Companion planting in the UK: what actually works

So what is companion planting, really?

Companion planting is the practice of growing different plants near each other so they do each other a favour. The favours come in a few flavours. One plant might pull in insects that eat the pests bothering its neighbour. Another might mask a crop's scent so a pest struggles to find it. Some plants earn their keep by shading, supporting or sheltering the ones next to them, and others simply cover bare soil so weeds can't get a foothold.

It's an old idea, and it sits right at the heart of the kind of mixed, productive garden most homesteaders are after. The trouble is that companion planting has picked up a lot of baggage over the years. Charts get passed around claiming this loves that and hates the other, often with no source and no reasoning. So before you rearrange your whole plot around a printout, it's worth asking a blunt question.

Does companion planting actually work, or is it folklore?

Both, and that's not a fudge, it's the accurate answer.

Some companion planting has proper research behind it. Some of it is tradition that has never really been tested, or has been tested and didn't hold up. The RHS, which is about as level-headed a source as you'll find, is careful to say that many companion planting claims are anecdotal, while a handful have solid trial data. That's the spirit to bring to your own garden.

The honest framing matters because credulous companion planting can waste your space and your time. If you fill a bed with a plant that "protects" your crop but actually just competes with it for water and food, you've done yourself no favours. On the other hand, being sniffy and dismissing the whole idea means missing the bits that clearly do work. The middle path is the sensible one. Try things, keep the ones that earn their place, and don't build your whole plot on faith.

Which companion planting methods have real evidence behind them?

If you want to spend your effort where it counts, focus on the mechanisms that stack up.

Pulling in beneficial insects. This is the strongest card in the deck. Flowers rich in accessible nectar and pollen bring in hoverflies, ladybirds, lacewings and parasitic wasps, and the larvae of those insects are ferocious aphid-eaters. A single hoverfly larva can get through hundreds of aphids. Poached egg plant (Limnanthes douglasii) is arguably the best hoverfly magnet you can grow, a low carpet of yellow and white flowers that self-seeds happily. Calendula (pot marigold), phacelia and single-flowered dahlias do a similar job. Dot them among your veg and you're building a small standing army of pest controllers.

Trap crops. The idea is to give a pest something it likes more than your crop, so it goes there instead. Nasturtiums are the classic in UK gardens, drawing blackfly (aphids) away from beans and other veg. The evidence here is mixed rather than airtight, and you do have to keep an eye on the trap crop so it doesn't just become an aphid nursery next to everything you're protecting. Still, nasturtiums are cheap, quick and edible, so there's little to lose in trying.

Masking scent with strong smells. Many pests find their host plants by smell. The theory goes that strong-smelling alliums and herbs confuse them. This is where the famous carrots-and-onions pairing comes from, on the basis that onion scent throws carrot fly off the trail. As we'll see, the evidence for that specific combo is wobbly, but the general principle of scent confusion has some support and does little harm to try.

Tall plants for shade, shelter and support. This one is just physics, so it's reliable. The "three sisters" system, sweetcorn with climbing beans scrambling up the stalks and squash sprawling below, uses tall plants to support the beans while the squash leaves shade the soil and keep moisture in. In a UK garden you can borrow the logic: use sweetcorn or sunflowers to give a bit of afternoon shade to lettuce that would otherwise bolt in a heatwave.

Ground cover to suppress weeds. Bare soil is an invitation to weeds. Low, spreading plants like trailing squash, or a living mulch of low-growing herbs, shade the ground and keep weeds down while holding moisture in the soil. No mystery to it, and it works.

The best companion planting isn't magic. It's ecology you can watch working from the kitchen window.

What about the classics gardeners swear by?

Some pairings are so embedded in British gardening lore that leaving them out would feel wrong. Here's the honest read on the big three.

