Companion Planting UK: The Complete Guide to What to Plant Together
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There’s a moment every gardener has — standing in a vegetable bed that looks perfectly fine on paper but has been quietly ravaged by carrot fly, blackfly, or cabbage white butterflies, wondering what went wrong. The answer is often not about what you planted, but what you planted together. Companion planting is one of the oldest, most elegant solutions in the gardener’s toolkit, and in the UK’s cool, damp climate — where pests and fungal issues thrive — it can make a dramatic difference to your harvests.
The principle is beautifully simple: grow two or more plants together because their combination achieves something neither could do alone. Some deter pests. Others attract the beneficial insects that keep your garden in balance. A few quietly enrich the soil beneath their neighbours’ feet. Done well, companion planting reduces your reliance on chemical intervention, improves biodiversity, and makes your growing space work harder for you from the first warm days of spring through to the last autumn harvests.
This guide covers everything you need to know about companion planting in a British garden — from the science behind why it works to a detailed, vegetable-by-vegetable breakdown of the best combinations for UK conditions.
Why Companion Planting Works: The Science Behind the Combinations
Before diving into which plants to grow alongside which, it’s worth understanding the mechanisms at play. Companion planting isn’t folk wisdom operating on faith alone — there are well-documented biological processes underpinning the most effective combinations.
Scent Confusion and Pest Deterrence
Many of the UK’s most damaging garden pests — carrot fly, cabbage root fly, aphids — locate their host plants primarily through scent. They’re exceptionally good at detecting the volatile compounds produced by the crops they feed on. The strategy behind scent-based companion planting is to overwhelm or disguise those signals with the stronger aromatics of nearby plants.
This is why the classic pairing of carrots and onions works so well. The pungent sulphur compounds from onions, leeks, or spring onions mask the carrot’s scent, causing carrot root fly to pass over the bed in confusion. The same principle applies to strongly aromatic herbs — rosemary, sage, thyme, and mint all emit compounds that interfere with pest navigation. Growing them throughout your vegetable beds rather than keeping them confined to a separate herb garden pays real dividends.
| Crop | Companion | Reason |
|---|---|---|
| Aubergines | Basil | Increase productivity and taste |
| Fruit bushes | Peas, beans and sweet peas | Releases nitrogen into soil to increase fruit crop |
| Fruit trees | Peas, beans and sweet peas | Releases nitrogen into soil to increase fruit crop |
| Lettuce | Basil | Improve the taste |
| Peppers | Basil | Increase productivity and taste |
| Tomatoes | Basil | Improve the taste |
| Vegetable beds | Yarrow | Fertilises the soil and can be composted |
Sacrificial and Trap Planting
Martyr plants — sometimes called trap crops — work on a different principle. Rather than deterring pests, they attract them away from your valuable crops. The nasturtium is the most famous example in British gardens: planted around brassicas, courgettes, or beans, it draws blackfly and cabbage white butterflies to itself, giving your food crops a fighting chance. You lose the nasturtium, but you keep your kale.
In greenhouse growing — a significant part of many UK vegetable gardens given our unreliable summers — basil planted alongside tomatoes and cucumbers performs this function beautifully. Whitefly are irresistibly drawn to basil, and keeping a pot of it in the greenhouse as a deliberate sacrifice protects the crops you’re actually growing for the table.
Attracting Beneficial Insects
Not all insects are the enemy. Hoverflies, lacewings, ground beetles, parasitic wasps, and of course bees are all working in your favour, and certain plants act as powerful beacons for them. This matters in two distinct ways: pollination of fruiting vegetables, and natural predation of the pests that damage your crops.
Hoverfly larvae are voracious aphid predators — a single larva can consume hundreds of aphids before it pupates. Adult hoverflies are attracted to flat, open flowers they can land on easily, which is why umbellifers like yarrow, fennel, and poached egg flower (Limnanthes douglasii) are such effective companions. Ladybirds, whose larvae similarly feast on aphids, are drawn to the same type of flower.
Nitrogen Fixing and Soil Health
The legume family — peas, beans, sweet peas, vetches, and lupins — has a remarkable relationship with certain soil bacteria. These bacteria colonise the plant’s roots and fix atmospheric nitrogen, converting it into a form that can be absorbed by plants. When legumes are grown near other crops, that nitrogen becomes available to the surrounding soil, acting as a slow-release, entirely natural fertiliser.
This is particularly valuable in the UK context, where our heavy clay soils can become depleted quickly. Growing broad beans or runner beans alongside brassicas or fruit bushes isn’t just convenient space management — it’s actively improving your soil for seasons to come.
