What Actually Works

Alliums (Onions, Garlic, Chives) with Carrots

The classic pairing, and there's genuine evidence behind it. The strong scent of alliums can confuse carrot fly, which locate carrots by smell. Interplanting rows of onions with carrots or edging carrot beds with chives provides some protection.

It's not foolproof, but it does help. For best results, combine with physical barriers like fleece or environmesh.

Marigolds (Tagetes) with Tomatoes and Brassicas

French marigolds (Tagetes patula) produce chemicals that deter whitefly and some aphids. Planting them near tomatoes in greenhouses or around brassicas can reduce pest pressure.

The scent also attracts hoverflies, whose larvae eat aphids. African marigolds (Tagetes erecta) have roots that suppress some soil nematodes, though you need a lot of them for noticeable effect.

Nasturtiums as Trap Crops

Nasturtiums are aphid magnets. Blackfly and other aphids love them even more than your beans or brassicas. Plant nasturtiums near susceptible crops and they'll often take the hit instead.

You can then either remove the aphid-covered nasturtiums or leave them to attract ladybirds and hoverflies that will spread to your other crops. This genuinely works.

Strong-Scented Herbs Throughout the Garden

Herbs like rosemary, sage, thyme, and lavender have strong scents that can mask the smell of vegetables, confusing pests that hunt by scent. Scattering them throughout beds provides some general pest deterrence.

They also attract beneficial insects: bees, hoverflies, and predatory wasps that all help with pollination and pest control.

Basil with Tomatoes

There's some evidence that basil can repel whitefly and aphids from tomatoes. Whether it's the scent or something else isn't clear, but many gardeners swear by it. Bonus: they're perfect culinary companions too.

What Probably Doesn't Work (Or Needs More Evidence)

Three Sisters (Corn, Beans, Squash)

This traditional Native American planting method is often cited as companion planting, but it's more about space efficiency than pest control. Corn provides support for beans, beans fix nitrogen, and squash shades the ground to suppress weeds.

It's a clever polyculture, but it doesn't specifically deter pests. In British gardens, it's also tricky because our summers often aren't warm enough for corn and squash to thrive together.

Cabbage with Tomatoes

Sometimes recommended to deter cabbage white butterflies, but there's no solid evidence that tomatoes repel them. Physical barriers (netting) are far more effective.

Most 'This Plant Repels Everything' Claims

You'll read that certain plants (often tansy, wormwood, or rue) repel all sorts of pests. The evidence is thin. Some plants contain compounds that are insecticidal in concentrated form, but that doesn't mean growing them nearby will protect your veg.

The Real Benefits of Companion Planting

Even when pest control claims are overstated, companion planting offers genuine benefits:

Attracting Beneficial Insects

Flowers throughout the veg garden bring in pollinators, hoverflies, ladybirds, and parasitic wasps. These insects help pollinate crops and prey on pests. This is probably the biggest benefit of companion planting.

Biodiversity Reduces Pest Pressure

Monocultures (large areas of a single crop) make it easy for pests to find and devastate plants. Mixed plantings make it harder for pests to locate their target crops and provide habitats for predators.

Efficient Use of Space

Fast-growing crops (radishes, lettuce) can be interplanted with slow growers (parsnips, brassicas). You harvest the quick crops before the slow ones need the space. This isn't pest control, but it's smart gardening.

Succession and Soil Health

Following heavy feeders (brassicas, squash) with nitrogen-fixing legumes (peas, beans) helps maintain soil fertility naturally. Again, not pest control, but good practice.

Practical Companion Planting for Pest Control

Here's how to use companion planting effectively:

Scatter Flowers Throughout Veg Beds

Don't create separate flower and veg areas. Tuck calendula, marigolds, alyssum, and phacelia into gaps between vegetables. They'll attract beneficial insects all season.

Edge Beds with Herbs and Alliums

A border of chives, thyme, or parsley around brassica or carrot beds provides some scent confusion and looks attractive.

Use Nasturtiums as Sacrificial Plants

Plant them near beans, brassicas, or anywhere aphids are a problem. Check them regularly and either remove heavily infested plants or leave them to attract predators.

Interplant Alliums with Carrots

Alternate rows of onions or spring onions with carrots. It won't eliminate carrot fly, but combined with good timing (sowing after peak carrot fly periods) and barriers, it helps.

Don't Rely on It Alone

Companion planting is one tool in your pest control toolkit. Use it alongside physical barriers (netting, fleece), good garden hygiene (removing pest habitats), crop rotation, and resistant varieties.

The Bottom Line

Companion planting isn't magic, and some of the traditional claims don't hold up to scrutiny. But it's not nonsense either.

The real value is in creating a diverse, flower-rich garden that supports beneficial insects and makes life harder for pests. Whether that's because of specific plant combinations or simply because biodiversity is healthier than monoculture is almost beside the point.

Plant flowers with your veg. Scatter herbs throughout beds. Use trap crops strategically. Enjoy the variety, the pollinators, and the reduced pest pressure. Just don't expect companion planting alone to solve all your pest problems.

Combine it with other methods, stay observant, and you'll have a healthier, more productive garden.