the only one taken through to field trap-crop trials, so arugula is a supported second choice rather than a proven substitute. |
| Collard Greens + Cabbage | Collards planted as a border protect a cabbage crop by absorbing the diamondback moth pressure. | Here collards are the trap crop, not the protected crop. Screenhouse trials found diamondback moth laid 3-18x more eggs on glossy collards, waxy collards and Indian mustard than on cabbage when given a choice, and UF/IFAS lists collards as a trap crop for diamondback moth on cabbage. Larvae do survive on collards, so unlike a dead-end trap this needs the trap row treated or the collards themselves take heavy damage. |
| Collard Greens + Sweet alyssum | Strips of flowering alyssum near collards build up hoverflies, lady beetles and parasitoid wasps that suppress aphids and caterpillars. | A replicated organic cabbage study using a glossy-collard trap mix found insectary alyssum significantly raised parasitism of the cross-striped cabbageworm and the abundance of predatory lady beetle eggs and larvae. Note this is measured as natural-enemy activity and pest suppression, not as a yield increase, and it works as a strip rather than a couple of scattered plants. |
| Collard Greens + White clover (undersown living mulch) | Collards undersown with a low living clover receive fewer cabbage root fly eggs and less aphid colonisation. | Repeated cage and field work reduced cabbage root fly egg counts in clover-undersown brassicas. The decisive control: brown desiccated clover gave no protection while living green clover did, so the mechanism is disrupted host-plant finding over a green background rather than any repellent chemistry. The trade-off is real competition for water and nitrogen, so undersow only once collards are established and expect to irrigate more. |
| Cucumber + Blue Hubbard squash | A perimeter of Blue Hubbard squash pulls striped cucumber beetles off the cucumber crop. | Blue Hubbard is far more attractive to striped cucumber beetle than cucumber because of its cucurbitacin content; Connecticut on-farm trials cut beetle infestation by up to 95 percent and squash vine borer by up to 88 percent. |
| Cucumber + Buckwheat | A buckwheat perimeter or strip lowers cucumber beetle pressure and feeds natural enemies. | Supported by preliminary perimeter-planting trials, but the yield result cited by extension was obtained on melon rather than cucumber, so treat the size of the effect as provisional. |
| Cucumber + Summer squash and melons | Massing cucurbits together concentrates cucumber beetle and bacterial wilt. | Striped cucumber beetle is the vector of Erwinia tracheiphila and moves freely between cucurbit hosts; extension IPM explicitly treats cucurbit consolidation and rotation distance as a control lever. |
| Eggplant + French Marigold | A dense marigold cover crop grown the season before eggplant suppresses root-knot nematodes. | Well documented for a solid preceding stand of Tagetes, which produces alpha-terthienyl and is a poor nematode host; root-knot nematode is a genuine eggplant problem. Interplanting a few marigolds among eggplants is not the tested method and there is no evidence it repels insects. |
| Eggplant + Radish | Radish or mustard sown early beside eggplant draws flea beetles onto a preferred host. | Flea beetle trap cropping with radish and mustard is supported by replicated trials, and flea beetle is eggplant's signature pest; the published work used brassica cash crops, so applying it to a nightshade is a reasonable but untested extension of the method. |
| Eggplant + Buckwheat | Buckwheat borders raise numbers of predators and parasitoids attacking eggplant pests. | Extension cites trials showing flowering buckwheat and cowpea increase natural enemies; the tested pest was corn earworm, so the eggplant benefit follows the mechanism rather than a direct trial. |
| Garlic + Carrot | Interplanting alliums with carrots reduces carrot rust fly damage. | Uvah and Coaker's field work is the most-cited support and reported real reductions, but later analysis attributes the effect to disrupted host-plant selection rather than repellent odour, and marigold intercrops tested the same way failed to reduce carrot fly at all. |
| Kale + Glossy Collard | Collards planted as a perimeter absorb diamondback moth pressure that would otherwise hit the kale. | The trap-crop effect is well demonstrated, though note the trial caveat: kale and cabbage grown within 30 cm of the trap crop weighed less, so leave about 60 cm between trap and cash crop. |
| Kale + White Clover | Undersowing kale with clover cuts colonization by cabbage aphid and caterpillar pests. | Demonstrated across brassicas at Warwick; the mechanism is disrupted landing behaviour rather than repellency, so any low green cover works. Watch competition, and keep clover trimmed back from young plants. |
| Kale + Radish | Radish sown a week or two early pulls flea beetles off kale, which is highly susceptible when young. | Radish and daikon trap cropping is supported by multiple trials; flea beetle is one of kale's worst seedling pests, making this among the more useful evidence-backed pairings in the dataset. |