Companion Planting by Vegetable: What to Grow with What
This is the section most gardeners actually need: a practical, crop-by-crop breakdown of which companions work best in UK conditions, and crucially, which pairings to avoid.
| Crop | Companion | Reason |
|---|---|---|
| Beans | Sweet peas | Attracts pollinating insects |
| Courgettes | Calendula | Attracts pollinating insects |
| Raspberries | Poached egg flower | Attracts pollinating insects when planted at base |
| Vegetable beds | Yarrow | Attracts hoverflies and ladybirds to eat pests |
Tomatoes
Tomatoes are one of the most rewarding crops to grow in the UK, particularly in a greenhouse or polytunnel, and they respond well to thoughtful companion planting. Growing basil alongside tomatoes is almost horticultural tradition — the aromatic oils in basil are thought to repel aphids and whitefly while some gardeners report improved flavour in nearby tomatoes, though this is harder to prove definitively. What’s certain is that basil in the greenhouse or on the same bed creates a powerful scent barrier that does appear to reduce aphid pressure.
Chives and garlic are excellent tomato companions for the same reason — their strong allium chemistry deters aphids and is also thought to help prevent certain fungal diseases, including the botrytis that plagues UK tomato crops in damp summers. French marigolds (Tagetes patula) are perhaps the most widely recommended companion for tomatoes in British growing guides, and for good reason: the roots of French marigolds produce a compound called alpha-terthienyl that deters root knot nematodes, while the flowers attract hoverflies and repel whitefly.
Avoid planting tomatoes near fennel, which produces allelopathic compounds that inhibit the growth of most vegetable crops. Tomatoes and brassicas also compete heavily for nutrients and are best kept well apart.
Brassicas (Cabbage, Broccoli, Cauliflower, Kale, Brussels Sprouts)
Brassicas are under almost constant threat in UK gardens — cabbage white butterflies, cabbage root fly, flea beetles, and aphids all target them from early spring onwards. Companion planting is one of the most effective defences available, especially when combined with physical barriers like fine mesh.
Nasturtiums are your first line of defence. Plant them liberally around and between your cauliflower and cabbage plants, and they’ll draw cabbage white butterflies away from your crops. The butterflies preferentially lay eggs on nasturtiums, which you can then remove and compost. Dwarf French marigolds planted as a border perform a similar function while also attracting hoverflies.
Strongly scented herbs are highly effective between brassica rows. Sage, rosemary, thyme, and mint all confuse the carrot root fly (which affects not just carrots but related alliums grown nearby) and help mask the strong glucosinolate scent of brassicas that attracts cabbage white butterflies. A row of sage interspersed with your cabbages is both practical and rather beautiful.
Avoid planting brassicas near runner beans or strawberries — they compete for similar nutrients and don’t benefit each other. Tomatoes planted close to brassicas will deplete the soil’s nutrient reserves rapidly.
Carrots
Carrot root fly is the defining problem for UK carrot growers, and companion planting addresses it directly. The classic solution — interplanting carrots with onions, leeks, or spring onions — exploits the scent-confusion mechanism described earlier. The key is to create genuine proximity: alternate rows of carrots and spring onions, or even interplant them within the same row, so the scent barrier is continuous.
Growing garlic near carrots provides similar protection. Chives work well too, and have the added advantage of flowering in a way that attracts beneficial insects. Rosemary planted at the ends of carrot rows creates a strongly scented border that carrot fly is reluctant to cross.
One important note: avoid planting carrots near dill. Though dill attracts beneficial insects beautifully, it’s in the same plant family as carrots and can cross-pollinate, leading to disappointing results if you’re saving seed. Parsnips present the same problem — both crops attract the same pests, so planting them together simply creates a larger target.
Beans (French, Runner, and Broad)
Beans are remarkable companions for two reasons: they’re nitrogen-fixers, enriching the soil for everything around them, and they’re also the basis of one of the most famous companion planting systems in horticultural history — the Three Sisters.
The Three Sisters combination (sweetcorn, climbing beans, and squash or courgette) was developed by Native American farmers over centuries. In the UK context it works beautifully: the sweetcorn provides a climbing frame for the beans, the beans fix nitrogen for the hungry corn and squash, and the large leaves of the squash act as living mulch, retaining moisture and suppressing weeds. It requires a sheltered, sunny spot — not always easy in Britain — but when it comes together it’s deeply satisfying.
Broad beans are particularly susceptible to blackfly, and summer savoury is the traditional companion to combat this. The aromatic herb repels blackfly when planted nearby, and has the added bonus of being a useful culinary herb in its own right.
