Using Companion Planting to Deter Common Pests

Why Scattering Beats Blocking Up

The old-fashioned allotment look — a tidy block of cabbages here, a neat row of carrots there — is a gift to pests. A pest that finds your brassicas lands on a perfect, unbroken buffet, and once it settles in, it spreads sideways with nothing to interrupt it. Companion planting works on a simple principle: break up the signal. Strongly scented flowers and herbs mask the smell of your crops, hide the shapes pests are searching for, and give the predators that eat those pests somewhere to live.

The key word is mix. Planting a separate bed of marigolds at the end of the plot does very little for the carrots in the middle. Dot them through the veg instead — between rows, at the corners of raised beds, tucked into the gaps beside a tomato plant in a pot. You are trying to make your plot confusing and crowded rather than tidy and obvious.

Marigolds: Small, Cheap and Genuinely Useful

Be clear about which marigold you mean, because they behave differently. French marigolds (Tagetes patula) are the compact, orange-and-yellow ones sold in trays for a few pounds, and they are the workhorse. Their roots release compounds that suppress soil nematodes, and their open, single flowers are easy for hoverflies and parasitic wasps to reach — both of which lay their eggs in aphid colonies. Double, frilly varieties look lovely but offer far less to insects.

Pot marigolds (Calendula officinalis) are a different plant altogether, but equally worth growing. They flower from early summer right into the first frosts, and those bright orange blooms pull in hoverflies at exactly the point when your broad beans are being hammered by blackfly. The petals are edible, too.

• Plant French marigolds between tomatoes, peppers and aubergines, in the ground or in the pot.
• Sow calendula in a rough row along the edge of a raised bed — it self-seeds freely once established.
• Let a few plants go to seed; the finches will thank you in autumn.

Nasturtiums: The Sacrificial Decoy

Nasturtiums are the companion plant that earns its keep by being eaten. Blackfly adore them, and if there is a nasturtium nearby they will often colonise it in preference to your beans, runner or French. Cabbage white butterflies will also lay on nasturtium leaves rather than on your kale, which means you are collecting and squashing caterpillars from one sprawling plant instead of hunting through the whole brassica patch.

The trick is to keep an eye on them. A nasturtium thick with blackfly quickly becomes a nursery that spills over onto everything else. Check the undersides of leaves twice a week, rub off the worst clusters with your fingers, and either cut the plant back hard or pull it out altogether if the infestation gets away from you. Trailing varieties are ideal for tumbling over the edge of a container or a wall, where they also give ground beetles and spiders some cover.

Alliums: A Scent Screen Against Carrot Fly

Carrot fly finds its target by smell, flying low and following the scent of crushed carrot foliage. Interplanting with onions, leeks, spring onions, garlic and chives muddles that trail. Chives are the most practical choice on a small plot: they are perennial, they come back year after year, and their purple pom-pom flowers are one of the best early nectar sources for bees.

Use them as a living hedge. A row of chives either side of your carrot row, or a ring of spring onions around a carrot patch in a pot, is far more effective than a single plant in the corner. The honest caveat is that alliums reduce carrot fly damage, they do not eliminate it — a fine mesh barrier, 60cm high, alongside the planting is still the most reliable defence.

One rule worth remembering: do not plant alliums next to peas and beans. They can suppress legumes, so keep your chives well away from the climbing frame.

Bringing In the Predators

Half of companion planting is about confusing pests; the other half is about feeding the creatures that eat them. Adult hoverflies, lacewings and parasitic wasps need nectar and pollen, and they need it in small, accessible flowers. Give them a continuous supply and they will patrol your plot for you.

• Umbellifers: let coriander, dill, fennel and parsley bolt. The flat flower heads are magnets for hoverflies.
• Borage: superb bee plant, self-seeds generously, and looks cheerful among courgettes.
• Phacelia: sow a pinch in any bare patch; it flowers in weeks and is arguably the single best hoverfly plant for a UK garden.
• Thyme, rosemary and sage: shrubby, drought-tolerant, and useful for masking the scent of brassicas.

Add a small log pile, a bundle of hollow canes, or a patch of long grass at the back of a border. Ground beetles, which patrol at night for slug eggs and caterpillars, need somewhere damp and dark to shelter during the day.

Making It Work on a Small Plot or in Containers

Space is tight, so be deliberate rather than scattering seed hopefully. Alternate rows in a raised bed rather than planting in blocks — a row of carrots, a row of spring onions, a row of carrots. Put marigolds in the gaps where a crop has failed, and tuck a nasturtium into the corner of every bed you can spare.

In pots, the principle still holds. A tomato in a 30cm container will happily share with a single French marigold and a clump of basil; a carrot container can take a fringe of chives around the rim. Keep the flowers small and the planting tight, and water carefully, because competition for moisture is fiercer in a container than in open ground.

Finally, treat companion planting as one tool among several, not a magic wand. It works best alongside healthy soil, sensible watering, good airflow, hand-picking of eggs and caterpillars, and a bit of fleece where the pressure is highest. Do those things, and the flowers and herbs around your veg will quietly tip the balance your way.