Companion planting chart
Companion planting means growing plants side by side so one helps the other. Pick a plant to see good neighbours and ones to keep apart, each with the reason and how well it stands up.
What grows well with it
Pick a plant. Each pairing says why, how good the evidence is, and where it comes from.
Tomatoes
Suggested neighbours
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French marigolds (Tagetes)
Tested in trials
French marigolds grown beside greenhouse tomatoes from the start slowed the build-up of glasshouse whitefly in two large trials, and a scent the marigolds give off (limonene) is the likely cause. Bringing marigolds in once whitefly were already bad did little. The trials were under glass.
Sources: Conboy and others (2019), PLoS ONE: French marigolds and glasshouse whitefly; RHS, Three ways to use companion planting
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Basil
Widely recommended, little evidence
A favourite pairing. In the marigold trials, a mix of plants that whitefly do not feed on, basil among them, also cut whitefly on tomatoes, and the RHS notes that mixing tomatoes with other plants had a similar effect to marigolds. Basil alone has not been shown to do it.
Sources: Conboy and others (2019), PLoS ONE: French marigolds and glasshouse whitefly; RHS, Three ways to use companion planting
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Nasturtiums
Widely recommended, little evidence
Grown in the hope that their flowers bring in hoverflies and other insects that eat aphids. Reasonable, but not tested for this pair.
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Pot marigolds (Calendula)
Widely recommended, little evidence
Pot marigolds are open, nectar-rich flowers that feed hoverflies, whose young eat aphids. Planted near tomatoes on hope more than evidence.
Keep apart
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Potatoes
Sound growing sense
Both catch blight, the same disease, which rots leaves, fruit and tubers in warm, wet summers. Most garden blight blows in on the wind, so keeping them apart is no guarantee, but it can also live on in potato tubers left in the ground. The RHS advises a rotation of at least four years, with no tomatoes in that soil in between.
Source: RHS, Potato and tomato blight
What the evidence ratings mean
- Tested in trials
- Published trials found it works, though often only in the conditions tested.
- Trials disagree
- It has been tested, and the results are mixed or only hold in some conditions.
- Sound growing sense
- Not a pest trick: the reason is plain gardening, such as sharing space or a shared disease.
- Widely recommended, little evidence
- Good gardening bodies suggest it, but we found no trial that tests it.
- Folklore
- An old saying that has been tested and not borne out, or that the evidence goes against.
How to use it
Choose a plant from the list. You see its suggested neighbours first, then any it's better kept apart from. Each pairing says why, gives our rating of the evidence, and links to where the claim comes from, so you can read it for yourself.
How to read the ratings
A lot of companion planting advice is handed down rather than tested, so we say how far each pairing has been put to the test. Tested in trials means published trials found it works, often only in the conditions they tried. Trials disagree means it has been tested and the results are mixed. Sound growing sense isn't a pest trick at all, but plain gardening, such as sharing space well or not spreading a disease. Widely recommended, little evidence means good gardening bodies suggest it but we found no trial of it. Folklore means an old saying that has been tested and not borne out.
A few examples. French marigolds grown beside greenhouse tomatoes from the start kept glasshouse whitefly down in two large trials. Onions beside carrots cut carrot fly in one trial, but only while the onions were young, and later trials have been mixed: fine mesh is the reliable cure. Nasturtiums grown to draw blackfly off beans are a sacrificial crop, one grown to take the pests, and are widely advised but little tested.
How it is put together
We kept the list short on purpose: twenty plants, and only pairings we could trace to a source. The main ones are the RHS guide to companion planting, the Garden Organic guide, and published trials on marigolds and whitefly, onions and carrot fly, ground cover and cabbage root fly, and the three sisters (sweetcorn, beans and squash grown together). The ratings and the wording are ours. If a pairing you have heard of is missing, it is because we could not find good grounds for it.
More for the plot
Good neighbours still need room and good timing. The seed spacing calculator works out how many fit, and when to plant turns your local frost dates into sowing dates for tomatoes, beans and squash.
Garden art prints to go with it
Questions people ask
Does companion planting really work?
Some of it does. Trials back French marigolds against greenhouse whitefly on tomatoes, and low ground cover against cabbage root fly. Much else is sensible but untested, and some old sayings have been tested and not borne out. The chart says which is which.
Do onions keep carrot fly away?
Only partly. A 1984 trial found fewer carrot fly attacks with carrots mixed among onions, but the effect faded once the onions began to bulb, and later trials have been inconsistent. Fine mesh or fleece over the carrots is the reliable answer.
What should not be planted next to tomatoes?
Keep tomatoes away from where potatoes have grown. Both get blight, the same disease, and it can live on in potato tubers left in the soil. The RHS advises a rotation of at least four years, with no tomatoes in that soil in between.
What are the three sisters?
Sweetcorn, climbing beans and squash grown together, an old way of growing from the Americas. The corn holds up the beans and the squash covers the ground. A field study found the mix gave more than each crop alone, but it is hard to make work in a short British summer, so give it plenty of room.