Eleanor Hartley
Why Mixing Plants Actually Works
Pests rarely stumble across your veg by accident. Most of them hunt by scent, homing in on the distinctive smell of a row of carrots or a stand of cabbages. Companion planting interrupts that process in two ways. First, strongly scented flowers and herbs mask the smell of the crops you are trying to protect, so the pest flies on past. Second, many of these plants offer food and shelter to the creatures that eat pests — hoverflies, lacewings, ladybirds and parasitic wasps — and those predators do a great deal of quiet work on your behalf.
There is a third mechanism worth knowing about: the trap crop. Some plants are simply more attractive to a pest than your prized brassicas or beans, so you sacrifice them deliberately and keep the damage away from the crop that matters. None of this is a magic cure, but in a small garden, where you cannot rotate crops over long distances, a mixed, bustling planting scheme is one of the most effective tools you have.
Marigolds: Small, Cheap and Genuinely Useful
When gardeners talk about marigolds for pest control, they usually mean French marigolds (Tagetes patula) rather than the taller African types. These compact, single-flowered varieties are the ones worth seeking out, and a packet of seed costs very little.
- Under glass: plant a ring of French marigolds around greenhouse tomatoes, peppers and aubergines to help keep whitefly at bay.
- In the soil: their roots release compounds that suppress root-knot nematodes, so they earn their place in beds where you grow tomatoes or potatoes.
- Getting started: sow under cover in April at about 20°C, harden off, then plant out after the last frost, roughly 15–20cm apart. Deadhead regularly to keep flowers coming until autumn.
They also look cheerful, which matters more than it sounds — a plot you enjoy tending is a plot you tend often, and frequent visits mean you spot problems early.
Nasturtiums: The Classic Sacrificial Plant
Nasturtiums are the best-known trap crop in British gardens, and for good reason. Blackfly will colonise them in preference to your broad beans, and cabbage white butterflies will often lay on them instead of your kale or sprouts.
Sow them direct from April onwards, about 30cm apart, or use the bushier 'Tom Thumb' in pots and at the front of beds. Keep them on the lean side: rich soil gives you masses of leaf and very few flowers. Check the undersides of leaves every few days and either squash the aphid colonies with your fingers or leave them for the ladybirds to find. The flowers and peppery leaves are edible, so nothing is wasted. Just keep an eye on trailing varieties, which can smother smaller neighbours if left unchecked.
Herbs That Earn Their Space Twice Over
Herbs are the hardest-working companion plants in a small garden because you get scent, flavour and pest disruption from the same patch of ground. Plant them among your crops rather than fencing them off in a separate herb bed.
- Chives, garlic chives and spring onions: a row between your carrots is the traditional defence against carrot root fly. The onion scent confuses the low-flying females as they search for a place to lay.
- Basil: grow beside tomatoes, in the greenhouse or on a sunny windowsill, to help with whitefly and thrips. Pinch out the tips to keep it bushy.
- Rosemary, sage, thyme and lavender: strongly aromatic evergreens that deter cabbage white butterflies and slugs. They are drought-tolerant and perfect for the dry edges of a raised bed.
- Dill, fennel, coriander and chervil: let a few plants bolt and flower. Their flat, open flower heads are magnets for hoverflies and parasitic wasps, whose larvae eat aphids by the hundred.
- Mint: useful against ants and aphids, but always grow it in a pot sunk into the ground, or it will take over.
- Borage: a bee favourite. A single plant near your strawberries or courgettes brings in pollinators all summer.
Fitting It All Into a Small Plot
You do not need to redesign the garden. Interplant as you sow. Put a line of chives between your carrot rows, dot marigolds along the front of the tomato bed, and let nasturtiums sprawl over the edge of a raised bed where they will not shade anything precious. In containers, a tomato, a basil plant and a marigold make a perfectly balanced trio in a 30cm pot.
Aim for roughly one flowering companion per square metre in a working bed, and leave a few herbs to flower permanently near paths and seating areas where predators will find them easily. Allow at least 30cm between your main crops so that air moves freely — overcrowding is the fastest route to mildew and botrytis, which will cost you more than the pests ever would.
Getting the Balance Right
Treat companion planting as one layer of a sensible strategy rather than a standalone fix. Combine it with netting over brassicas, barriers around carrots, a nightly slug patrol with a torch, and a garden that welcomes birds and hedgehogs. Above all, resist the urge to spray broad-spectrum insecticides, because these kill the hoverflies and ladybirds that are doing your pest control for free.
Tolerate a few aphids — they are the food supply that keeps your predators in residence. Keep a notebook, jot down what you planted where and how the season went, and after two or three years you will have a planting plan tailored precisely to your own patch of Britain. That local knowledge is worth more than any general rule.
