General principle — not a specific pairing
Grow a mix, not a monoculture
Reviewing decades of published studies, roughly half of pest species had measurably lower populations in diverse, mixed plantings than in a single-crop block.
"Roughly half" is the honest number, not "always" — the underlying research itself found real variability between pest species, not a uniform effect. A 2011 re-analysis of the same body of work using stricter statistical methods found a more consistent reduction. Not a specific pairing to plant — more a reason to mix crops in the same container or bed where you have the room, rather than one crop alone. The RHS's own advice independently backs the same general finding: mixing tomatoes with other plants reduced unwanted insect populations even without a specific companion like French marigolds involved — the diversity itself was doing some of the work.
Sources: Vegetational Diversity and Arthropod Population Response (Andow, 1991, Annual Review of Entomology) · Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
French marigolds release limonene, a compound that measurably slowed glasshouse whitefly population development on nearby tomatoes. The RHS also credits the same compound with deterring thrips on tomatoes and other crops.
The whitefly result is a specific, controlled, published trial (below). The thrips claim is the RHS's own institutional guidance rather than the same controlled trial — real, but a notch less rigorously tested than the whitefly result it sits alongside.
Sources: Companion planting with French marigolds protects tomato plants from glasshouse whiteflies (PMC) · Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
African marigold intercropped with kale increased natural pest-predator numbers 3.1-fold over a 13-week trial — the strongest result of four companion plants tested against the same pest. A distinct, taller species from French marigolds — the two aren't interchangeable despite the name.
Source: Companion plants associated with kale increase the abundance and species richness of natural enemies of Lipaphis erysimi (African Journal of Agricultural Research, 2016)
Calendula intercropped with kale increased natural pest-predator numbers 2.1-fold in the same trial.
Source: Companion plants associated with kale increase the abundance and species richness of natural enemies of Lipaphis erysimi (African Journal of Agricultural Research, 2016)
Coriander intercropped with kale increased natural pest-predator numbers 2.0-fold in the same trial.
Source: Companion plants associated with kale increase the abundance and species richness of natural enemies of Lipaphis erysimi (African Journal of Agricultural Research, 2016)
Dill intercropped with kale increased natural pest-predator numbers 1.6-fold in the same trial — the smallest of the four effects measured, but still real and measured.
Source: Companion plants associated with kale increase the abundance and species richness of natural enemies of Lipaphis erysimi (African Journal of Agricultural Research, 2016)
Sweet alyssum's small, nectar- and pollen-rich flowers draw in aphid-predator insects — the RHS specifically credits alyssum grown with lettuce with reducing aphid populations.
Sweet alyssum isn't a food crop and doesn't have its own page on this site — it's included here only because it's a real, RHS-documented way to support a crop that is: lettuce. Grow it in the same container purely as a working flower, not for eating.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
A well-established pollinator magnet — planting it near a fruiting crop draws in more bees, which genuinely improves fruit set on the crop it's grown alongside.
A controlled trial growing lettuce and radish together versus separately found genuinely higher land-use efficiency in the intercrop, with radish specifically showing higher total weight, root size and yield when grown alongside lettuce.
Not free of trade-offs: the same trial found water-use efficiency was actually higher when the two were grown apart — the yield benefit comes from using the same container/bed space more fully, not from either crop needing less watering.
Source: Lettuce and radish grown in single crop and intercropping systems under different irrigation water depths in a protected environment (Revista Caatinga, 2022)
A real space-efficiency technique, not a pest or nutrient one: sowing a fast-maturing crop (radish, lettuce) into the gaps around a slower, bigger one (Brussels sprouts, sweetcorn) that's been spaced for its full mature size — the fast crop is harvested before the slow one grows large enough to shade it out.
