Companion planting: a practical guide

A vegetable garden that includes flowers, herbs and a thoughtful mix of crops is more resilient, more interesting and often more productive than one planted in neat rows of single crops. Companion planting is how you build that mix intentionally.

What companion planting is — and realistic expectations

Companion planting is the idea that certain plants grown near one another will do better than they would alone. The benefits are real, but they're more modest and more varied than the folklore sometimes suggests. No combination is a guaranteed pest repellent or a substitute for good soil, watering and crop rotation.

The most reliable benefits of companion planting are:

• Attracting useful insects — predators that eat pests, and pollinators that improve fruit set.
• Covering bare soil so weeds can't establish and moisture is retained.
• Making efficient use of space by combining plants with different canopy heights and root depths.
• Fixing nitrogen from the air (legumes), reducing how much additional feeding other plants need.

Less reliable, though often repeated, are claims that specific plants chemically "repel" pests through scent alone. Some evidence supports this in limited cases, but the effect is often smaller than gardeners hope, and it depends heavily on timing, density and specific pest populations. Approach companion planting as a way to build a more diverse, ecologically functional garden — not as a quick fix.

How companions help each other

Understanding the mechanisms helps you make better decisions than just following lists of pairings. The main ways plants support each other include:

• Confusing pest insects. Many pests find their host plants by smell. A mix of strongly scented plants — herbs, alliums, flowers — can make it harder for pests to locate the crop they're looking for. The evidence for this varies; it seems more effective against some pests (carrot root fly, for example) than others.
• Attracting predatory and parasitic insects. Flowers with open, accessible nectar and pollen — particularly flat-headed or shallow flowers like umbellifers, pot marigolds and phacelia — attract hoverflies, parasitic wasps and lacewings. These insects prey on or parasitise aphids, caterpillars and other pests. This is one of the most well-supported benefits of flower-rich companion planting.
• Attracting pollinators. More pollinators visiting a diverse planting means better fruit set on beans, squash, courgettes and other crops that depend on pollination.
• Ground cover and living mulch. Low-growing companions — nasturtiums, clover, lettuce — shade the soil between taller plants. This suppresses weeds, retains moisture and can reduce soil temperature in hot weather.
• Structural support. Tall plants can support climbers; wide-leafed plants shade the roots of sun-sensitive neighbours.
• Fixing nitrogen. Legumes (peas, beans, clover) host bacteria in their root nodules that convert atmospheric nitrogen into a form plants can use. This feeds the soil and benefits neighbouring plants — and, after the legumes are removed, the following crop.
• Deep roots improving soil structure. Taprooted plants like dandelions, chicory and some herbs draw minerals up from deeper layers and create channels that improve drainage and aeration for shallower-rooted crops.

Classic pairings explained

These pairings have a good practical rationale, whether well-evidenced or based on solid horticultural logic:

• Carrots and alliums (onions, leeks, chives). Carrot root fly locates carrots by smell and lays its eggs at the base of the plant; the larvae then tunnel into roots. The scent of alliums is thought to confuse or deter the fly, though physical barriers and timing are also important. Growing rows of onions and carrots alternately is a long-established recommendation. Similarly, the scent of carrots may deter onion fly.
• Tomatoes and basil. This is one of the most cited pairings in kitchen garden tradition — basil is said to improve tomato flavour and deter aphids. The flavour claim is not reliably evidenced; the insect deterrence is mild at best. What basil does do is attract pollinators and provide a useful harvest right next to the tomatoes. It's a pleasant and practical combination even if the magic is uncertain.
• Brassicas and nasturtiums. Nasturtiums are sometimes described as "trap crops" for aphids — they attract aphids so strongly that aphids colonise the nasturtium rather than the nearby brassica. In practice, this is inconsistent; aphids may simply colonise both. However, nasturtiums also attract aphid predators such as hoverflies, which genuinely helps. They also provide continuous flowers that brighten the bed through the season.
• Lettuce and tall crops. Lettuce bolts (runs to seed) quickly in hot weather. Grown in the light shade of tall crops like climbing beans or sweetcorn, it stays productive for longer. This is a good use-of-space pairing rather than a pest-management one.
• Beans and squash. Beans fix nitrogen; squash leaves cover the ground and suppress weeds. Grown together (as in the three sisters — see below), they complement each other structurally and nutritionally.

The three sisters

The three sisters — maize (corn), climbing beans and squash — is one of the most celebrated and well-documented companion planting systems, originating with Indigenous peoples of North America who developed it over many generations.

The system works on several levels at once:

• Maize provides the vertical structure — a living pole for the beans to climb. It grows straight and tall quickly, reaching heights that give the beans plenty of support by the time they need it.
• Climbing beans fix nitrogen from the air through the bacteria in their root nodules. As roots and leaf litter break down, this nitrogen feeds the maize and squash. The beans also use the maize for support, saving the need for separate poles or canes.
• Squash spreads wide, paddle-shaped leaves across the ground between the other plants. This dense canopy shades out weeds, keeps the soil moist and cool, and — with some squash varieties — the scratchy leaf surface discourages ground-level pests.

To grow three sisters successfully, you need a reasonably warm climate and enough space for maize to grow well (it's wind-pollinated and does better in blocks than single rows). In cooler climates, the squash in particular may struggle. Adapt the planting to your conditions — climbing beans with a tall sweet corn variety work in temperate climates given a sheltered warm spot; butternut squash needs more warmth than a courgette or a small trailing marrow.