| Kale + Mustard | Mustard and arugula divert brassica pests away from the kale planting. | Extension names mustard, arugula, rapeseed and napa cabbage as effective brassica trap crops; the trap must be destroyed or sprayed or it simply raises the next generation. |
| Kale + Buckwheat | Buckwheat feeds parasitoids and predators that attack aphids and cabbageworms on kale. | Extension cites trials showing flowering buckwheat and cowpea raise natural-enemy numbers; the kale-specific effect is inferred from the mechanism. |
| Kohlrabi + White Clover (undersown living mulch) | A low clover living mulch between brassica plants reduces colonization by cabbage root fly and aphids. | Warwick Crop Centre field work behind the 'appropriate/inappropriate landings' model found that green ground cover between brassicas sharply increases wasted landings by host-seeking flies; the caveat is that clover competes for water and nitrogen and can cut yield if not managed. |
| Kohlrabi + Cabbage and other brassicas | Planting kohlrabi with or after other brassicas concentrates shared pests and soilborne disease. | This one is real: clubroot resting spores persist in soil for years and cabbage maggot, flea beetle and diamondback moth all move freely between brassica hosts, which is why extension guidance is a multi-year rotation away from the whole family. |
| Leek + Carrot | Interplanting leeks with carrots is said to cut carrot rust fly on the carrots and thrips on the leeks. | Uvah & Coaker (1984) showed mixed cropping carrots with ONIONS reduced both carrot fly attack on carrots and Thrips tabaci on onions versus monoculture, attributed to allium volatiles disrupting host-plant finding — but leek itself was not the tested species, the protection was strongest only while the alliums were young, and the effect is density-dependent, so a few leeks scattered among carrots should not be expected to reproduce it. |
| Lettuce + Sweet alyssum | Alyssum interplanted with lettuce delivers biological control of lettuce aphids via hoverflies. | Backed by roughly a decade of USDA-ARS commercial-scale organic work in the Salinas Valley; as few as 500-1000 alyssum transplants per acre distributed through the field supplied enough pollen and nectar for hoverflies to control aphids in romaine. |
| Okra + Marigold (Tagetes) | Marigold suppresses root-knot nematodes, the most serious pest of okra. | The evidence supports marigold only as a dense, weed-free, full-season COVER/ROTATION crop grown on the same ground for at least two months BEFORE the okra, at roughly 7-inch spacing (~131,000 plants/acre); UF/IFAS states plainly that 'intercropping marigold with other crops to reduce plant-parasitic nematodes does not appear to be effective,' so a few marigolds tucked among okra plants is not the tested practice. French marigold (T. patula) cultivars such as 'Single Gold' and 'Tangerine' are effective; signet marigolds are not, and marigold cannot eradicate a severe infestation. |
| Onion + Broccoli and cabbage | Onions interplanted with brassicas reduce caterpillar damage. | In an Iowa study cited by extension, thyme, onion and nasturtium each reduced cabbage looper and imported cabbageworm damage in broccoli; note the effect may require a substantial planting of the companion, not a token row. |
| Parsnip + Onion | Onions intercropped with parsnip reduce carrot rust fly damage. | The strongest-evidenced pairing in this crop: UMass Extension's New England guide states outright that intercropping with onion has been shown to reduce carrot rust fly damage, backed by Uvah and Coaker's mixed-cropping work, but the effect scales with onion density and fades once the onions stop actively growing, so a token row of onions should not be expected to do much. |
| Parsnip + White Clover (undersown living mulch) | A low non-host groundcover between parsnip rows disrupts carrot rust fly host-finding. | Rämert and Ekbom's replicated field study found carrot rust fly damage was consistently lower under intercropping and traced it to the resource concentration effect rather than to predators, so any low green cover, not clover specifically, is doing the work; the trade-off is competition for water and nitrogen. |
| Parsnip + Carrot | Carrots and parsnips grown together or in succession build up shared root pests. | Well supported as a rotation rule: carrot rust fly and carrot weevil overwinter in the soil and attack both hosts, and UMass Extension specifically advises rotating carrot and parsnip crops to new ground to escape overwintering adults. |
| Pea + Buckwheat | A buckwheat strip near the pea row feeds parasitoids and predators of pea aphid. | Extension cites trials in which flowering buckwheat and cowpea raised predator and parasitoid populations; the measured target was corn earworm, so the pea aphid benefit follows the mechanism rather than a direct trial. |
| Pea + Sweet Corn | In a Three Sisters-style planting, corn supplies support for climbing legumes and the mixture out-yields the same area in monoculture. | Land equivalent ratios above 1 are documented for the maize/bean/squash polyculture, driven by root-niche complementarity — but the trials used climbing common bean, not garden pea, and corn's summer timing rarely overlaps a spring pea crop. |
