Companion Planting to Reduce Common Garden Pests

Why Companion Planting Earns Its Keep

Companion planting gets oversold in some quarters and dismissed in others. The truth sits comfortably in the middle: it will not banish pests from your plot, but it will tip the balance in your favour. There are three broad ways it works, and it helps to know which one you are relying on.

The first is scent masking. Pests such as carrot fly and cabbage white hunt largely by smell, following a chemical trail until they find a plant worth laying eggs on. Surround a crop with strongly scented herbs — chives, garlic chives, sage, rosemary, mint — and the signal becomes muddled. The pest is still out there, but it has to work much harder.

The second is breaking up the search. Tall, feathery or sprawling plants interrupt the neat block of green foliage that flying insects home in on. A brassica bed with nasturtiums spilling over the edges and dill standing between the rows is simply harder to read from above.

The third, and probably the most valuable, is habitat for beneficial insects. Hoverflies, lacewings, ladybirds and tiny parasitic wasps do more pest control in a fortnight than you will manage with a spray bottle all summer. The adults need nectar and pollen, so a few flowers left to bloom are an investment, not a waste of bed space.

The Herbs and Flowers Worth Growing First

You do not need an encyclopaedic collection. Start with a handful that earn their place in a UK garden:

• Chives and garlic chives — the classic carrot fly deterrent, useful in a row between carrots. Both are perennial, cut-and-come-again, and the flowers are edible.
• Salad onions — sow a row alongside carrots and pull them as you thin. Cheap, quick and effective.
• Nasturtium — a sacrificial trap crop for blackfly and cabbage white. Grow it on poorer soil; rich soil gives you leaves, lean soil gives you flowers.
• Pot marigold (calendula) — easy from seed, flowers for months, and a magnet for hoverflies and bees.
• French marigolds — especially useful under glass, where their roots release compounds that suppress root-knot nematodes in containers.
• Sweet alyssum — low-growing, fragrant, and a favourite of hoverflies and parasitic wasps. Perfect at the front of a bed or under greenhouse staging.
• Dill and coriander — let a few plants bolt. The flat flower heads are a landing pad for beneficial insects.
• Basil — traditionally planted with tomatoes, and genuinely helpful against whitefly and aphids.
• Borage — tall, hairy and covered in bees. Good near strawberries and courgettes.
• Sage, rosemary, thyme and lavender — woody herbs that double as a low boundary hedge around a vegetable bed.

Pairings for the Pests You Will Actually Meet

Carrot fly. Sow salad onions or chives in the row beside your carrots, and add a second line of them as a barrier. The fly is a weak flier that stays low, so a 60cm barrier of fleece or a dense herb hedge will stop most of them. Rosemary works well if you have a spare plant.

Cabbage white butterflies. Dot sage, rosemary and thyme among your brassicas and let nasturtiums flower nearby as a decoy. Be realistic: a female will still find your sprouts. Check leaf undersides for yellow eggs every few days and squash them — companion planting buys you time, not immunity.

Aphids and blackfly. Pinch out the growing tips of broad beans in late May, which removes the softest growth the blackfly love. Keep nasturtiums nearby as a trap crop, and encourage ladybirds with dill, calendula and alyssum. A healthy predator population clears a colony far faster than you can.

Whitefly. A greenhouse problem, mostly. French marigolds and basil grown among tomatoes help; alyssum under the staging brings in parasitic wasps. Yellow sticky traps are a useful extra in a closed space.

Slugs and snails. No companion plant will solve these. Encourage ground beetles with log piles and undisturbed corners, raise your seedlings in modules and plant them out sturdy, and accept that a scattering of nasturtium may become slug food — which is, in a grim way, the point.

Making It Work on a Small Plot or Allotment

Space is precious, so do not give a whole bed over to flowers. Instead, work companions into the gaps:

• Run a row of salad onions between carrot rows, and radishes through them as a quick marker crop.
• Tuck lettuce under tall brassicas — they appreciate the shade and the space would otherwise be bare.
• Plant a herb border along the edge of a raised bed; it is productive, ornamental and pest-deterrent in one.
• On an allotment, keep perennial herbs along the boundary and leave a strip of phacelia or calendula to flower. Ten per cent of the plot is a fair price for better pollination and fewer aphids.
• Grow mint in pots, always — in open ground it will take over everything within a season.

Under Glass: Greenhouses and Polytunnels

Still, humid air makes greenhouses ideal for whitefly, red spider mite and mildew. Basil, French marigolds and alyssum in pots at the ends of the bed or under the staging do real work here without competing with your tomatoes for light and water. Keep basil in the same pot as a tomato if space is tight; they grow happily together and you will notice the difference in airflow and scent.

The golden rule is not to crowd. Flowering herbs left to grow into the crop will shade it and cut the ventilation that prevents botrytis. Open vents early, water in the morning, and let the companions sit at the margins.

A Few Honest Cautions

Not everything gets along. Fennel releases compounds that suppress nearby plants, so keep it to a corner of its own. Dill and fennel cross readily and neither enjoys being crowded by the other. Mint spreads; keep it contained. And no companion scheme will rescue a crop grown in exhausted soil or planted far too closely.

Rotation still matters, airflow still matters, and a late May frost will still catch you out. Use companions as one layer of a sensible approach — good soil, clean tools, sturdy transplants, and a willingness to inspect your plants properly twice a week. Keep a notebook of what you sowed where and which combinations actually reduced damage on your plot. After two or three seasons you will have a system tailored to your soil, your pests and your pests' predators, which is worth more than any chart.