Flowers and herbs among vegetables

Mixing flowers and herbs into a vegetable garden is one of the most practical things you can do to support natural pest management. Many of the most useful companions are flowers.

• French marigolds (Tagetes patula). These have the best documented companion effect: when grown as a dense cover crop before susceptible vegetables, they suppress root-knot nematodes in the soil — a significant pest of tomatoes and other crops in warmer regions. They also attract hoverflies and parasitic wasps that prey on aphids. Their effect on above-ground pests through scent alone is overstated in popular gardening advice, but the insect-attraction benefit is real and worth having.
• Pot marigolds (Calendula officinalis). Easy to grow, long-flowering and attractive to a wide range of beneficial insects including hoverflies, lacewings and bees. A border of calendula around a vegetable bed makes a meaningful difference to the number of aphid predators present. They also self-seed freely, so one initial sowing can maintain itself for years.
• Nasturtiums (Tropaeolum majus). Grown as a flower-and-foliage ground cover, nasturtiums attract aphids and pollinators, are edible (flowers, leaves, young seeds), and fill gaps quickly. In some gardens they act as a sacrificial crop drawing black aphids away from beans; in others, aphids occupy both nasturtium and bean regardless. Either way, they earn their place and cost almost nothing.
• Phacelia (Phacelia tanacetifolia). Often grown as a green manure cover crop and turned in before vegetables, but also brilliant left to flower. The vivid blue flowers attract hoverflies in large numbers and are one of the best single bee plants you can grow. Excellent sown at the edges of beds or in gaps.
• Dill and fennel. The flat flower heads (umbels) of the carrot family are magnets for parasitic wasps, hoverflies and lacewings. Dill is the safe choice — it gets along with most vegetables. Fennel is the exception: it is allelopathic to many crops (see below) and is best grown in a separate spot or its own pot.
• Chives and other alliums. Chives produce attractive purple flowers that attract pollinators, and the strong scent of the whole plant may deter some pests. They're perennial, low-maintenance and useful in the kitchen — excellent edging plants for a vegetable bed.

For more on attracting predatory insects and wildlife to your garden, see our guides on natural pest control and creating a wildlife-friendly garden.

What to keep apart

Just as some plants support each other, others inhibit their neighbours. The most practically important cases:

• Fennel and almost everything. Fennel releases allelopathic compounds — chemicals that suppress germination and growth in other plants. Tomatoes, peppers, coriander, basil, beans and many others grow poorly near fennel. Give it a dedicated spot away from the main vegetable garden, or grow it in a large container.
• Onions and beans or peas. Alliums can inhibit the growth of legumes when planted in close proximity. This doesn't mean they can't be in the same garden, but avoid interplanting rows of onions directly among bean plants.
• Brassicas and brassicas (over time). Growing the same brassica family crops in the same soil year after year — even with different brassica companions — allows clubroot and other brassica-specific soil pathogens to build to damaging levels. This is why rotation matters more than companion planting for this family.
• Potatoes and tomatoes. Both are in the Solanaceae family and share disease susceptibility, particularly blight (Phytophthora infestans). Growing them adjacent makes it easier for disease to spread between crops; keep them separated and rotate them separately.
• Competitive root systems. Some plants simply compete aggressively for water and nutrients. Sunflowers, for example, are allelopathic to some crops and their roots are very competitive — useful on margins but not ideal crammed into a productive vegetable bed.

How to plan a companion bed

1. Decide on your main crops first. Base your companion choices around the vegetables you most want to grow. Companions support the main crop, not the other way around.
2. Identify the main pest pressures in your garden. If aphids are your main problem, prioritise flowers that attract hoverflies and parasitic wasps — calendula, phacelia, dill. If root pests are an issue in the soil, consider French marigolds as a pre-crop or border planting.
3. Consider heights and spacing. Tall companions should go to the north (or south in the Southern Hemisphere) of shorter crops so they don't cast excessive shade. Low-growing companions fill gaps between taller plants without competition.
4. Add flowers and herbs at the edges and in gaps. You don't need to redesign the whole bed. A border of calendula and chives around an existing vegetable plot already makes a difference. Tuck nasturtiums into corners.
5. Keep allelopathic plants out of the main bed. Move fennel to a separate container or a far corner of the garden where it can do no harm to your vegetable crops.
6. Include at least one nitrogen-fixer. Even if you're not growing a full three sisters combination, including climbing beans, dwarf beans or peas somewhere in the bed adds nitrogen to the soil as a free benefit.
7. Observe and adjust over seasons. Companion planting is not precise. Note what works in your specific garden — pest populations, soil type and microclimate all affect results. The best companion plan is one refined over a few seasons of observation.

Companion planting checklist

• Main crops chosen and sited; companions planned around them.
• At least one good pollinator flower included — calendula, phacelia or chives.
• At least one hoverfly-attracting plant near pest-susceptible crops.
• Fennel isolated from the main vegetable bed.
• Alliums kept away from beans and peas in the same row.
• Legume included somewhere in the bed to fix nitrogen.
• Low-growing companions filling gaps between taller plants to suppress weeds.
• Notes kept on what works — companion planting improves with observation.