Companion planting: what actually holds up

What the evidence supports

Flowers and flowering herbs (alyssum, dill, cilantro left to bloom, buckwheat) genuinely pull in hoverflies, parasitic wasps and other predators that reduce aphids and caterpillars.

Interplanting and diversity make it harder for specialist pests to find and stay on their host crop, compared with a large single-crop block.

Tall or open crops can shelter shade-tolerant ones — lettuce in the afternoon shade of pole beans or corn.

Legumes fix nitrogen, but most of it becomes available to later crops after the legume dies, not to a neighbor growing at the same time.

What is mostly folklore

Most specific "plant X with Y for better flavor / bigger yield" pairings have little or no controlled evidence. The famous "tomato and basil" pairing is fine to do — they like the same conditions — but basil does not measurably improve the tomatoes.

The classic antagonist claims (beans hate onions, carrots hate dill) are weak. The real reasons to keep some plants apart are competition for light and water, shared diseases within a family, and cross-pollination (dill and carrot, different squash).

A practical layout approach

Group by water and sun needs. Keep members of the same family out of the same square year to year (rotation). Give heavy feeders room for airflow. Edge beds with flowers and let a few herbs bolt for the pollinators. Check the companion planting chart for a specific pair. Do not lose sleep over the charts.

Trap crops: planting something for the pests

A trap crop is a plant pests prefer, grown nearby so they gather there instead of on the crop you care about. It is one of the better-supported ideas in this area. Blue Hubbard squash planted around the edge of a squash or cucumber patch, for example, draws striped cucumber beetles and squash bugs, where they can be picked off or the trap plants removed.

The catch is that a trap crop only works if you deal with the pests once they've gathered - otherwise it becomes a nursery that sends a bigger population into the main crop.

Pairings that really do matter

A few separations are well founded. Black walnut trees release juglone from their roots, and tomatoes, peppers and potatoes planted within the root zone often wilt and die. Potatoes and tomatoes share early and late blight, so keeping them apart - and rotating both - limits disease. Squash varieties of the same species cross-pollinate, which matters only if you save seed.

The traditional Three Sisters planting - corn, pole beans and squash together - does work when timed so the corn is a few inches tall before the beans go in, and the beans and squash don't swamp it. It needs more space than it looks, and yields per plant are usually lower than growing each crop on its own.

Putting it into a real bed

Start with the non-negotiables - sun, spacing, and not repeating a plant family in the same spot - then use companion ideas to fill gaps: a row of flowering herbs along the edge, lettuce tucked into the shade of taller crops, quick radishes marking a slow row of carrots. The garden bed planner flags neighbouring pairs listed as poor companions and repeated families as you lay out a bed.

Common questions