Marigolds Next To Your Tomatoes: Where That Advice Actually Comes From

Plant marigolds next to your tomatoes. Onions keep carrot fly off your carrots. Basil makes tomatoes taste better. Companion planting charts repeat these pairings so often that they sound like fact. Checked against RHS, Garden Organic and published trials, a few pairings hold up, many are untested, some are false, and two make things worse.

The short version: borage beside strawberries and French marigolds beside glasshouse tomatoes, planted from the start of the season, have real evidence. Outdoor tomatoes are marginal in much of Scotland, so that evidence mostly applies in a polytunnel or greenhouse. So does growing flowers among your crops to feed the insects that eat pests. Almost everything else, from onions protecting carrots to herbs repelling cabbage white, is unproven or has been tested and found false. Chard among leeks, and a herb border relied on instead of netting against cabbage white, have both been shown to make things worse.

Much of this folklore comes from US advisory services, herbals written for warmer, drier places, or English trial gardens. Several of the studies below are from Virginia, West Virginia or California, so where a result comes from outside the UK, the text says so. None of the studies below is described as a Scottish trial, so treat each result as UK or overseas evidence, not a Scottish finding.

What holds up

Flowers feeding pest predators. The adults of many natural enemies of pests need nectar and pollen. With nothing in flower they are shorter-lived and produce fewer offspring, and the benefit fades with distance from the flowers (UC IPM, California). RHS reports that alyssum grown with lettuce has been shown to reduce aphid populations by attracting the predators of aphids. Garden Organic suggests mixing coriander or fennel flowers, Californian poppy, poached-egg plant and calendula among vegetables. Plant them close among the crops, not as a border at the far end of the plot, and aim for something in flower through the whole season. See calendula in the veg patch.

French marigolds against whitefly, from the start. Glasshouse trials by Newcastle University, Stockbridge Technology Centre and Northumbria University found that French marigolds grown among short-vine tomatoes from the start of the season slow whitefly build-up, and identified limonene as the active chemical. Marigolds introduced after whitefly had built up gave minimal reductions. The same trials found that basil, nasturtium and Chinese cabbage, which are not whitefly hosts, also reduced whitefly on tomato, and mixing tomatoes with other plants, without Tagetes, had a similar effect (RHS). So the lesson is to mix plants rather than to rely on marigolds alone. Garden Organic also advises growing marigolds in the greenhouse with tomatoes to draw whitefly away, as advice rather than a tested result. The trials were in glasshouses, so the result applies to tomatoes under cover; see growing tomatoes and the whitefly page.

Borage and strawberries. A UK study (Griffiths-Lee, Nicholls and Goulson, Ecological Entomology, 2020) combined researcher-led experiments with citizen scientist data from gardens and allotments. Strawberries grown with borage produced an average of 35% more fruits and 32% more yield by weight, with better market quality. The cause was pollination, not soil: more flies visited the strawberries. UK data at this scale is rare in this field.

Radish as a catch cropA fast-maturing crop sown into a gap between two main crops, to use ground that would otherwise sit empty. Full definition →. A catch crop is a fast crop sown into gaps that a slower crop has not grown into yet. It is different from a trap crop, which draws pests away from the main crop. Sow radish between slower plants such as Brussels sprouts or sweetcorn: Radish and lettuce are intercropped this way (RHS), and the radishes are harvested before the bigger plants overshadow them. Summer radishes are ready in about three to six weeks, and they quickly get woody if left in the ground. RHS also notes that deep-rooting carrots and shallow-rooting onions can share a bed without much competition, as neither casts heavy shade.

Where marigolds don't help. The limonene effect is on whitefly and thrips. Aphids are not among the pests named in RHS's marigold entry, so the popular advice to plant marigolds against aphids has no institutional source behind it.

Where the mechanism falls apart

Half of every companion-planting chart rests on one idea: strong-smelling plants confuse pests and hide the crop. Warwick Crop Centre tested this and found insects hunting a host land on green surfaces and avoid brown soil. Artificial green plants, and green paper releasing no chemicals, were as effective as living companion plants. Colour, size and shape rather than odour appear to decide the effect. Any claim built only on "strong smell confuses the pest" therefore needs a different kind of evidence, and most do not have it.

Basil and tomato flavour. Doctoral research at West Virginia University (Bomford, 2004) ran double-blind taste tests over three years and found no consistent preference for tomatoes grown with companions. But the study found a mean 20% yield advantage from growing tomato or Brussels sprout with basil, most pronounced at the highest densities and with the most basil. The highest yields in the monocultures came at 25cm spacing for tomato and basil and 40cm for Brussels sprout. So basil with tomatoes is worth doing for yield, but not for flavour. It is a single US garden-scale study.