| Pepper (sweet/bell) + Potato | Potatoes near peppers spread nightshade-family disease and Colorado potato beetle. | Both are Solanaceae and share Verticillium wilt, early blight and Colorado potato beetle; extension disease guidance is built around rotating all nightshades on a 3-4 year cycle. |
| Pole bean + Sweet corn | Corn provides the living pole for climbing beans in the Three Sisters polyculture. | The polyculture genuinely out-yields the component monocultures on a land-equivalent basis; the measured driver is complementary root foraging, and the widespread claim that the beans supply the corn's nitrogen within the same season is not supported. |
| Pole bean + Squash | Sprawling squash suppresses weeds beneath the corn-and-bean canopy. | The full three-crop system is documented to out-yield monocultures; extension reviewers note the practical trade-off is difficult harvest access. |
| Potato + Tomato | Potatoes and tomatoes grown together amplify early blight, late blight and Colorado potato beetle. | Documented rather than folkloric: Alternaria solani and Phytophthora infestans both infect the two crops, inoculum survives in soil and debris, and extension management for both is built on separation and a 3-4 year nightshade rotation. |
| Pumpkin + Blue Hubbard Squash | A perimeter row of Blue Hubbard concentrates striped cucumber beetles away from the pumpkin crop. | Strongly supported: over 94% of beetles remained on the perimeter in replicated trials, main-crop beetle numbers dropped by up to 95%, and treating only the border cut insecticide costs 92-96% with no yield loss. Cucurbita maxima types are more attractive to beetles than C. pepo pumpkins, which is why the system works. |
| Pumpkin + Nasturtium | Nasturtium planted among pumpkin vines reduces squash bug pressure. | Listed by University of Minnesota Extension among the companion claims research does support; the effect size is not published, so treat it as a supplement to hand-picking and row cover rather than a control measure. |
| Pumpkin + Sweet Corn | In the Three Sisters polyculture, corn, beans and squash together yield more per unit area than the same crops grown separately. | Land equivalent ratios above 1 are documented and attributed to root-niche complementarity rather than to nitrogen sharing; note corn grain holds up well while the bean and squash components individually yield less than in monoculture. |
| Pumpkin + Buckwheat | Flowering buckwheat draws pollinators for pumpkin flowers and supports predators of aphids and beetles. | Insectary strips raising natural-enemy numbers is trial-supported; pumpkins are entirely bee-pollinated, so the pollinator half of the claim is the more dependable one. |
| Pumpkin + French Marigold | Marigold grown as a preceding cover crop reduces root-knot nematodes in pumpkin ground. | Supported only for a dense, full-season preceding stand producing alpha-terthienyl; scattering marigolds between hills is not the tested practice and should not be expected to reproduce the result. |
| Rhubarb + Curly Dock / Wild Dock | Wild dock growing near the garden sustains rhubarb curculio and should be eliminated. | This one is grounded in documented host biology rather than folklore: Illinois Extension states that rhubarb curculio 'bores into the stalks, crowns, and roots' and instructs gardeners to 'destroy all wild dock growing around the garden' because the beetle also attacks these related plants. It is extension pest-management guidance rather than a replicated field trial, but the alternate-host relationship is established, and dock is a Rumex relative of rhubarb in the same family. |
| Spinach + Radish | Fast-maturing radishes use the between-row space before spinach fills in, and pull flea beetles off the spinach. | Radish and daikon are repeatedly confirmed as effective flea beetle trap crops in replicated trials, but nearly all of that work used brassica cash crops rather than spinach, so the transfer is an inference. |
| Spinach + Buckwheat | A nearby buckwheat strip feeds hoverflies, lacewings and parasitoid wasps that attack spinach aphids. | University of Minnesota Extension cites trials in which flowering buckwheat and cowpea measurably increased predator and parasitoid numbers; the tested target pest was corn earworm, so the aphid benefit is extrapolated from the same mechanism. |
| Summer squash / zucchini + Blue Hubbard squash | A perimeter of Blue Hubbard around summer squash intercepts striped cucumber beetle and squash vine borer. | The best-documented trap-cropping result in home vegetable growing: Connecticut on-farm work cut cucumber beetle infestation by up to 95 percent and squash vine borer by up to 88 percent, and a later trial showed buttercup squash works as well as Blue Hubbard while being marketable. |
| Summer squash / zucchini + Nasturtium | Nasturtium planted with squash reduces squash bug numbers. | UMN Extension states this age-old practice is supported by research; an Iowa study found nasturtium reduced squash bug and cucumber beetle damage, though effect size may depend on planting a substantial quantity rather than a token few. |
| Summer squash / zucchini + Sweet alyssum | Sweet alyssum increases populations of natural enemies of aphids in squash. | Cited by extension alongside African marigold as raising aphid natural-enemy numbers in squash; note this is a biological-control effect on beneficials, not direct repellence of pests. |
