The grid of good and bad neighbours is the most-copied thing in garden writing, and one of the least examined. Here's what happens when you actually check it.
The chart is the product everywhere else
Open almost any garden planner and you'll find a companion-planting chart: crops down one side, crops across the top, and every cell marked "plant together" or "keep apart." Basil with tomato. Carrots love tomatoes. Never plant beans near onions.
It's the easiest feature in the category to build. The data is just a list of pairs, nobody asks where the pairs came from, and anyone who follows it and has a good season gives the chart the credit.
This planner doesn't have one, and it never will. That's on purpose - not a gap we're getting around to.
Where the pairs actually come from
Most of the modern companion-planting canon traces back to early-twentieth-century biodynamic and anthroposophical writing, spread later through mid-century gardening books. It wasn't put together from trials. The individual pairings are rarely tested, and when someone does test them, most of them don't hold up.
The corpus records that finding as a claim in its own right, with the same grade and the same sources as anything else it says:
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
A grid can't tell you why, and that's the whole problem with it. "Plant together" isn't something you can check, because it never says what is supposed to happen, to which plant, by what route, or how much. The cell can't be proven wrong - which is exactly how it has survived a century without anyone really examining it.
What replaced it here
Every rule in this planner has to name the reason it works in one sentence, without leaning on the word "helps". If it can't, it gets downgraded or dropped. That one rule is what makes the chart impossible: almost no cell in a companion-planting grid can pass it.
What does pass is a much shorter list. Corn shades and supports. Squash leaves catch the light at soil level and slow evaporation. A legume pulls nitrogen out of the air and leaves it in the soil, through a partnership with bacteria on a timescale you can actually name. Those are real reasons. They're modelled here, and each one carries its evidence.
Some of these reasons are genuinely weak, and we show them weak rather than drop them. The one about aromatic herbs among brassicas is the honest edge case: something gets reported, nobody has pinned down why, and the trials disagree with each other.
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.
Read the full rule → · open in the planner ↗
That's what a real companion claim looks like when you write it down honestly. It's a lot less satisfying than a green cell in a grid. It's also the truth.
The specific beliefs, answered
A chart is also where a lot of individual folklore quietly hides. The corpus keeps 35 of these as records in their own right, each with our answer and, where one exists, a source that flatly refutes it. Two of the most commonly charted:
ContestedMarigolds repel most garden pests and protect the whole garden.
Marigolds don't repel pests in general. Their one real effect is narrow: a dense planting of the right Tagetes, grown before a crop, suppresses root-knot nematodes in the soil (not a scent that wards off flying insects). Enjoy them for colour and pollinators; use the nematode trick on purpose if you need it.
what is actually known
Marigolds have one real, narrow effect: some Tagetes suppress root-knot nematodes in the soil when grown as a dense preceding stand, via the root compound alpha-terthienyl — a soil biofumigant effect on a specific soilborne pest, handled as a role in this corpus, not a scent that wards off insects. The broad folk claim — that a few marigolds dotted among the vegetables repel aphids, whiteflies, cabbage moths, or rabbits by smell — has no reliable field support.
what this rests on - one pointer
University of Florida IFAS Extension, ENY-056 'Marigolds (Tagetes spp.) for Nematode Management' (read 2026-07-14): the effect is a dense preceding cover crop, not a scent repellent — 'alpha-terthienyl inhibits the hatching of nematode eggs'; 'Planting should be dense to ensure the best nematode control'; 'Marigolds cannot eradicate nematodes.' Backs the narrow root-knot-nematode mechanism; the general pest-repellent claim is unsupported.
Basil improves the flavour of adjacent tomatoes.
There's no evidence for this and no mechanism anyone has proposed. Plant basil next to tomatoes anyway — you'll want them at the same time in the kitchen.
what is actually known
No proposed mechanism. No trials. Extremely widely believed.
Neither one is a scandal. Marigolds do something real and narrow; basil is lovely next to a tomato, but the claim that it improves the flavour isn't supported. The point is that you can see which is which, instead of reading a cell that just says "companion" and filling in a meaning nobody ever claimed.
What you get instead
When this planner tells you to do something, it points to the rule behind it, and that rule names the reason it works, how strong the evidence is, and where it came from. When the evidence is thin, the page says thin. When nothing is known, it says that too, and treats the silence as the finding.
That's a slower thing to build and a harder one to sell than a grid. It's also the only version of this that is worth trusting.