This is straightforward space-and-timing logic, endorsed by the RHS as standard practice, rather than a claim needing a controlled trial to make sense of.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
Growing lettuce in the light shade cast by a taller neighbour (staked tomatoes, climbing beans, sweetcorn) keeps it cooler through summer, which genuinely delays bolting — lettuce struggles once daytime temperatures climb past about 24-27°C, and even partial shade lowers the heat load on the plant.
This one doesn't need a companion-planting trial to stand up — it's basic plant physiology (heat triggers bolting; shade reduces heat), not a claim about a special pairing effect. The only real judgement call is not shading it so much it grows leggy and pale.
Sweetcorn+Climbing French beans+Runner beans+Squash (summer) Moderate evidenceA real structural benefit, not a nutritional one: corn stalks give climbing beans a natural trellis, and squash's broad leaves shade the soil, suppressing weeds and holding moisture.
The commonly repeated part of this claim — that the beans' nitrogen feeds the corn and squash in the same season — is overstated. Legumes fix nitrogen for their own growth; almost none of it reaches the soil around a live neighbour until the bean plant is cut down and left to break down.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
Cabbages+Cauliflower
Moderate evidenceGrowing a low, dense cover of clover or trefoil under cabbages and cauliflower disrupts cabbage root fly's ability to find its host plants by sight — the fly locates crops partly by the visual contrast of bare soil around them, and a green groundcover removes that cue.
Clover/trefoil aren't food crops and don't have their own page on this site — included here as a real, RHS-documented technique for protecting cabbages and cauliflower specifically. Keeping soil covered rather than bare is also independently good practice for soil health.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
Two separate real reasons to pair them, not one: alliums' stronger scent is commonly used to help mask carrots from egg-laying carrot fly, and separately, deep-rooted carrots and shallow-rooted onions genuinely use the same container or bed's underground space more efficiently, with little competition for light above ground.
The pest-masking half is widely used in practice and mechanistically plausible, but not backed by a controlled trial testing this exact pairing the way the marigold/kale results above are — the RHS itself describes it as a claim, not a proven result. The root-depth space-efficiency half is closer to straightforward physical fact than a claim needing testing.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
Coriander has been found to sometimes deter unwanted insects in crops of basil and parsley, per the RHS's own advice.
The RHS's own hedged wording ("sometimes") is the honest version — this is genuine institutional horticultural guidance, not a fabricated statistic, but it isn't a claim of reliable, tested protection either.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
Some strawberry growers plant garlic in with strawberries, cutting it down and spreading the chopped leaves over the growing medium every couple of weeks — the scent is intended to repel aphids.
Real, reported practice from working growers, per the RHS's own advice — not a claim invented for this list, but not a controlled result either.
Source: Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
Dwarf/bush beans+Broad beans (dwarf)+Climbing French beans+Runner beans+Dwarf peas Moderate evidenceLegumes host nitrogen-fixing bacteria on their roots, a real, well-established mechanism.
Most of the nitrogen fixed goes to the legume's own growth while it's alive, not straight into the soil for a neighbour — the real soil benefit mostly comes later, once the plant is cut down and left to break down (a genuine reason to compost bean plants in place rather than pull and bin them).
French marigolds
Moderate evidenceMarigold roots release a compound shown to suppress some root-knot nematode species by preventing their eggs from hatching.
More relevant to in-ground, small-garden growing than a typical container setup, where soil-borne nematodes are less of an established risk to begin with.
Commonly described as a "sacrificial" trap crop, drawing aphids and blackfly away from nearby vegetables — gardeners commonly plant it near runner beans and tomatoes specifically in the hope of this effect, per the RHS's own advice.
One of the most repeated companion-planting claims there is, but — checked directly — it has little to no controlled testing behind the specific claim, as distinct from the general observation that diverse planting can support more predator insects overall. Even the RHS's own phrasing ("in the hope of similar effects") stops short of endorsing it as reliable.
Sources: Do Companion Plants Actually Deter Pests on a Small Balcony? · Three ways to use companion planting (RHS, Guy Barter, RHS Chief Horticulturist)