| Summer squash / zucchini + Sweet corn | Squash is the ground-cover sister in the corn-bean-squash polyculture. | The three-crop system is documented to out-yield the component monocultures on a land-equivalent basis; the yield advantage traces to complementary root foraging rather than to any pest effect. |
| Summer squash / zucchini + Buckwheat | A buckwheat border reduces cucumber beetle pressure and feeds beneficial insects. | Supported by perimeter-planting trial work, but the strongest published result was on melon rather than summer squash, so treat the magnitude as provisional. |
| Sweet corn + Buckwheat | Buckwheat interplanted with sweet corn raises populations of corn earworm predators. | In replicated field trials, Orius (minute pirate bug) abundance was significantly greater in buckwheat-interplanted corn than in monocrop, apparently because the flowers supplied both prey and nectar. |
| Sweet corn + Cowpea | Cowpea interplanted with sweet corn increases parasitism of corn earworm eggs. | The same trials found significantly higher Trichogramma parasitism of Helicoverpa zea eggs in cowpea-interplanted corn, because cowpea hosts alternate insect eggs that sustain the wasp population. |
| Sweet corn + Sunn hemp | Sunn hemp interplanted with sweet corn raises corn earworm egg parasitism. | Performed like cowpea in the same replicated trials, providing alternate host eggs for Trichogramma; extension summarises buckwheat, cowpea and sunn hemp together as the flowering plants shown to boost corn earworm biocontrol. |
| Sweet corn + Other corn types (supersweet, standard, field, popcorn) | Corn types that shed pollen at the same time cross-pollinate and ruin kernel quality. | The best-documented antagonism in the vegetable garden and pure genetics, not folklore: pollen from a different type produces hard, starchy kernels on supersweet (sh2) ears, so extension specifies isolating sh2 from all other corn by 500 feet or more, especially downwind, or staggering maturity dates instead. |
| Sweet Potato + French Marigold (Tagetes patula) | A solid stand of French marigold grown as a cover crop before sweet potato reduces root-knot nematode populations in the soil. | Real but narrow: UF/IFAS reports Tagetes suppresses root-knot (Meloidogyne) and lesion (Pratylenchus) nematodes, but only as a dense solid planting held at least two months before the crop, repeated every season; it states outright that 'intercropping marigold with other crops to reduce plant-parasitic nematodes does not appear to be effective' and that marigold raises stubby-root, sting, spiral and awl nematode numbers. Scattering a few marigolds among sweet potato slips does nothing. |
| Sweet Potato + Sunn Hemp | A summer cover crop of sunn hemp or velvetbean ahead of sweet potato lowers southern root-knot nematode pressure. | Schloemer et al. (2025) screened summer cover crops against Meloidogyne incognita on sweetpotato; velvetbean cut nematode populations roughly 94% relative to elbon rye and sunn hemp was among the poorest hosts. Limits: greenhouse and microplot only, not replicated field-scale sweet potato yield trials, and sunn hemp must be terminated before the crop rather than intercropped. |
| Sweet Potato + Daikon Radish | Daikon radish grown as a winter cover crop before sweet potato keeps root-knot nematode populations low. | In the same 2025 Journal of Nematology screening, daikon radish and elbon rye were poor hosts and supported minimal M. incognita reproduction, and winter grasses plus daikon radish in mixes suppressed populations best. Evidence is greenhouse/microplot, and the benefit comes from the rotation slot, not from radishes growing alongside the crop. |
| Sweet Potato + Cereal Rye | A winter cover crop of cereal rye with crimson clover, turned down before planting, suppresses early-season weeds in sweet potato. | Werle et al. (2023) found weed biomass lower in cover-cropped plots than bare soil at one of two Arkansas sites, and cover crop biomass correlated positively with several yield grades. Weed interference itself cut sweet potato yield 60-80%, so weed control matters far more than any decorative companion. Single-year, two-site trial. |
| Sweet Potato + Field Pea | Field pea is often suggested as a soil-building winter legume, but planting it immediately before sweet potato makes root-knot nematode damage worse. | Schloemer et al. (2025) found field pea supported significantly more Meloidogyne incognita per gram of root than any other winter cover crop tested, with a reproductive factor of 15.3 — it multiplies the nematode fifteenfold right before a highly susceptible crop. Greenhouse and microplot data. |
| Sweet Potato + Signet Marigold (Tagetes tenuifolia) | Signet marigold planted for nematode control actually feeds the nematode instead. | UF/IFAS advises that signet marigolds should be avoided because nematodes will feed and reproduce on them, and notes that interplanting marigold with a susceptible crop is 'very risky'. Species and cultivar choice matters more than the plant simply being a marigold. |
| Swiss Chard + Radish | Radish sown alongside chard acts as a preferred host that draws flea beetles away. | Radish and daikon trap cropping for flea beetles is supported by replicated trials and extension IPM guidance, though the published work used brassica cash crops; efficacy depends on sowing the trap 1-2 weeks early and devoting roughly 10% of the area to it. |
