Companion Planting Guide: What to Grow Together (and What to Keep Apart)

Companion planting — growing two or more plant species in close proximity for mutual benefit — is one of the oldest agricultural practices in human history. The Three Sisters polyculture of corn, beans, and squash fed Indigenous North American civilisations for centuries. European cottage gardens combined aromatic herbs with vegetables long before anyone understood why it worked. Today we have the science to explain many of these traditional pairings and to identify which folk claims are real versus wishful thinking.

This guide covers the mechanisms behind companion planting, the best-evidenced plant combinations, what to keep apart, and how to plan your garden to take advantage of plant partnerships.

The Science Behind Companion Planting

Companion planting works through several distinct mechanisms. Understanding which mechanism applies to each combination helps you evaluate which pairings are worth trying:

Allelopathy: Chemical Communication

Plants release chemical compounds from their roots and leaves that can inhibit or stimulate the growth of neighbouring plants. This is called allelopathy. The most dramatic example is black walnut trees, which release juglone — a compound toxic to many vegetable crops, roses, and rhododendrons — from their roots and decaying leaves. Keep any black walnut tree at least 15 metres from your vegetable garden.

Positively, some allelopathic compounds benefit neighbours. Alliums (garlic, onions, chives) release sulphur compounds that deter fungal diseases in nearby plants. Marigolds release thiophenes from their roots that are toxic to root-knot nematodes — this is one of the best-evidenced companion planting mechanisms in the scientific literature.

Nitrogen Fixation

Legumes (beans, peas, clover, vetch) form symbiotic relationships with Rhizobium bacteria that live in nodules on their roots. These bacteria fix atmospheric nitrogen — converting N₂ gas to ammonium compounds that the legume can use. When the legume dies or roots turn over, this nitrogen becomes available to neighbouring plants.

This is the mechanism behind the Three Sisters: corn and squash are heavy nitrogen users, while beans restore nitrogen to the soil. Modern variants include using crimson clover as a living mulch beneath brassicas, or planting peas before sweet corn in a rotation.

Pest Confusion and Disruption

Many garden pests locate host plants using olfactory cues — they smell the volatile compounds their host plants produce. Dense monocultures broadcast these scent signals strongly and clearly. Mixed plantings dilute and disrupt these scent cues, making host plants harder for pests to locate.

Research on carrot fly (Psila rosae) showed that interplanting carrots with onions reduced carrot fly damage compared to monoculture rows. The onion scent interfered with the carrot fly's host-finding ability. Similar effects have been observed with cabbage white butterflies being confused by interplanted aromatic herbs.

Beneficial Insect Attraction

Flowers attract pollinators (bees, butterflies, hoverflies) and beneficial predatory insects (parasitoid wasps, lacewings, ground beetles) that prey on garden pests. Companion flowers planted around vegetable beds create a habitat diverse enough to support these populations year-round.

Parasitoid wasps — which lay eggs inside aphids, caterpillars, and whiteflies — are particularly valuable. They require nectar and pollen for adult nutrition but need small flowers they can access with their short mouthparts. Dill, fennel, sweet alyssum, phacelia, and yarrow are excellent parasitoid wasp attractors.

Physical Interaction

Some companion effects are purely structural. Tall corn provides shade for heat-sensitive lettuce growing in its shadow. Climbing beans use corn as a trellis. Squash leaves shade the soil, reducing moisture evaporation and weed germination. Ground covers planted beneath taller crops reduce splash-back of soil-borne fungal spores onto lower leaves.

Trap Cropping

Trap crops are sacrificial plants placed to attract pests away from valuable crops. Nasturtiums are classic aphid magnets — aphids preferentially colonise nasturtium over neighbouring plants. Instead of reaching for pesticide, you monitor the trap crop and remove or treat it when pest numbers peak, protecting the main crop with minimal intervention.

The Best Companion Planting Combinations

Tomatoes

Tomatoes have more research-backed companions than almost any other vegetable:

• Basil: Anecdotal evidence is strong that basil confuses aphids and whitefly through its volatile compounds. Plant every 45–60 cm around tomato plants.
• French marigolds (Tagetes patula): Root exudates deter root-knot nematodes. Effectiveness requires planting densely around the tomato bed, not just occasional plants.
• Borage: Flowers attract bees (improving pollination) and are anecdotally believed to deter tomato hornworm. Also accumulates calcium, which tomatoes need.
• Carrots: Loosen compacted soil around tomato roots; the relationship is largely structural.
• Parsley: Attracts parasitoid wasps that prey on aphids and caterpillar eggs.

Avoid with tomatoes: Fennel, brassicas (competing heavy feeders), corn (shares blight vectors), potatoes (same family, share late blight).

Brassicas (Cabbage, Broccoli, Kale, Brussels Sprouts)

• Dill: Attracts parasitoid wasps that parasitise cabbage white caterpillars. One of the most effective brassica companions.
• Nasturtium: Trap crop for cabbage aphids. Inspect and treat nasturtium rather than brassicas.
• Aromatic herbs (sage, rosemary, thyme): May confuse cabbage white butterflies with strong scent masking.
• Celery: Repels cabbage white butterflies; the mechanism is not fully understood.

Avoid with brassicas: Tomatoes, strawberries, other brassicas in the same family (rotation gaps are more important than companion planting for this family).

