Why companion planting earns its keep

Companion planting is often oversold as a magic bullet, but strip away the folklore and the useful mechanisms are surprisingly practical. Strongly scented plants can mask the smell of a crop that pests home in on. Dense or tall plantings can act as a physical barrier, breaking up the silhouette of a row of brassicas. Flowers provide nectar and pollen for hoverflies, lacewings and parasitic wasps, which then hunt the aphids and caterpillars on your vegetables. And sacrificial "trap" plants lure pests away from the crops you actually want to eat.

The honest expectation is not zero damage. It is less pressure, fewer sprays, more predators, and a plot that looks and smells like a garden rather than a battlefield. On a smallholding, allotment or kitchen garden where space is tight, that is worth having.

Carrots with spring onions: the pairing that works

Carrot root fly is the classic reason to try companion planting. The female fly travels low — often under 50cm — and finds carrots by scent. Anything that muddles that scent trail helps. Spring onions, bulb onions, leeks, chives and garlic all release sulphurous compounds that do the job well.

Sow a row of carrots and a row of spring onions alternately, roughly 20–30cm apart, rather than block-planting each in separate beds. You can also mix the seed and broadcast thinly. If you want belt and braces, stretch a fine mesh or fleece barrier 60cm high around the carrot bed; the fly rarely clears it. The onions will be lifted before the carrots finish, which reduces competition for water and nutrients.

Brassicas and nasturtiums

Nasturtiums are the great sacrifice of the brassica bed. Cabbage white butterflies and blackfly both find them irresistible, and a row of nasturtiums at the edge of your kale or sprouting broccoli will pull egg-laying females away from the crop. Site them a couple of metres upwind of the brassicas rather than right among them, and inspect them weekly — squashing clusters of blackfly by hand is far easier on nasturtiums than on your winter greens.

Strong-smelling herbs help too. Hyssop, thyme, sage and rosemary planted between or alongside brassicas seem to interrupt the butterflies' search. None of this replaces netting. A fine 7mm mesh over hoops is still the single most effective defence against pigeons and cabbage white, and companions work best around the edges of a netted crop rather than instead of it.

Scent and confusion among the herbs

Herbs earn their place twice — as food and as scent. Plant chives, garlic chives and coriander among tomatoes to blur the scent of the foliage. French marigolds around tomatoes and potatoes are a well-tried partner, and the roots of some varieties release compounds that suppress soil nematodes.

Keep mint in pots sunk into the ground; it will otherwise romp through a bed within a season. One useful rule of thumb: alliums and legumes dislike each other. Onions, garlic and leeks can slow the growth of peas and beans, so keep them in different parts of the rotation.

Build habitat for the garden's helpers

Deterrence is half the job; the other half is making your plot welcoming to the things that eat pests. Hoverfly larvae eat aphids by the hundred, and adult hoverflies need shallow, open flowers. That means phacelia, borage, poached egg plant, calendula, sweet alyssum and buckwheat.

• Let a few parsley, dill and fennel plants bolt — their umbels are magnets for hoverflies, ladybirds and tiny parasitic wasps.
• Sow a short strip of phacelia or borage near the vegetable beds, and re-sow every three or four weeks for a steady supply.
• Leave a small log pile or clump of long grass in a corner for beetles and frogs.
• Put a shallow dish of water with pebbles in it, so beneficial insects can drink without drowning.
• Stop spraying. Even gentle sprays knock back predators as readily as pests.

Making it work on a small plot

On an allotment or in a kitchen garden, forget neat blocks of single crops. Interplanting in alternate rows is easier to manage and gives you the scent confusion you are after. A few practical habits make the difference:

• Plan companions into your rotation rather than adding them later.
• Keep plants well spaced — stressed, overcrowded plants attract pests.
• Sow little and often, so an attack never wipes out a whole harvest.
• Grow a few companions in pots at the ends of beds if space is tight.
• Keep a notebook. What works on your soil, in your microclimate, is the only advice that really counts.

Companion planting will not rescue a badly timed crop or a neglected row, but used alongside netting, rotation and healthy soil, it makes a real difference — and it turns a working plot into a far more interesting place to spend a Saturday morning.