Companion Planting: What Grows Well Together (and What the Evidence Says)

Companion planting asks what grows well together, and what the evidence says, because some pairings control pests while others only sound good in theory.

Companion planting works when it uses a real mechanism, such as feeding predatory insects, drawing pests to a sacrificial crop, or providing shade, and it fails when it relies on a folk rule. Pest and predator effects have the best support. Flavour, “chemical repellent” and root-communication claims mostly don’t. This guide sorts well-supported pairings from tradition, then shows how to build a mixed bed around the ones that hold up.

It sits within the Vegetables & Herbs guides. Planning a bed from scratch is covered in How to Start a Vegetable Garden from Scratch, and the insects that do the pest control are profiled in Beneficial Insects.

Companion planting at a glance

What the research on companion planting does and does not show

Reviews of field trials generally find that mixed plantings carry fewer plant-eating insects and more natural enemies than single-crop blocks. Yield results are less consistent: sometimes higher, often unchanged, occasionally lower because plants compete. That is a useful baseline. Diversity helps with pests, but it doesn’t guarantee a bigger harvest.

Most individual pairings have not been tested properly. A gardener who plants basil beside tomatoes and sees a good crop has not tested anything, because the tomatoes might have done as well alone. When a pairing is untested, the decision rule is simple: it’s fine if both plants want the same conditions, and not worth a space if it replaces something proven.

Companion planting pairings graded by evidence

Choosing flowers that feed the pest controllers

The best-supported mechanism needs the most care to get right. Adult hoverflies and tiny parasitic wasps feed on nectar, and their larvae or offspring do the killing. Their mouthparts are short, so they can only reach nectar in small, open, shallow flowers. Showy double-flowered garden forms and deep tubular flowers are poor value for this job.

A practical target is one flowering plant for every few vegetable plants, with the flowers at the bed edge or in the gaps rather than in a separate border across the garden. Predators can only protect what they can find.

Real companion planting conflicts to plan around

Some “bad neighbour” rules are real, others are tradition, and a few reflect competition rather than chemistry.

• Black walnut: roots and fallen leaves release juglone, which harms tomato, potato, pepper, aubergine and apple. The zone of damage reaches at least the drip line and often further, so raised beds on a barrier or a different spot are the fix.
• Tomatoes and potatoes: they get the same late blight, so a nearby infected crop is a source of spores. Separate them, and rotate.
• Dill and fennel: they cross-pollinate, which affects saved seed. The claim that fennel poisons all neighbours is poorly documented, but it grows 1.5 to 2 m (5 to 6 ft) tall and self-seeds, so give it its own corner.
• Onions with peas and beans: a long-standing rule with little trial backing. Onions are shallow-rooted and poor competitors, so the practical issue is weeds and shade, not chemistry.
• Heavy feeders side by side: squash, sweetcorn and tomatoes in one tight bed will compete for water and nutrients regardless of any “affinity”.
• Mint and lemon balm: they spread by runners, so plant in a pot (see Herb Garden Essentials).

How to build a companion planting mixed bed

1. Choose crops by shared needs. Group sun-loving, thirsty crops (tomato, pepper, basil) apart from drought-tolerant herbs (thyme, rosemary, oregano).
2. Set height layers. Trellised beans or tomatoes on the north side in the northern hemisphere (south side in the southern), medium crops in the middle, low leaves at the front.
3. Add a flower line. Sow alyssum along bed edges and dill or calendula every 3 to 4 m (10 to 13 ft) so no crop is far from nectar.
4. Fill gaps with fast crops. Sow radish or baby salad between slow crops. They finish in 25 to 45 days, before the slow crop needs the room.
5. Keep families together. Put tomato-family plants in one zone and brassicas in another so rotation stays workable.
6. Leave airflow. Crowding helps leaf diseases such as blight and powdery mildew spread. Follow the spacing on the seed packet, not the smallest gap that fits.

Three combinations worth trying

• Tomato, basil, French marigold, alyssum: shared sun and water needs. The marigolds only reduce nematodes if used as a dense cover in infested soil, so treat them here as flowers. The tomato itself is covered in How to Grow Tomatoes.
• Brassicas, dill, nasturtium: dill supports parasitic wasps, nasturtium soaks up aphids. Inspect the nasturtium weekly and bin heavily infested plants.
• Carrots under fleece with calendula nearby: the barrier does the protecting, the flowers feed predators of aphids on other crops.

Troubleshooting companion planting in a mixed bed

Common mistakes

• Treating companions as a substitute for barriers. Scent confusion is unreliable, so fleece and netting remain the dependable defence against carrot fly and cabbage white. Use both.
• Using one marigold as a nematode fix. The effect needs a dense planting across a full season, then digging in. A single plant does nothing measurable.
• Ignoring competition. If two plants want the same water and nutrients, “friendly” makes no difference. Space for the larger plant’s mature size.
• Planting a trap crop and never checking it. It then multiplies the pest. Inspect weekly and remove it when infested.
• Dropping rotation. Mixed beds still need rotation. Soil diseases build up in family groups, whatever grows beside them.

Next steps

Choose crops that reward a first mixed bed from 7 Easiest Vegetables to Grow, build the soil with Composting at Home, and learn which pests are worth the effort of controlling in Common Garden Pests and Organic Control. Back to the Vegetables & Herbs guides.