French marigolds are excellent general companions for beans, deterring whitefly and aphids while attracting pollinators to help set pods. Sweet peas grown on the same wigwam or obelisk as climbing beans are a classic UK combination — they attract bees to pollinate the beans and make the structure look rather lovely in the process.
Courgettes and Squash
Courgettes are generally undemanding companions, but they benefit enormously from the presence of pollinating insects — without pollination, those first yellow flowers set no fruit. Calendula (pot marigold) planted nearby is a reliable way to draw in bees and hoverflies. Nasturtiums serve double duty here, attracting pollinators while also distracting aphids away from the courgette plants.
Borage is an exceptional companion for courgettes and squash. It deters the tomato hornworm (less of a problem in the UK than in warmer climates, but still present) and attracts bees with unusual efficiency — the flowers produce abundant nectar and are irresistible to bumblebees, which are the most effective pollinators for courgettes in British conditions. Borage self-seeds freely, which means once you’ve grown it in a spot it tends to return year after year with minimal effort.
Potatoes
Growing potatoes benefits significantly from the presence of French marigolds, which deter wireworm and the aphids that transmit potato virus diseases. Horseradish planted at the corners of potato beds is a traditional UK companion — it’s thought to increase disease resistance in potato plants, and the roots are easy to contain by growing them in a buried pot.
Avoid planting potatoes near tomatoes, aubergines, or peppers — they’re all in the nightshade family and share several serious diseases, including blight. Planting them together simply increases the disease load on both crops. Fennel and raspberries are also problematic neighbours for potatoes.
Onions, Leeks, and Garlic
Alliums are companion planting workhorses. They’re effective companions to almost every other vegetable in the garden, deterring aphids, carrot fly, and various other pests through their strong sulphur chemistry. Roses are a classic pairing in British gardens — alliums planted beneath rose bushes are widely believed to reduce aphid pressure and help prevent black spot, and the combination looks beautiful in early summer when allium flowers open alongside the first rose blooms.
The main caution with alliums is their incompatibility with the legume family. Onions, garlic, and leeks all inhibit the growth of peas and beans — keep them well separated in the vegetable garden.
Salad Leaves and Lettuce
Salad leaves are relatively easy going as companions, but they do benefit from the light shade provided by taller plants in the height of summer, when lettuce can bolt quickly in British heat waves. Growing them alongside sweetcorn, climbing beans, or even tall brassicas provides natural protection.
Basil grown near lettuce is reported to improve flavour, though the evidence is anecdotal. What’s more certain is that strongly aromatic herbs planted nearby deter the slugs and aphids that devastate salad crops. Avoid planting lettuce near parsley — they appear to inhibit each other’s growth, and the combination consistently produces poor results for both.
The Best Companion Flowers for UK Vegetable Gardens
Flowers deserve specific attention in any companion planting guide, because the right flowering plants dotted through a vegetable garden do more for overall health and productivity than almost any other intervention.
French Marigolds (Tagetes patula)
The single most useful companion flower for UK vegetable gardens. French marigolds should be in every vegetable plot, full stop. Plant them in spring after the last frosts — typically mid-May across most of the UK — and they’ll flower continuously until October, providing pest deterrence and pollinator attraction throughout the entire growing season. The variety ‘Naughty Marietta’ is particularly good, and seeds are widely available from UK suppliers including Suttons, Thompson & Morgan, and Jekka’s.
Nasturtiums (Tropaeolum majus)
Direct-sow in April or May where they’re to grow — nasturtiums dislike root disturbance and transplant poorly. They’re extremely easy from seed and will spread enthusiastically, so choose your position thoughtfully. As a martyr plant they’re without equal in UK gardens, and as a bonus the flowers and leaves are edible, with a pleasant peppery flavour.
Poached Egg Plant (Limnanthes douglasii)
This unassuming little annual is one of the best hoverfly attractors available to British gardeners. Sow direct in autumn or early spring — it tolerates UK frosts well — and it will carpet the ground between vegetable plants with cheerful yellow-and-white flowers from April onwards, just when hoverfly populations are beginning to build. Like nasturtiums, it self-seeds freely once established.
Calendula (Pot Marigold)
Calendula is particularly valuable in the early season when other companion flowers haven’t yet opened. It germinates readily in cool UK spring conditions and produces flowers from as early as March if sown under cover in February. It’s an excellent companion for courgettes, beans, and brassicas alike, and the petals are edible and make a striking addition to salads.
