Most companion-planting chart entries are tradition, not research. Here's the short list of pairings that hold up, the famous ones that don't, and how to tell which is which.
Companion planting is the most popular garden idea with the least evidence behind it. The familiar charts - carrots love tomatoes, this plant "protects" that one - are largely tradition and hearsay copied from book to blog to book, and most of the pairings have never been tested. That doesn't mean none of it is real. It means you have to sort the few real effects from the many folk ones, which is exactly what this planner is built to do.
The chart itself is the folk belief
Start with the thing the charts assume - that a grid of "good neighbours" encodes real relationships:
RefutedCompanion planting charts encode real relationships.
Most companion planting advice you'll find online is folklore. Here's what we can actually defend, and why.
what is actually known
The bulk of the modern companion-planting canon descends from early-20th-century biodynamic and anthroposophical writing, propagated through mid-century popular gardening literature. Individual pairings are rarely trialled. A minority have real named mechanisms — volatile masking, living mulch, insectary provisioning — and are recorded here as a good hunch. Most have nothing.
what this rests on - one pointer
history of the companion planting literature
And the deeper assumption underneath it, that simply putting plants together makes them a functioning team, is one the corpus refutes outright:
Well establishedAny co-planted set of vegetables is a guild.
This is a claim the planner refuses to enforce — recorded here with what is actually known, rather than dropped.
none if ignored → refute
What is actually known: A guild asserts a mechanism between its members. A culinary bundle asserts only that you will want these things in the same bowl. Both are legitimate. Only one is a polyculture.
Evidence: settled without a trial - definition.
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A real guild names a mechanism you can state and check. A chart names a pairing and asks you to trust it.
The short list that does hold up
A few things in this territory are genuinely supported, and they earn their place by naming a mechanism:
- The Three Sisters' structure is real - corn is a living trellis, the squash's sprawl shades out weeds. Those are physical facts, not chemistry, and the milpa guild is built on them.
- Flowers that pull in beneficial insects help, because more pollinators and predators visit.
- Perimeter trap cropping - a sacrificial border crop that draws a pest off the main one - works in specific, tested systems.
The planner carries the honest, hedged versions of the companion claims that have some support, each graded for exactly how sure we are:
A good hunchInsectary plants near fruiting crops increase pollinator and predator visitation.
suboptimal if ignored → advise
Mechanism: The flowers supply nectar and pollen that adult predators and parasitoids feed on, so a patch of them near a fruiting crop keeps those beneficial insects fed and close by to work the crop. The effect is real but not guaranteed — an insectary patch does not always suppress pests.
Evidence: confirmed against 1 source in the research-based literature.
the source
UC Statewide IPM Program (UC IPM), 'Insectary Plants', ipm.ucanr.edu/environmental-protection-and-pesticide-stewardship/insectary-plants (read 2026-07-11): 'Insectary plants are those grown to attract, feed, and shelter insect parasites (parasitoids) and predators to enhance biological pest control. Insectary plants provide nectar and pollen, which the adults of many natural enemy species need to consume.' (UC IPM also notes 'insectary plants may not always help' suppress pests — consistent with the 'reasonably supported' framing.)
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A good hunchAromatic species interplanted with brassicas are reported to reduce cabbage-worm damage. The mechanism is unresolved and trial results are mixed.
suboptimal if ignored → advise
Mechanism: Unresolved, and the rule says so rather than picking one. Volatile masking of host-finding cues and disrupted landing are both proposed. The trials that report an effect measured DAMAGE, which cannot separate fewer eggs laid from fewer larvae surviving or more predators arriving - so no mechanism here is established, only candidates.
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
Univ. of Minnesota Extension, 'Companion planting in home gardens', extension.umn.edu/planting-and-growing-guides/companion-planting-home-gardens (read 2026-07-25): 'In an Iowa study, thyme, onion and nasturtium helped to reduce cabbage looper and imported cabbageworm damage in broccoli' - citing Riesselman, Leah B. (2006), 'Companion Planting: A Method for Sustainable Pest Control', Iowa State Research Farm Progress Reports. Thyme is this guild's canonical masking species, so the one trial found is on the plant the corpus actually prescribes.
Same page, on the wider claim base: 'While gardening charts online will provide you with long lists of plants that repel insects, these are not always accurate or backed by research', and of marigolds specifically, 'Sometimes yes and sometimes no.' That is the mixed picture this rule's effect_size asserts, now cited rather than claimed.
PRIMARY READ: the maintainer read Riesselman (2006) itself on 2026-07-25, at dr.lib.iastate.edu/farms_reports/295, and promoted this rule on it - not on Minnesota's summary alone. The trial's own measurements are not transcribed into this pointer; what the corpus asserts is still the DAMAGE claim the summary reports, and the read did not overturn it.
GAP that survives the read: the effect is measured as damage, and damage cannot separate fewer eggs laid from fewer larvae surviving or more predators arriving. That is why this claim does not mention egg-laying and why the mechanism is stated as unresolved. Verified means the source was read, not that the mechanism is known.
NEGATIVE RESULT, recorded so it is not cited by its title: Morkeliune et al. (2024), 'Effect of White Cabbage Intercropping with Aromatic Plant on Yield, Mineral and Biochemical Composition', Plants 13(13):1870, intercrops cabbage with calendula, French marigold, thyme and sage across 2017-2019 and measures NO pests, eggs or oviposition at all - yield and biochemistry only. It cannot support this rule. Nor can the '42% fewer egg clusters under sage' figure circulating in search results, which traces to commercial content sites and to no study.