Beans and Peas (Legumes)

• Corn: Classic Three Sisters pairing. Beans climb corn stalks; corn receives nitrogen from bean root nodules.
• Squash and courgette: Third member of the Three Sisters. Benefits from bean-fixed nitrogen and shares the canopy structure.
• Carrots: Complementary root zones (beans in upper soil, carrots deeper) with no competition.
• Summer savoury: Traditional companion claimed to deter bean beetles and improve bean flavour.

Avoid with legumes: Alliums (onions, garlic, leeks) — allicin compounds inhibit the Rhizobium bacteria that form nitrogen-fixing root nodules. Planting garlic next to beans reduces the bean's ability to fix nitrogen.

Carrots

• Onions and chives: Classic pairing — each deters the other's primary pest (carrot fly and onion fly). Plant in alternating rows.
• Leeks: Similar effect to onions. Traditional cottage garden pairing.
• Rosemary and sage: Aromatic herbs mask carrot scent from carrot fly.
• Lettuce: Uses the space between wider-spaced carrot rows with no competition for resources.

Roses

• Garlic: Root exudates are believed to repel aphids from roses. Plant a clove at the base of each rose bush.
• Chives: Similar effect to garlic; also provides visual appeal with purple flowers.
• Lavender: Repels aphids and provides habitat for beneficial insects. Requires the same well-drained alkaline conditions as many rose cultivars.
• Marigolds: Deter aphids and nematodes; low-growing and non-competitive with rose roots.
• Catmint (Nepeta): Classic rose underplanting — attractive ground cover that deters aphids and attracts pollinators.

Plants to Keep Apart: The Bad Companions

Beneficial Flowers for Every Garden

These flowering plants benefit nearly every neighbouring crop and should have a place in any vegetable garden:

• French marigold (Tagetes patula): Best nematode deterrent; also deters whitefly and repels some beetles. Non-negotiable in any vegetable garden bed.
• Nasturtium (Tropaeolum): Trap crop for aphids; edible flowers and leaves. Attracts predatory hoverflies. Let some self-seed for continuous cover.
• Sweet alyssum (Lobularia maritima): Low-growing; powerful attractor of parasitoid wasps and hoverflies. Ground-cover benefit between larger plants.
• Phacelia (Phacelia tanacetifolia): One of the highest-rated bee-attracting plants; also attracts hoverflies and parasitoid wasps. Blue flowers visible to UV-sensitive insects.
• Borage (Borago officinalis): Star-shaped blue flowers attract bees; self-seeds freely. Accumulates minerals including calcium. Edible flowers.
• Calendula (pot marigold): Attracts beneficial insects; root exudates deter nematodes; flowers edible. Easy from seed, long blooming season.
• Yarrow (Achillea millefolium): Flat-topped flowers attract parasitoid wasps; perennial; accumulates potassium, copper, and other minerals in leaf tissue.

Companion Planting for Pest Control

The most practically useful companion planting applications are pest-oriented. Three strategies work best:

Strategy 1: Scent Masking

Plant strongly aromatic herbs throughout the vegetable bed. Insects locate host plants by scent; a bed that smells strongly of multiple aromatic species makes host-finding harder for specialists. Mix at least three or four aromatic herbs through your vegetable plantings: basil, thyme, sage, rosemary, and chives all contribute.

Strategy 2: Habitat for Beneficials

Maintain a continuous flow of small flowers to support parasitoid wasp and hoverfly populations. If these populations crash between flower flushes, pest populations can explode unchecked. Plan for overlapping bloom times — sweet alyssum from spring, then phacelia, then calendula, then late-season borage and nasturtium.

Strategy 3: Physical Barriers and Trap Crops

Use physical companion plants as barriers. A row of tall basil between the garden path and your tomatoes may reduce the number of aphids that drift in from outside. Nasturtiums planted at the garden's perimeter intercept incoming aphid populations before they reach valuable crops.

Companion Planting by Crop: Quick Reference

Planning a Companion-Planted Garden

A companion-planted garden is more complex to design than a simple row vegetable garden, but several principles simplify the planning:

1. Start with your main crops — tomatoes, beans, brassicas — and place them first.
2. Add beneficial flowers as border plants and in the gaps between crop groups. French marigolds and sweet alyssum can edge every bed.
3. Interplant aromatics throughout the bed — not just at the edge. One basil plant every 60 cm among tomatoes is more effective than a row of basil at the border.
4. Reserve a trap crop zone — nasturtiums along one edge of the garden intercept pests arriving from outside.
5. Respect antagonistic pairs — keep fennel in its own isolated corner or in pots, well away from vegetable beds.

Posts in This Series

• Tomato Companion Plants: Best and Worst Combinations
• Three Sisters Companion Planting: Corn, Beans, and Squash
• Companion Plants for Pest Control: A Science-Backed Guide
• Companion Plants for Roses: Garlic, Lavender, and Catmint
• Basil Companion Plants: What Grows Well With Basil
• What Not to Plant Together: The Complete Incompatibility Guide
• Companion Plants for Peppers: Boost Yield and Repel Pests
• Companion Planting for Beginners: Simple Combinations That Actually Work
• Vegetable Companion Planting: Crop-by-Crop Pairing Guide
• Companion Planting Chart: Complete Quick-Reference Table