Why herbs earn their place among the vegetables

Walk round a well-tended allotment in July and you will notice that the healthiest plots are rarely the ones given over entirely to straight rows of cabbage and carrot. There is usually a scruffy fringe of herbs — rosemary by the water butt, mint threatening to escape its bucket, basil tucked beside the tomatoes in the greenhouse. These are not there by accident. Herbs release volatile oils from their leaves, and those oils do two useful jobs: they mask the scent pests use to find their favourite crops, and they attract the hoverflies, ladybirds and parasitic wasps that eat the pests for us.

Companion planting will not wipe out aphids single-handed. What it does is tilt the balance — fewer pests find the crop, more predators arrive to deal with the ones that do, and you reach for the spray far less often.

Basil: the whitefly confuser

Basil's great partnership is with tomatoes. Its clove-and-anise scent interferes with whitefly and aphids as they hunt for a host, so a tomato plant with basil growing among it is simply harder to find. In a greenhouse, where whitefly can build into a serious nuisance by August, that is worth real money in saved fruit.

Basil is tender, so treat it as a summer visitor. Sow indoors in April, harden off carefully and plant out after the last frost, usually the end of May. Give it sun, shelter and free-draining soil. Pinch out the growing tips every fortnight to keep plants bushy and the oils strong, and set two or three between your tomato vines — or in pots at the end of the row, moved about as trouble appears.

Mint: vigorous, aromatic — and best kept in a pot

Mint is the most useful deterrent for the brassica patch. Its menthol-rich scent is disliked by aphids and flea beetles, and ants — which farm aphids for honeydew — tend to keep away from it. Plant spearmint or peppermint near cabbages, kale and broccoli and you will see fewer grey-green colonies clustering on the growing points.

The catch is that mint runs underground and will take over a bed within two seasons. Sink it in a bottomless bucket or a large pot plunged into the soil, rim just above the surface, and cut any runners that escape. Water well and cut it hard two or three times a season; young leaves carry the strongest oils. Let a separate clump flower for the hoverflies and bees, though the leaves turn bitter once it is in bloom.

Rosemary: an evergreen sentinel

Rosemary is the herb to plant along a row of carrots. Carrot fly finds its host by smell, homing in on the finely divided foliage, and a hedge of rosemary's resinous scent makes that much harder. Grow it as a low hedge at the end of the bed, or lay prunings along the row and brush them as you pass. The same scent is said to deter cabbage white butterflies, and sprigs scattered round young seedlings can save a few from a night's nibbling.

It is also the most permanent of the three. Rosemary is a shrub, not a bedding plant, and in a sunny, free-draining spot it will live for years. It hates wet feet, so on heavy clay dig in grit and plant on a slight mound. Take cuttings from non-flowering shoots in late summer; they root easily. Left to flower in spring, it is one of the best early nectar sources for bees.

A few more reliable companions

• Chives — a ring around carrots or under fruit trees is the traditional defence against aphids, and the pink flowers pull in hoverflies.
• Thyme — low and drought-tolerant; useful edging for brassicas, and thought to put off cabbage root fly.
• Sage — plant near cabbage and carrots, or let it sprawl around courgettes.
• Coriander — let some bolt; the flowers are magnets for hoverflies, whose larvae eat aphids by the hundred.
• Dill — good near cucumbers and lettuce, and its umbels bring in parasitic wasps.
• Fennel — disappointing as a companion. It suppresses nearby plants, so give it its own corner.

Making it work on a working plot

Companion planting only pays off if it fits the way you actually garden. A few practical rules help.

• Dot, don't block. Scatter herbs through the bed rather than planting solid rows. One every metre or so among the veg is enough to confuse pests.
• Use them alongside other defences. Netting over brassicas, crop rotation and a daily squash-and-drop session still do most of the heavy lifting.
• Keep plants growing strongly. Stressed, dry, hungry plants attract pests; water and mulch, and your herbs will pump out far more scent.
• Sow successionally. Basil and coriander fade fast, so a second sowing in mid-July keeps the scent going into autumn.
• Harvest as you go. Picking for the kitchen keeps herbs compact and fragrant. A herb you never cut becomes a tired, woody shrub that pests ignore.

Start with one or two partnerships this season — basil with tomatoes, mint with brassicas — and see how you get on. Most growers find that after a year or two the herb patch is no longer a decorative extra; it is the first thing they plant.