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Why scent is a pest's worst enemy

Aphids, whitefly and cabbage white butterflies all find their dinner the same way: by smell. They drift on the breeze, pick up the volatile compounds given off by your broad beans or your brassicas, and home in. This is why a solid block of identical plants is such an easy target — it reads like a beacon from several metres away.

Companion planting works on that weakness. By mixing strong-smelling flowers and herbs among your vegetables, you mask the scent of the crops you actually want to eat. Pests still arrive, but they take longer to find a host plant, waste energy investigating the wrong things, and often move on. It is not a force field, and anyone who tells you otherwise is selling something. What it does is reduce pressure, so your plants stay healthy enough to shrug off the aphids that do turn up — no spraying required.

Marigolds: the hardest-working flower in the veg patch

If you plant only one companion this year, make it a French marigold (Tagetes patula). These are compact, cheap to raise from seed and remarkably hard-working. Their roots release compounds that suppress root-knot nematodes in the soil, and their foliage carries a pungent scent that whitefly dislike intensely.

Choose single-flowered varieties such as 'Tangerine Gem' or 'Linnaeus' rather than the big frilly doubles. Single flowers have open centres that hoverflies and tiny parasitic wasps can actually reach, and those insects are your unpaid workforce — hoverfly larvae hoover up aphids by the dozen.

Practical spacing: sow indoors in March or April, harden off in May, then plant out every 30–45cm along a row of tomatoes or between brassicas. In a greenhouse, pop one or two in the border at the door end, where whitefly tend to enter first.

Nasturtiums: plant them to be eaten

Nasturtiums work differently. Instead of repelling pests, they distract them. Blackfly find nasturtium foliage irresistible and will colonise it in preference to your broad beans and runner beans, leaving the crop clean. Cabbage white butterflies will also lay on nasturtium rather than your kale, and you can simply pick off and bin the eggs.

Treat them as a sacrificial crop and site them deliberately:

• Sow direct in late April to May, once the soil has warmed, spacing clumps about a metre apart.
• Place them a few paces upwind of the row you are protecting, not tucked behind it.
• Choose a bush variety near the veg so it does not swamp neighbouring plants — the trailing types can smother a bed by August.
• Check the undersides of leaves every few days and squash any clusters of yellow eggs before they hatch.

Nasturtiums are also edible, so a few flowers in a salad is a tidy reward for your trouble.

Basil, and the aromatic herb brigade

Basil is the classic partner for tomatoes, and there is sound reasoning behind it. Its strong scent interferes with whitefly and thrips locating their host, and interplanted basil appears to reduce the number of whitefly settling on tomato foliage. Grow it as a genuine companion rather than a lone pot on the windowsill — one basil plant roughly every 30cm between tomatoes works well.

Other herbs earn their place too. Coriander and dill run to flower quickly in a UK summer, and their umbels are magnets for hoverflies and predatory wasps. Mint, kept firmly in a pot so it does not take over, masks scents effectively along a path edge. Chives and garlic chives around carrots and lettuces add another layer of confusing aroma.

Making companion planting work under glass

Protected growing brings its own pests. Greenhouses and polytunnels stay warm and still, which suits whitefly, red spider mite and aphids perfectly. In here, companion planting is about airflow as much as scent.

Plant basil between peppers, aubergines and tomatoes, and keep marigolds in pots that you can move to wherever a problem is building. Ventilate properly — open doors and vents on warm days even in April — because stuffy, humid air is what lets whitefly populations explode. Yellow sticky traps hung among the plants will tell you what is arriving, which is far better than spotting an infestation three weeks late. And never let a sacrificial nasturtium sit there seething with blackfly for weeks; once it has done its job, pull it out and compost it.

A simple planting plan for a UK plot

Start with a handful of reliable partnerships rather than trying to companion-plant everything at once:

• Brassicas — a nasturtium clump upwind, plus marigolds along the row edge.
• Tomatoes, indoors or out — basil between plants, a French marigold at the end of each row.
• Broad and runner beans — nasturtiums nearby as a blackfly trap, chives at the base.
• Carrots and lettuce — chives and coriander sown in the gaps to confuse aphids and carrot fly.

Keep the flowers deadheaded so they carry on blooming until the first frosts, and leave a few to set seed for next year. None of this replaces good soil, sensible rotation and a bit of hand-picking, but it does mean you can walk away from the chemical bottle. A garden that smells of marigold, basil and nasturtium is, happily, one where pests have to work a great deal harder for their supper.