| Swiss Chard + Buckwheat | Flowering buckwheat nearby recruits predators and parasitoids that suppress aphids on chard. | Extension cites trials showing buckwheat and cowpea raise predator and parasitoid numbers; the effect on chard aphids specifically has not been quantified. |
| Tomato + Basil | Interplanted basil reduces thrips pressure on tomatoes and may improve tomato growth. | UMN Extension states a few studies show basil can reduce thrips populations in tomatoes; the separate folk claim that basil improves tomato flavor has never been tested. |
| Tomato + French marigold | French marigold planted with tomato reduces whitefly settling on the crop. | A controlled PLOS ONE study showed the effect is real and is caused by airborne limonene from the marigold, not by generic 'pest repellence'; the separate root-knot nematode benefit requires marigold grown as a pre-crop, not as an interplant. |
| Turnip + Collards | A border or block of turnip planted near collards or cabbage pulls harlequin bug off the main crop. | Harlequin bug demonstrably prefers mustard, turnip, kale, rutabaga and Chinese cabbage over cabbage, collards, broccoli and radish (Sullivan & Brett 1974, reviewed by Wallingford et al.), and border rows of mustard cut harlequin bug injury in collard by roughly 50% without insecticide in Virginia field trials — but the replicated trap-crop trials used mustard, not turnip, separation distance changed the result fourfold, and an unmanaged trap crop becomes a nursery unless it is treated or destroyed. |
| Turnip + Broccoli | Turnip sown as an edge strip acts as a trap crop that keeps flea beetles off broccoli and other cole crops. | George, Port & Collier (Insects 10:286, 2019) found turnip rape borders took significantly more flea beetle feeding than the cauliflower they surrounded — a genuine trap effect — but simply placing the trap at the crop edge did not reliably reduce damage to the main crop; a 3-6 m bare-soil gap worked better, and the trap crop also raised overall beetle pressure in those plots. Benefit was clearest late in the season under high pest pressure. |
| Winter Squash (Butternut) + Blue Hubbard Squash (perimeter trap crop) | An unbroken perimeter of Blue Hubbard or buttercup squash pulls striped cucumber beetles off the butternut main crop. | The strongest research-backed pairing on this list, and it was tested with butternut as the main crop specifically. UMass Extension field trials (2003-2006, Massachusetts) found beetles concentrated on the Cucurbita maxima border, the never-sprayed main butternut crop fared as well as a conventionally sprayed field, and insecticide savings were 92-96%. A peer-reviewed comparison (Adler & Hazzard 2009, Environmental Entomology 38(1):207, DOI 10.1603/022.038.0126) found butternut flowers had the fewest beetles when ringed by Blue Hubbard or buttercup, with no penalty to pollinator visits; buttercup works as well as Blue Hubbard and is more marketable. Limit: the border must be sprayed, vacuumed or otherwise cleaned of beetles, or it becomes a beetle and bacterial-wilt nursery. Utah State ran a parallel 2021 trial for squash bug and also found significantly fewer adults, nymphs and eggs on the protected main crop. |
| Winter Squash (Butternut) + Corn | The Three Sisters intercrop of corn, pole bean and squash produces more total food per unit of land than the same crops grown separately. | Supported by measured land-equivalent ratios. Zhang et al. (2014, Annals of Botany) found LERs for maize/bean/squash polyculture were greater than 1 in all but one of the nutrient-level and season combinations tested, and the three-species mix outperformed maize/bean by 15.5-17.8% in 2007, attributed to complementary rooting depths. Important limit: the advantage is in TOTAL land productivity, not in any one crop - individual squash and bean yields in polyculture are often lower than in monoculture, so a gardener growing squash for maximum squash should not expect a gain. |
| Winter Squash (Butternut) + Pole Bean | Pole beans climb the corn and fix nitrogen while squash vines shade out weeds below - the third element of the Three Sisters. | Same land-equivalent-ratio evidence as above: the three-way polyculture beat monocultures on a land basis. But in that study bean biomass in polyculture was the same as or LESS than in monoculture, so the bean is the component that gives ground. The system pays off in total productivity per area, not per species. |
| Cantaloupe (Muskmelon) + Blue Hubbard Squash (perimeter trap crop) | An unbroken perimeter row of Blue Hubbard or buttercup squash concentrates striped cucumber beetles on the border and keeps them out of the melon planting. | This is one of the few companion-planting practices with real field-trial data behind it. UMass Extension ran small-plot and whole-field trials in Massachusetts from 2003-2006: beetles concentrated on the Cucurbita maxima border, the unsprayed main crop fared as well as a conventionally sprayed field, and insecticide savings ran 92-96%. The trials were done with butternut squash as the main crop, not melon, so extending it to cantaloupe is an inference from the same beetle preference. Critically, the border only works if beetles in it are actually killed or removed - an unmanaged trap crop becomes a beetle and bacterial-wilt nursery. |