Beans and peas feeding neighbours. Legumes fix nitrogen, but the transfer between two living plants is small and hard to measure. The larger benefit comes when the residues decay: pulse residues supply roughly 20 to 80 kg of nitrogen per hectare (review in Frontiers in Sustainable Food Systems, 2025). Follow peas or beans with a hungry crop the next season instead of expecting this year's cabbage to benefit from beans beside it.

Pairings in limbo

Several well-known claims have a real biological basis but no test of the effect claimed.

Nasturtiums and blackfly on beans. Nasturtium is a host plant for black bean aphid, along with broad, French and runner beans, so aphids will feed on it. What is untested is whether that diverts them from a nearby bean crop or just gives them a second food source. Garden Organic lists the pairing as a trap-crop practice, while saying companion planting often has unproven benefits. Even if it works, a trap crop concentrates blackfly rather than removing it.

Nasturtiums and cabbage white. Cabbage white (Pieris rapae) lays eggs on nasturtium, and caterpillars hatching on it develop normally. But caterpillars transferred from cabbage refused to feed and starved, because nasturtium foliage contains strong antifeedants, with chlorogenic acid the most prominent (Renwick et al., Journal of Chemical Ecology). So nasturtium can attract egg-laying adults but will not rescue a bed with caterpillars already feeding. No field trial shows a nasturtium border reducing damage on brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition →. See nasturtiums as an edible companion and the cabbage white caterpillar page.

Onions and carrot fly. The theory is that onion scent masks the carrots. The scent claim is repeated by RHS as a claim, Garden Organic trials gave inconsistent results, and RHS's carrot guide recommends covering the crop with insect-proof mesh instead. Carrot fly is patchy, so a year without damage proves nothing either way. See the carrot fly page.

Savory and blackfly. Summer and winter savory are often named as blackfly deterrents for broad beans, but RHS's blackfly page and its savory guide mention no such effect. No trial isolates it.

Mint and onion fly, and chervil with radish. Both rest on a single secondary source or general repetition, with no RHS or Garden Organic mention. Mint also spreads aggressively by runnersLong, trailing stems a plant sends out that root where they touch soil, producing a genetically identical new plant for free. Full definition →, so planting it carries a cost either way.

Where companion planting backfires

Garden Organic's 2024 members' trial interplanted leeks with chard and clover to test whether companions would reduce allium leaf miner. Chard showed a consistent trend to less leaf miner damage, but the result was not statistically significant, and the chard was clearly competing against the leeks from July onwards. Using chard as a companion significantly reduced marketable yield by 69%, and the competitive effects far outweighed any reduction in pest damage. See the allium leaf miner page.

A Virginia field trial (run in 1981 and 1982, published in The Canadian Entomologist in 1983) interplanted collards with mixed herbs to test whether strong scent repels cabbage white. Eggs were laid in denser clusters on the interplanted collards, and collards bordered by companion plants received significantly more eggs than unbordered controls. The most eggs were on collards between sage. The herbs did not repel the butterfly and encouraged it instead. Fine netting or horticultural fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition → keeps adult butterflies off brassicas (RHS), which a herb border does not.

Mustard or Chinese cabbage as a flea beetle trap crop, and plants said to deter slugs, are left ungraded because neither RHS nor Garden Organic makes a tested claim about them. Garden Organic does say Chinese cabbage will attract cabbage white butterflies away from neighbouring brassicas, as a practice and not as trial data.

What works on blackfly

Pinch outRemoving the soft growing tip of a stem to stop upward growth and make the plant branch out sideways instead. Full definition → the shoot tips of broad beans once the lowest flowers have formed small pods. This puts the plant's energy into pods and reduces blackfly (RHS), which colonise soft young shoot tips; Garden Organic gives the same advice, timed after the first flowers set. Time it by the plant, not the calendar: in a cool Scottish spring the pods set later, so push the pinch-out back rather than follow an English date. If aphids appear, squash colonies between finger and thumb and encourage ladybirds, hoverflies and other natural enemies. Hoverfly and lacewing larvae eat aphids, and the adults feed on nectar and pollen, which is why flowers nearby help. RHS also advises tolerating some blackfly: they are food for wildlife, rarely kill the plant, and the beans stay undamaged. See the black bean aphid page.

The three sisters

Corn, beans and squash grown together is the best-known companion system, and neither RHS nor Garden Organic makes a tested claim about it. Sweetcorn is marginal and short-season in much of Scotland, which makes the system harder here. It is also a different kind of claim: an Indigenous American agricultural system developed over generations for a particular climate, not a pest-deterrent pairing tested in a UK trial. It gets no verdict here. If you want to try it, the sweetcorn guide covers how the corn itself copes with a Scottish season.

Summary

Common questions

Sources