Borage
A powerfully effective pollinator plant that bees find irresistible. Borage is best grown from direct-sown seed in May; it dislikes transplanting. Once established it’s vigorous and self-seeds prolifically — in a well-established kitchen garden you’ll find it returning reliably year after year. The bright blue star-shaped flowers are edible and make a striking garnish.
Sweet Peas (Lathyrus odoratus)
Often treated purely as an ornamental, sweet peas are in fact excellent vegetable garden companions. Grow them on the same supports as climbing beans and they’ll attract bees throughout summer while their roots fix small amounts of nitrogen. UK sowing time is October to February under cover, or March to April direct.
Seasonal Companion Planting in the UK: A Month-by-Month Framework
One of the most important things to understand about companion planting in Britain is that timing matters enormously. Our growing season is compressed compared to continental Europe, and the pest pressure calendar is quite specific. Here’s how to approach it through the seasons.
Spring (March–May): Setting the Foundation
This is the time to establish your companion framework before the main crops go in. In March, as March garden jobs pick up pace, sow calendula and poached egg plant directly into beds — both will germinate in cool conditions and establish quickly. Pot up or direct-sow French marigolds for planting out after the last frosts.
April is the key month for allium planting alongside brassica seedlings — get onion sets or leek plants in early to give them time to establish and start producing the scent barrier you’re relying on. Interplant spring onions between carrot rows as carrots are sown direct from mid-April in most UK regions.
By mid-May, when frosts have passed for most of England and Wales (allow until late May or early June in Scotland and upland areas), it’s time to plant out French marigolds, nasturtiums, and basil alongside your tomatoes, courgettes, and beans.
Summer (June–August): Active Management
Summer is when companion planting pays off most visibly. By June your French marigolds should be in full flower, nasturtiums spreading vigorously, and the beneficial insect population building. This is also peak pest season — carrot fly has its first generation in late May to June, cabbage white butterflies are active from May through August, and aphid populations explode in warm weather.
Watch your martyr plants carefully. If nasturtiums are absolutely covered in blackfly or aphids, their job is working — but check that the infestation isn’t beginning to spread to crop plants. Remove heavily infested nasturtium shoots and compost or dispose of them away from the garden.
Succession-plant calendula and borage through summer to ensure a continuous flower supply. Both can run to seed quickly in warm weather, so having a fresh sowing coming through maintains the flowering display.
Autumn (September–October): Winding Down and Looking Ahead
As the growing season ends, leave some companion plant flowers to set seed. Calendula and borage self-seed reliably in UK conditions; nasturtiums less so but worth trying. Collect French marigold seeds for next year — they’re easy to save and one plant produces hundreds of seeds.
As you clear beds after harvest, consider sowing a green manure of winter tares or phacelia. These act as companion plants for the soil itself, fixing nitrogen and providing a habitat for overwintering beneficial insects beneath the foliage. They’re dug in the following spring, improving soil structure and fertility ready for the next season’s companions.
Plants to Avoid Planting Together
Understanding incompatible combinations is just as valuable as knowing good ones. Some pairings actively inhibit growth through allelopathy — the release of chemical compounds that suppress neighbouring plants — while others simply compete too intensely for the same resources.
Fennel deserves special mention here because it’s one of the most problematic plants in the vegetable garden. It produces allelopathic compounds that suppress the growth of almost everything around it — tomatoes, beans, brassicas, and most herbs all fare poorly near fennel. Grow it in an isolated spot away from your vegetable beds, or in a container. The beneficial insect attraction of fennel flowers is real, but the trade-off isn’t worth it in a productive growing space.
The full picture of incompatible combinations worth knowing:
| Crop | Avoid Planting With | Reason |
|---|---|---|
| Asparagus | Garlic, onion, potato | Competition and allelopathy |
| Beetroot | Runner beans, tomatoes | Inhibit each other’s growth |
| Brassicas | Runner beans, strawberries, tomatoes | Nutrient competition |
| Carrots | Dill, parsnips | Same family, same pests |
| Fennel | Almost everything | Allelopathic compounds |
| Lettuce | Parsley | Mutual growth inhibition |
| Onions/Garlic | Peas, beans | Alliums inhibit legumes |
| Peas | Chives, garlic, onion, shallots | Alliums inhibit legumes |
| Potatoes | Tomatoes, aubergines, peppers | Shared diseases |
| Tomatoes | Fennel, brassicas, potatoes | Disease and allelopathy |
Companion Planting in Small UK Gardens and Containers
It’s worth addressing the reality that most UK gardeners — particularly those with urban plots, allotment half-plots, or small back gardens — don’t have the luxury of large, separated vegetable beds. The good news is that companion planting works beautifully at small scales, and container gardening opens up further possibilities.