| Strawberry + Borage | Borage planted with strawberries increases fruit set, yield and berry quality. | The strongest garden-scale companion planting result found: a replicated 2020 study combining researcher-led trials and citizen science recorded 35 percent more fruits and 32 percent greater yield by weight, driven by increased pollinator visitation. |
| Watermelon + Blue Hubbard Squash | A perimeter row of Blue Hubbard intercepts striped cucumber beetles before they reach the melon crop. | The strongest companion-planting evidence in this dataset. In UConn and UMass trials over 94% of beetles stayed on the perimeter, main-crop beetle numbers fell by up to 95%, and growers cut insecticide costs by 92-96% while the unsprayed main crop yielded as well as a conventionally sprayed one. |
| Watermelon + Nasturtium | Nasturtium interplanted among vines reduces squash bug numbers. | University of Minnesota Extension lists nasturtium reducing squash bug populations among the insect-management claims that research does support; note it is far weaker evidence than the Blue Hubbard perimeter system and most trial work is on squash rather than watermelon. |
| Watermelon + French Marigold | A solid marigold cover crop grown before melons lowers root-knot nematode populations. | Well documented, but only for a dense preceding stand grown for a full season; root-knot nematode is a real watermelon problem, and interplanting a handful of marigolds among vines is not the tested practice. |
| Watermelon + Buckwheat | Buckwheat borders supply pollen and nectar to pollinators and to predators of aphids and beetles. | Flowering insectary strips are demonstrated to raise natural-enemy numbers; watermelon also depends heavily on bee visitation, so the pollinator benefit is meaningful even where the pest effect is not measured. |
| Cilantro / Coriander + Cabbage | Flowering cilantro feeds parasitoid wasps that attack cabbage aphids and caterpillars. | Cilantro nectar measurably extended parasitoid wasp survival in lab feeding trials (median +3.5 days vs water), but no trial has measured reduced cabbage damage from a cilantro interplanting. |
| Common Thyme + Broccoli | Thyme interplanted with broccoli reduces cabbage looper and imported cabbageworm damage. | The strongest aromatic-herb result in this dataset, and unusually it comes from the field: UMN Extension cites an Iowa study in which thyme, onion and nasturtium reduced cabbage looper and imported cabbageworm damage in broccoli. |
| Common Thyme + Brussels Sprouts | Thyme reduces diamondback moth populations on brassicas. | Supported, with the caveat that the sage and thyme diamondback moth result was obtained in a greenhouse rather than an open garden. |
| Dill + Cabbage | Dill flowers are among the best nectar sources for the parasitoid wasps that attack brassica pests. | Dill was the single best floral nectar source tested, raising parasitoid median survival by 17.5 days over water and sharply increasing wasp glycogen reserves; the crop-level payoff still needs field confirmation. |
| Dill + Broccoli | Flowering dill recruits parasitic wasps and hoverflies that suppress aphids and loopers. | Clemson Extension explicitly recommends dill as a small-flowered insectary plant; the nectar benefit to parasitoids is measured, the yield benefit is not. |
| Garden Sage + Brussels Sprouts and Brassicas | Sage reduces diamondback moth populations on brassicas. | One of the few aromatic-herb claims with trial support: UMN Extension reports sage and thyme reduced diamondback moth populations on Brussels sprouts. Read the caveat carefully though, because that result was obtained in a greenhouse, which matches WSU's finding that aromatic volatiles accumulate in closed systems but disperse too fast in open gardens. |
| French Marigold + Squash | Marigolds interplanted with squash reduce pest pressure. | UMN Extension cites a Florida study in which marigolds were effective with squash; UMN's overall verdict on marigolds is candid, sometimes yes and sometimes no. |
| Sunflower + Spring Wheat and Barley | Sunflower residue suppresses germination of following cereal crops. | Aqueous sunflower extracts reduced germination of spring barley and wheat by 33-44 percent in laboratory assays; the author notes extract results may not fully reflect living-plant field conditions. |
| Sweet Alyssum + Broccoli | Strip-intercropped alyssum supplies floral resources for aphid predators in broccoli. | USDA-ARS found broccoli outcompetes alyssum, so bed sections planted with alyssum alone are recommended for all-season flowers, with additive intercropping used for early-season bloom. |
| Sweet Alyssum + Aphid-Prone Crops Generally | A border of alyssum raises beneficial insect numbers and suppresses aphids across the bed. | UC Davis reported aphid suppression extending up to 50 feet from the alyssum planting, with parasitic wasps, lacewings and hoverflies as the responsible predators. |
Widely repeated, not supported
32These claims appear on companion planting charts everywhere. They have been tested, and they did not hold up — or the research behind them says something much narrower than the claim.