Tomatoes and basil. Gardeners adore this one, partly because they taste so good together on a plate. The claim is that basil repels whitefly and improves tomato growth and flavour. The insect side has some support, since strong-scented plants can deter pests, but the flavour claim is more kitchen romance than science. Grow them together anyway. They enjoy the same conditions and you'll always have basil to hand. Worth reading alongside our full guide to growing tomatoes in the UK.

Carrots and spring onions. The theory is that each masks the other from its pest, onion scent confusing the carrot fly and vice versa. Some mixed-cropping studies did show fewer carrot fly attacks, but controlled trials, including work by Geoff Hamilton, found no real protection. Any effect seems to fade as the season goes on. It's a lovely-looking combination and does no harm, but if carrot fly is a serious problem for you, a 60cm barrier of fine mesh or fleece is far more dependable.

French marigolds for whitefly. As covered above, this one has the best evidence of the lot for greenhouse tomatoes. In a UK greenhouse or polytunnel, marigolds are a genuinely sensible planting, not just a splash of colour.

Where is the evidence weaker?

Plenty of the companion planting charts you'll find online list dozens of "friends" and "enemies" with no reasoning attached. Claims that a particular herb will supercharge a crop's flavour, or that two plants "hate" each other, are usually tradition rather than tested fact. The same goes for a lot of the very specific pairings, like a single herb protecting one exact vegetable.

None of this means the pairing is useless. It means we don't have good data either way, so treat it as an experiment rather than a rule. Keep a few notes each season on what you tried and what actually happened in your own soil and climate. Your plot is the only trial that fully applies to your plot.

Which combinations should you avoid?

A shorter list, but a useful one.

Alliums with legumes. Keep onions, garlic, shallots and leeks away from peas and beans. Alliums release sulphur compounds that can inhibit the soil bacteria legumes depend on to fix nitrogen, so you can end up with weaker beans and peas for no gain. This is one of the better-supported "avoid" pairings.

Two heavy feeders together. Planting two hungry crops shoulder to shoulder, say brassicas next to squash, or sweetcorn crammed against courgettes, just sets up a fight for the same food and water. Give greedy crops their own space, or pair a hungry plant with a light feeder.

Anything that shades out a sun-lover by accident. Tall plants are useful when the shade is deliberate. When it's not, a row of sweetcorn on the sunny side of your peppers or tomatoes will quietly rob them of the light they need. Think about the sun's path across your plot before you commit.

How do I start companion planting in a UK garden?

You don't need to redesign everything. Start small and build on what works.

1. 1

   Sort out the basics first

   Companion planting can't rescue poor soil, bad spacing or a slug apocalypse. Get your soil healthy and your crops properly spaced before you rely on any pairing.

2. 2

   Add an insectary strip

   Sow a row or edge of poached egg plant, calendula and phacelia near your veg. This is the highest-return, best-evidenced move you can make, and it brings in hoverflies and ladybirds within weeks.

3. 3

   Slot in a trap crop

   Plant nasturtiums a little away from your beans to draw blackfly off them. Keep an eye on them and pull heavily infested growth if it gets out of hand.

4. 4

   Protect greenhouse tomatoes with marigolds

   Put French marigolds in from the start of the season, not as an afterthought once whitefly appear.

5. 5

   Note what actually happens

   Keep a simple season diary of what you paired and the result. Over a couple of years your own notes will beat any generic chart.

Treat companion planting as one thread in a bigger, healthier garden rather than the whole rope. Combine it with good rotation, decent soil and the odd physical barrier, and you'll get the best of it without kidding yourself about the bits that don't deliver. For more on building a productive UK plot, head back to the Grow section.

The honest bottom line

Companion planting is real, useful, and worth doing. It's also over-sold. Lean hard on the parts with evidence, chiefly flowers that bring in beneficial insects and French marigolds for greenhouse whitefly. Give the traditional pairings a go with clear eyes, knowing some will help and some won't. Avoid the handful of combinations that actively work against you. Do that, and you'll have a prettier, livelier, more productive garden, with a fair few pests quietly dealt with by insects you never had to buy.