A single large container can combine tomatoes with basil and a French marigold at its base, creating a functional companion trio on a balcony or patio. Growing herbs outdoors in pots positioned throughout a small bed provides many of the same aromatic deterrence benefits as in-ground planting. Nasturtiums spilling from a hanging basket at the edge of a patio vegetable bed will still perform their martyr function for crops nearby.
In raised beds — increasingly popular in UK gardens for their excellent drainage and soil control — companion planting is particularly effective because the controlled environment makes it easier to plan precise combinations. A raised bed containing tomatoes, basil, French marigolds, and chives is essentially a complete companion ecosystem in a small space.
Companion Planting and Wildlife-Friendly Gardening
There’s a significant overlap between companion planting and eco-friendly garden ideas that’s worth acknowledging. The principles are deeply complementary: both encourage biodiversity, reduce chemical intervention, and treat the garden as an ecosystem rather than a monoculture.
Companion planting with flowers rich in nectar and pollen — calendula, borage, phacelia, poached egg plant, and umbellifers — directly supports pollinator populations that are under significant pressure across the UK. The RHS recommends growing plants specifically for beneficial insects as one of the most impactful things home gardeners can do for biodiversity, and the bonus is that this approach simultaneously improves your vegetable harvests.
Growing a diverse range of companions also supports the ground beetle and spider populations that prey on slugs — one of the most damaging pests in British gardens. A garden rich in structural diversity, with companion plants at different heights and densities, provides the habitat these predators need to thrive.
Getting Started: A Simple Companion Planting Plan for UK Gardeners
If you’re new to companion planting or returning to it after a break, the temptation is to try to implement every combination at once. A more useful approach is to start with a handful of reliable, multi-purpose companions that work across many different crops.
| Vegetable | Companion | Reason |
|---|---|---|
| Beans | French Marigolds | Repels whitefly and aphids |
| Broad beans | Summer savoury | Repels blackfly |
| Carrots | Onions, spring onions, leeks | Smell deters carrot root fly |
| Potatoes | French Marigolds | Repels whitefly and aphids |
| Sweetcorn | French Marigolds | Repels whitefly and aphids |
| Tomatoes | Chives, onions | Smell deters aphids |
| Vegetable bed | Tansy | Deters ants |
Begin with French marigolds: buy or grow a tray of seedlings and plant one between every other vegetable plant across your entire growing space. Their pest deterrence and pollinator attraction benefits apply to virtually everything in a UK vegetable garden. Next, direct-sow nasturtiums at the edges of your brassica and bean beds in April or May. Finally, tuck a pot of basil into your greenhouse or polytunnel if you grow tomatoes under cover.
These three plants alone — French marigolds, nasturtiums, and basil — address the majority of common UK vegetable garden pest problems and provide a meaningful foundation for more sophisticated companion combinations as your experience grows.
Starting a vegetable garden with companion planting already integrated from the outset, rather than adding it retrospectively, gives you the best possible results. And the more you observe how your garden responds to different combinations — which pests reduce, which beneficial insects appear, which crops seem to thrive — the more intuitive it becomes.
Putting It All Together
Companion planting is not a magic solution to every garden problem, and it works best as one layer of a broader, ecologically thoughtful approach to growing food. But as part of a garden that also benefits from good soil management, composting, and careful crop rotation, it’s one of the most powerful tools available to UK gardeners.
The combinations that work in British conditions have been refined over generations of gardening in this specific climate — cool, damp, with a compressed growing season and a very particular cast of pest species. Working with that knowledge, and observing what happens in your own plot across successive seasons, is one of the genuine pleasures of vegetable growing.
Start simple, observe carefully, and let your garden tell you what’s working.
Continue Your Journey
If you found this guide useful, these posts will help you put companion planting into practice across the seasons:
- How to Plan a Vegetable Garden: A Beginner’s Guide — Getting the layout right from the start makes companion planting far easier to implement
- Starting a Vegetable Garden: 9 Steps — Build the foundations before you add the companion layer
- Eco-Friendly Garden Ideas for UK Gardens — Companion planting sits within a broader sustainable gardening approach
- March Garden Jobs UK — Spring is the key companion planting setup season in British gardens
- How to Grow Broad Beans — One of the most versatile nitrogen-fixing companions in the UK vegetable garden
Do you have a companion planting combination that’s made a noticeable difference in your UK garden? Drop a comment below — whether it’s a classic that’s served you well or something more unusual you’ve experimented with, I’d love to hear what’s been working for you.
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