| Pairing | The claim | What's actually true |
|---|---|---|
| Arugula + Thyme | Aromatic thyme is said to repel cabbage moths and flea beetles with its scent. | This one has actually been tested. When aromatic plants were compared against non-aromatic green plants around brassicas, the aromatic species were no more effective: any living green ground cover reduced egg-laying, and dried brown plant material did not. The reduction comes from disrupted host-finding over a green background, not from thyme's volatiles, so the scent-repellency claim as usually stated is not supported. |
| Bok Choy + Thyme | Aromatic thyme is said to repel cabbage moths and flea beetles from bok choy. | Tested and not supported as stated. Experiments comparing aromatic and non-aromatic companion plants around brassicas found aromatics were no more effective than plain green plants, and desiccated plants gave no protection at all — the reduction in egg-laying tracks green ground cover, not aroma. Interplanting thyme may still help slightly, but not for the reason the folklore gives. |
| Broccoli + Mint, rosemary and other aromatic herbs | Strongly scented herbs are said to mask the smell of broccoli and repel cabbage root fly and cabbageworm. | Directly tested and not supported. Finch, Billiald and Collier screened 24 non-host plants and found aromatic species — curry plant, marigold, mint, sage, thyme and onion — were no better than non-aromatic ones at disrupting cabbage root fly and onion fly. Green paper models emitting no volatiles worked as well as living clover, showing the effect is visual, not olfactory. Any low green ground cover does the job; the aroma is irrelevant. |
| Bush bean + Marigold | Marigolds planted with beans repel Mexican bean beetle. | No study supports it and the general marigold repellency premise fails when tested: Rutgers work found marigolds failed to repel cabbage, carrot and onion pests, and UMN reports multiple studies disproving the parallel Colorado potato beetle claim. |
| Cabbage + Mint, sage, thyme and other aromatic herbs | Pungent herbs are said to mask cabbage's scent and drive away cabbage root fly and cabbageworm. | Tested and not supported. Across 24 non-host plants, aromatic species including mint, sage, thyme, marigold and onion were no more effective than non-aromatic ones at disrupting cabbage root fly, and green paper models releasing no chemicals performed as well as living clover. The protective effect comes from green ground cover interfering with landing, not from smell. |
| Carrot + Tomato | Tomatoes improve carrot growth and flavor. | The famous claim behind Carrots Love Tomatoes (Riotte, 1975), a folk compilation containing no experiments or citations; no controlled trial has ever supported it. |
| Collard Greens + Thyme | Aromatic herbs like thyme, sage and mint are said to repel cabbage moths from collards. | Tested and not supported in the form usually claimed. Experiments comparing aromatic against non-aromatic companion plants around brassicas found aromatic species were no better than ordinary green plants, and aromatic plants did not repel the flies so much as interrupt their host-selection sequence; dried plant material produced no effect at all. Any benefit comes from green ground cover, not from thyme's scent. |
| Cucumber + Sage and other strong aromatic herbs | Strongly aromatic herbs planted nearby make cucumbers taste bitter. | Bitterness comes from cucurbitacin, and extension attributes its concentration to the cucumber variety itself (fruits are bitterest at the blossom end); neighbouring plants cannot cause it, and cucumbers cannot cross-pollinate with herbs. |
| Garlic + Tomato | Garlic is claimed to repel spider mites and aphids from tomatoes. | The one controlled test located — leek grown as a companion to sweet pepper — found allium volatiles disrupted aphid feeding but increased aphid survival from 65% to 91% and raised fecundity, and the authors concluded the allium was unsuitable for pest management. Allium 'repellency' toward sap-feeders should not be assumed. |
| Kohlrabi + Thyme | The strong scent of thyme masks kohlrabi and repels cabbage moths and root flies. | Thyme was one of the aromatic species directly tested by Finch, Billiald and Collier; aromatic plants were no more effective than non-aromatic ones, and green paper models with no scent at all worked equally well, so the scent-masking mechanism is tested and unsupported. |
| Kohlrabi + Marigold | Marigolds planted among kohlrabi repel cabbage root fly and cabbageworms by smell. | Marigold was also in the Warwick aromatic-plant trials and performed no better than any other green plant; marigold's genuine, well-supported use is as a full-season cover crop against root-knot nematodes, which is a different practice entirely. |
| Parsnip + Dill | Dill cross-pollinates with parsnip and ruins the roots. | Botanically impossible: dill is Anethum graveolens and parsnip is Pastinaca sativa, different genera that cannot cross, and in any case cross-pollination would only affect seed, never the root you harvest in year one. |
| Potato + Marigold | Marigolds planted among potatoes repel Colorado potato beetle. | Directly contradicted: UMN Extension states that multiple studies have shown this to be untrue, and Rutgers work found marigolds failed to repel cabbage, carrot and onion pests as well. |
| Swiss Chard + Bush Bean | Beans are said to enrich the soil for leafy chard through nitrogen fixation. | Fixed nitrogen stays overwhelmingly in the legume's own tissue during the season; reviews of isotope-tracer work find same-season belowground transfer to non-legume neighbours is small and inconsistent, with the real benefit arriving after residues decompose. |
| Turnip + Marigold | Marigolds interplanted among turnips and other brassicas repel flea beetles. | UMN Extension states directly that companion plants such as marigolds and green onions are listed for flea beetle control on gardening sites and even some extension websites, 'but there is little research to support this and some contradict it'; Illinois Extension likewise reports the marigold-as-beetle-deterrent claim tested and not supported. Marigold's genuinely evidenced use is as a solid-stand pre-plant cover crop against root-knot nematodes, not as a few interplanted flowers. |