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A good hunchAlliums interplanted with carrots reduce carrot rust fly damage.
suboptimal if ignored → advise
Mechanism: The carrot rust fly finds its host by the smell of carrot foliage. Onion and other allium volatiles growing among the carrots mask that scent, so fewer flies locate the row to lay eggs. Because the protection is the smell, it lasts only while the alliums are in the ground — once they are pulled, the masking goes with them.
Evidence: confirmed against 1 source in the research-based literature.
the source
New England Vegetable Management Guide (regional land-grant extension), 'Carrot and Parsnip - Insect Control' (Carrot Rust Fly, Psila rosae), nevegetable.org/crops/carrot-and-parsnip/insect-control (read 2026-07-11): 'Intercropping with onion has been shown to reduce damage by carrot rust flies.' Listed among partial/suppression measures (with row covers and staggered planting), consistent with the 'modest' effect_size. The Guide does not state the post-harvest collapse; that is the masking mechanism, not a claim from this source.
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Notice the grades: these are "a good hunch," not "well established." That is the point - they're offered with the uncertainty attached, not sold as fact.
The famous claims that don't survive checking
Two of the most-repeated deserve calling out by name. The first is the Three Sisters' own nitrogen story - that the beans feed the corn this season:
A good hunchBeans fix nitrogen that feeds the corn growing beside them, that season.
This is a claim the planner refuses to enforce — recorded here with what is actually known, rather than dropped.
none if ignored → refute
What is actually known: Rhizobia in legume root nodules do fix atmospheric N. Most of it is incorporated into the legume's own tissue. Some does move to a neighbouring non-legume the same season via direct root contact and rhizodeposition, but the amount is small — a few percent of the neighbour's N uptake — and does not meet a heavy feeder's demand. The bulk of the benefit arrives the following season, after residues decompose.
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
Univ. of Arkansas Extension, 'Creating a Three Sisters Garden', uaex.uada.edu (read 2026-07-11): 'Most of the nitrogen converted by the beans will not be available to the corn and pumpkins the first year; the bean roots have to break down to release nitrogen. Corn is a heavy nitrogen [feeder].'
Hupe et al. 2021, Sci Rep 11:11424, 'Evidence of considerable C and N transfer from peas to cereals via direct root contact but not via mycorrhiza', pmc.ncbi.nlm.nih.gov/articles/PMC8169652 (read 2026-07-11): reports 'a mean N transfer rate of 8.4%, with a range of 2.7 to 22.5%, ... from N2-fixing legumes to non-N2-fixing mycorrhizal cereals' and 'N transfer rates of between 3.1 and 6.0% ... from soybean to maize'.
'Nitrogen uptake and transfer in a soybean/maize intercropping system in the karst region of southwest China', pmc.ncbi.nlm.nih.gov/articles/PMC5648690 (read 2026-07-11): 'N transferred from soybean to maize account for 2.4%–3.3% of the N uptake of maize.'
Beans will improve this ground for next year. They will not feed your corn this year.
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Most of a legume's fixed nitrogen goes into the legume itself and reaches its neighbours only when its residue breaks down, largely the following year. The beans improve the ground; they are not fertilising this year's corn.
And the marigold's reputation as a general pest-repellent is mostly folk - its one real effect (suppressing certain nematodes) needs it grown as a full-season cover crop and tilled in, not dotted along a border.
A real negative pairing, for contrast
Lest this seem like nothing works: allelopathy - one plant chemically suppressing another - is real in a few well-documented cases, and the strongest is a plant to keep your vegetables away from:
Well establishedDo not plant a juglone-sensitive species within the root zone of Juglans. The sensitive list is the hazard's own, and it is wider than the nightshades - it also carries apple and cabbage.
fatal if ignored → block
Mechanism: Juglans roots exude hydrojuglone, oxidized to juglone, a respiration inhibitor. nightshades are highly sensitive: wilt and death.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UW-Madison / Wisconsin Horticulture (Joy, Hudelson & Jull, rev. 2024-02-28), 'Black Walnut Toxicity', hort.extension.wisc.edu/articles/black-walnut-toxicity (read 2026-07-13): 'Vegetables such as tomato, potato, eggplant and pepper ... are particularly sensitive to juglone'; 'The toxic effects of a mature black walnut tree can extend 50 to 80 feet from the trunk of the tree, with the greatest toxicity occurring within the tree's dripline.'
Iowa State Univ. Extension & Outreach, Yard and Garden FAQ, 'What plants are sensitive to the juglone produced by black walnuts?', yardandgarden.extension.iastate.edu (read 2026-07-13): juglone-sensitive plants include 'members of the tomato family (tomato, potato, eggplant, pepper).'
remedy
Plant it outside the walnut's root zone — the whole drip line and beyond. There is no soil treatment for juglone.
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How to tell, every time
When a chart claims plant X protects plant Y, ask one question: by what mechanism, and has it been shown? Usually the honest answer is that no one knows, and the pairing is tradition. The explainer why there's no companion-planting chart here makes the fuller case. The short version: grow the Three Sisters for its real, structural mutualisms - not because a grid told you to.
