Does companion planting really work?

Some pairings do, through clear mechanisms: trap crops, pest confusion, shade, nitrogen fixing and attracting predators. Others are folklore. Treat it as a way to build diversity, not as a magic fix.

Which plants should never be grown together?

Fennel is a poor neighbour for most vegetables. Keep onion family members away from peas and beans, and avoid planting tomatoes near potatoes (they share blight) or brassicas.

What are the best companion flowers for the vegetable garden?

Nasturtiums (aphid trap crop), French marigolds (against some soil nematodes), calendula, alyssum and borage to attract pollinators and predatory insects.

Can I interplant everything in one bed?

Mixed planting is good, but give each crop its needs for sun, water and nutrients. Tall plants can shade low ones, and heavy feeders compete with each other.

Companion Planting

Companion planting is one of the oldest ideas in gardening, and one of the most over-promised. Some combinations genuinely help each other, but others owe more to tradition than to evidence. The good news is that the working principles are simple and well supported: grow a variety of plants, include flowers, and put every plant to work.

This guide separates what is proven from what is folklore and gives a practical planting plan. It forms part of the Vegetables & Herbs pillar and builds on How to Start a Vegetable Garden, with pest ideas from Beneficial Insects.

How companion planting actually works

Companions help in a handful of ways. Knowing which one a pairing relies on tells you whether to trust it.

Trap cropping: a more attractive plant draws pests away from the main crop (nasturtium for aphids, radish for flea beetle).
Predator support: flowers such as alyssum, dill and calendula feed hoverflies, lacewings and parasitic wasps that eat pests.
Nitrogen fixing: peas and beans host bacteria that capture nitrogen from the air, helping the next crop.
Shade and shelter: tall plants shade lettuce in summer; sweetcorn supports climbing beans.
Pest confusion: strong scents or mixed plantings make it harder for pests to find their host.
Soil benefits: deep-rooted plants bring up nutrients; some, like French marigold, suppress certain nematodes when grown as a dense cover.

Pairings and the strength of the evidence

Pairing	Claimed benefit	Evidence
Corn + climbing beans + squash (“Three Sisters”)	Beans fix nitrogen; corn supports beans; squash shades soil	Strong, an established traditional system
Nasturtium near brassicas and beans	Aphids and caterpillars prefer nasturtium (trap crop)	Good, when trap plants are replaced or treated
Alyssum or dill near any crop	Feeds hoverflies and parasitic wasps that eat aphids	Strong
Carrots + onions or leeks	Strong scents confuse carrot fly and onion fly	Moderate; physical barriers are more reliable
French marigold (Tagetes patula) as ground cover	Reduces root-knot nematodes in soil	Good, only if planted densely as a full season cover
Basil with tomatoes	Pest deterrent and “better flavour”	Weak; they do like the same conditions, though
Lettuce under tall plants	Shade extends the season in hot weather	Strong in hot climates
Radish with cucumber or squash	Deters cucumber beetle	Mixed
Garlic around roses and fruit trees	Deters aphids and fungal disease	Weak to mixed

When a pairing has weak evidence it is still harmless if both plants like the same conditions. Just do not rely on it to replace good hygiene, rotation and physical barriers.

Plants that make poor neighbours

Fennel: releases compounds that inhibit many plants; grow it alone or in its own pot.
Onion family near peas and beans: may slow their growth.
Tomatoes near potatoes: share blight, so separate them, and rotate.
Brassicas near strawberries: compete heavily for nutrients.
Black walnut: produces juglone, toxic to tomatoes, peppers and many others within its root zone.

How to plan a companion bed

Start with crop families. Group by family (tomato family, brassicas, legumes, roots) so rotation stays simple (see garden planning).
Match needs. Put heavy feeders (tomatoes, squash, corn) next to nitrogen fixers (beans, peas), and light feeders (lettuce, herbs) in the gaps.
Add flowers. Plant a line of alyssum, calendula or nasturtium every 3–4 m, or along bed edges.
Layer by height. Trellised beans or tomatoes at the back, medium crops in the middle, low leafy crops at the front.
Keep paths clear for airflow; crowded beds invite fungal disease.
Observe and adjust. Note which pairs thrive, and keep a rotation plan.

Three tested combinations to try

Tomatoes + basil + French marigold + alyssum: shared sun and water needs, with flowers to feed predators.
Brassicas + dill + nasturtium: dill brings parasitic wasps; nasturtium takes caterpillar and aphid pressure.
Carrots + onions + calendula: alternating rows break up the scent trail; cover with fleece for real carrot fly protection.

Slip-ups to steer clear of

Expecting companions to replace pest barriers, rotation and sanitation
Overcrowding beds so plants compete for light and nutrients
Pairing thirsty and drought-loving plants together
Planting mint or lemon balm in the ground, where it takes over; use a pot
Believing every folk pairing without testing it in your own garden

Shortcuts worth knowing

Plant herbs such as thyme, oregano and chives at bed corners to attract bees.
Trap crops must be monitored; remove infested plants to prevent them becoming a pest nursery.
Draw a rough bed plan each year and compare it with results; it is the best companion-planting book you will own.

Keep going from here

Learn organic pest control in Pests & Diseases, grow your own compost with Composting at Home, and plan the harvest with Harvesting and Storing Vegetables. Back to Vegetables & Herbs.