| Winter Squash (Butternut) + Buckwheat | A flowering buckwheat border feeds parasitoids and predators that then control squash bugs in the crop. | Tested directly and not supported. Cornelius, Vinyard & Gates (2019, Insects 10:318) ran a two-year field study with a 3 m flowering buckwheat border around squash fields and concluded 'flowering buckwheat did not increase parasitism rates of squash bugs in squash fields' - no significant effect on egg parasitism (about 12% either way) or adult parasitism, despite pollinators visiting the buckwheat heavily. Egg predation was under 1% overall. Graded myth for this specific biocontrol claim only; buckwheat remains a legitimate cover crop and pollinator forage. |
| Winter Squash (Butternut) + Cucumber | Cucumbers planted near butternut will cross-pollinate it and make the squash bitter or misshapen. | Wrong twice over. Butternut is Cucurbita moschata and cucumber is Cucumis sativus - different genera entirely; Iowa State Extension states female flowers can only be fertilized by pollen from the same species, and SDSU adds that melons and squashes cannot cross with cucumbers. And even a genuine within-species cross does not change the fruit you harvest this year - only the seed inside it. Butternut will cross with other C. moschata (cheese pumpkins, Seminole), but not with zucchini or acorn (C. pepo) or hubbard and buttercup (C. maxima), which matters only for seed saving. |
| Cantaloupe (Muskmelon) + Cucumber | Keep cucumbers away from cantaloupe or they will cross-pollinate and ruin the melon's flavor. | Flatly contradicted on two independent grounds. First, cucumber (Cucumis sativus) and melon (Cucumis melo) are different species and cannot cross - Iowa State Extension: 'the female flowers of each crop can be fertilized only by pollen from male flowers of the same species.' Second, even where a cross IS possible, it does not change the fruit you eat this year: 'when crosses occur between members of the same species, we do not see the effect of the cross in the first year.' Only saved seed is affected. There IS a legitimate reason to separate the two, but it is shared cucumber beetles and bacterial wilt, not flavor. |
| Watermelon + Bush Bean | Beans are said to feed melons nitrogen while the vines are still small. | Same-season nitrogen transfer from legumes to neighbours is small and inconsistent in isotope-tracer studies; fixed nitrogen mostly stays in the bean until residues break down, so this benefits next year's crop, not this year's melons. |
| Chives + Carrot | Chives mask the carrot scent and deter carrot rust fly. | The allium-masks-carrot-fly belief is the aromatic-repellency claim WSU Extension reports field research has not borne out. |
| Common Thyme + Eggplant | Thyme is said to protect eggplant from flea beetles. | No field support for aromatic repellency; flea beetles are best excluded with row cover. |
| Dill + Fennel | Dill and fennel should not be planted together because they cross-pollinate. | Extension guidance does say keep them apart, but the cross-pollination rationale is wrong: Anethum and Foeniculum are different genera and do not interbreed. |
| Garden Sage + Carrot | Sage is said to deter carrot rust fly. | Unlike the diamondback moth result, this one has no trial behind it and falls under the aromatic-repellency belief WSU reports field research does not support; exclusion netting is the control that works. |
| Greek Oregano + Tomato | Oregano repels tomato pests and improves tomato flavour. | Aromatic repellency is not supported by field research, and a three-year double-blind taste test found no consistent flavour preference for companion-grown tomatoes. |
| Greek Oregano + Eggplant | Oregano is said to deter flea beetles on eggplant. | Falls under the aromatic-repellency belief WSU Extension reports is not borne out in field research. |
| Rosemary + Cabbage | Rosemary's scent repels cabbage moths and carrot flies. | This is the specific belief WSU Extension reports field research has not borne out for any aromatic species in an open garden. |
| Rosemary + Bean | Rosemary is said to deter Mexican bean beetle. | No field evidence; volatile oils disperse too quickly outdoors to protect a neighbour. |
| Rosemary + Carrot | Rosemary is said to mask the scent carrot rust fly uses to find carrots. | Scent-masking by aromatic companions is not supported by field research. |
| Spearmint + Cabbage | Mint's aroma repels cabbage moths and flea beetles. | WSU Extension is explicit that field research has not borne this out for mint or any other aromatic species, because the volatile compounds dissipate too fast in an open garden. |
| Spearmint + Tomato | Mint planted nearby deters aphids and improves tomato vigour. | Same aromatic-repellency failure; aromatic plants may work in closed greenhouses but not in open gardens. |
| Spearmint + Carrot | Mint is said to mask the scent that guides carrot rust fly. | Scent-masking by aromatic herbs is the specific belief WSU Extension reports field research does not support. |
| Spearmint + Kale | Mint is planted near kale to reduce aphid colonisation. | No repellent effect demonstrated in the field; sweet alyssum is the evidence-backed choice for aphid suppression. |
How to use these records
Start with the chart for an overview, then open the record for whatever you're actually planting — each one carries the full pairing tables, growing data, planting windows, and sources. If you're laying out a bed, the planner checks these same records square by square as you place crops.
And read the evidence review before you take any chart, including ours, too seriously.
Common questions
What is companion planting?
Growing certain plants near each other in the belief that one benefits the other — by repelling pests, attracting beneficial insects, improving flavour, providing shade or support, or feeding the soil. Some of those mechanisms are real and measured. Many of the specific pairings people repeat have never been tested.
How do you decide what counts as evidence?
A pairing is graded research-backed only when we could find a controlled trial, extension research, or peer-reviewed study measuring the effect. Traditional means the advice is widespread and plausible but we found no controlled test. Not supported means someone tested it and it failed. Full criteria are on the methodology page.
Which companion planting claims are actually proven?
Of 542 pairings recorded here, 129 met the research bar. They are listed in full on this page. The honest headline is that companion planting as practised is mostly untested tradition, with a small core of genuinely documented effects around trap cropping, insectary flowers, and physical support and shade.