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COMPANION PLANTING

Companion Planting Guide — Which Vegetables to Plant Together

Use plant relationships to reduce pests naturally, improve yields, and make the most of Canada's short growing season.

Companion planting is one of the most effective tools available to organic gardeners — and one of the most misunderstood. It's not magic, and not every claimed pairing has strong scientific backing. But the best-documented combinations genuinely work: basil does repel certain insects around tomatoes, marigolds do suppress nematodes, the Three Sisters system does improve yields of all three crops. Done well, companion planting reduces your pest problems, improves soil, and makes your garden more resilient — without any chemical inputs.

For Canadian gardeners, companion planting also offers an important space-efficiency benefit. With a shorter season than most of the world, we want every square foot of our beds working hard from May to October. Good companions maximize that space through vertical layering, succession timing, and mutual support.

Complete Companion Planting Chart

This chart covers the most common Canadian vegetable garden crops. "Good companions" share space beneficially — through pest repulsion, nitrogen fixing, ground shading, or simply not competing for the same root depth or nutrients. "Avoid" combinations actively inhibit each other's growth or share disease and pest vulnerability.

Vegetable Good Companions Keep Apart From Why It Works
🍅 Tomatoes Basil, carrots, marigolds, borage, parsley Fennel, cabbage, corn, potatoes Basil repels aphids & whiteflies; marigolds deter nematodes
🌶️ Peppers Basil, carrots, tomatoes, marigolds Fennel, brassicas Basil improves flavour and repels aphids
🥕 Carrots Onions, leeks, lettuce, tomatoes, peas, rosemary Dill, parsnips Onions repel carrot fly; carrots repel onion fly
🥒 Cucumbers Beans, peas, radishes, sunflowers, nasturtiums Potatoes, sage, aromatic herbs Radishes repel cucumber beetles; nasturtiums act as trap crop
🫘 Beans Corn, squash, cucumbers, carrots, potatoes Onions, garlic, shallots, fennel Beans fix nitrogen into soil for corn & squash
🧅 Onions & Garlic Carrots, tomatoes, lettuce, beets, roses Beans, peas, asparagus Strong scent confuses and repels many pest insects
🥬 Lettuce Carrots, radishes, strawberries, tall plants (shade) Parsley, celery Shaded by tall neighbours in summer heat — extends season
🥦 Brassicas Nasturtiums, dill, thyme, onions, beets Tomatoes, strawberries, peppers, pole beans Nasturtiums trap aphids away from plants
🌽 Corn Beans, squash, pumpkins, cucumbers, melons Tomatoes, celery Three Sisters — classic polyculture with mutually reinforcing benefits
🥔 Potatoes Beans, peas, cabbage, marigolds, horseradish Tomatoes, peppers, eggplant, cucumbers, squash Keep away from tomato family — shares blight disease
🌿 Basil Tomatoes, peppers, asparagus, oregano Sage, thyme Repels aphids, whiteflies, spider mites — plant throughout garden
💐 Marigolds Almost everything — especially tomatoes, peppers None significant Suppress soil nematodes; repel aphids, whiteflies, cabbage moths
🌸 Nasturtiums Cucumbers, squash, brassicas, fruit trees None significant Trap crop — aphids prefer nasturtiums, leaving vegetables alone

Planning Your Companion Garden?

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The 6 Best Companion Planting Combinations for Canadian Gardens

🌽 1. The Three Sisters — Corn, Beans & Squash

The most famous companion planting system in North America, developed by Indigenous peoples over thousands of years. Corn provides a vertical trellis for beans to climb. Beans fix atmospheric nitrogen into the soil, feeding corn and squash. Squash spreads across the ground, shading out weeds and keeping soil moist. Together, they produce more food per square foot than any of the three grown alone.

How to plant: Create a mound 60 cm wide. Plant 4–6 corn seeds in a circle. When corn is 20 cm tall, plant 4–6 bean seeds around the corn. One week later, plant 2–3 squash seeds around the base of the mound. Works well in all Canadian zones — plant after last frost.

🍅 2. Tomatoes & Basil

The most popular companion pair in Canadian vegetable gardens — and one with real scientific backing. Basil volatiles (the aromatic compounds responsible for its scent) confuse and repel aphids, whiteflies, and thrips. Some research also suggests that volatile compounds from basil can improve tomato flavour. Planting basil beneath tomatoes also shades the soil, conserving moisture during dry July and August.

How to plant: One basil plant per tomato plant, 30–40 cm from the base of the tomato. Also plant a border of basil around the perimeter of your tomato bed. Harvest basil regularly — this triggers new growth and keeps the plant producing aromatic compounds all season.

🥕 3. Carrots & Onions (The Mutual Repellent Pair)

This is one of the most well-documented companion relationships. Carrot fly (Psila rosae) and onion fly (Delia antiqua) are both serious pests in Canadian gardens. The scent of onions masks the smell of carrots from carrot flies, and vice versa — when planted in alternating rows, each crop essentially hides the other from its primary pest. Studies in Scandinavia (where both pests are a major problem) have confirmed the effect.

How to plant: Alternate rows of carrots and onions 20–25 cm apart. Works best with direct sown carrots and transplanted onion sets. Also works with leeks in place of onions — leeks and carrots are an excellent combination.

🥬 4. Lettuce Under Tall Plants (Shade Companions)

Lettuce bolts (goes to seed) in hot summer weather, ending its harvest season prematurely. In Canadian gardens this typically happens in July-August. By planting lettuce on the north or east side of tall plants like tomatoes, corn, or climbing beans, the shade from the taller plants extends your lettuce harvest by 2–4 weeks. This also uses otherwise unproductive shaded space beneath your tall crops.

How to plant: Succession sow lettuce on the north-facing side (shaded) of your tomato bed from June through mid-July. The tomatoes will provide afternoon shade as they grow. Also works with spinach, arugula, and cilantro — all bolt-prone crops that benefit from afternoon shade.

💐 5. Marigolds Throughout the Whole Garden

French marigolds (Tagetes patula) are the single most useful companion plant in a Canadian vegetable garden. Their roots produce thiophenes — chemicals that actively suppress root-knot nematodes in the soil. Their flowers attract hoverflies and parasitic wasps that prey on aphids and caterpillars. Their scent confuses aphids, whiteflies, and cabbage moths. And they're edible — the petals are mild and peppery in salads.

How to plant: Plant marigolds at the end of every raised bed row, in the corners of beds, and dotted throughout your entire vegetable garden. French marigolds (smaller, bushier) are more effective than African marigolds (taller). For nematode suppression, they need to grow in the same spot for a full season — so plant early and let them run.

🌸 6. Nasturtiums as Trap Crops

A trap crop is a plant that pests prefer over your vegetables — it "traps" them away from your food crops. Nasturtiums are the classic example: aphids strongly prefer nasturtiums over nearly everything else. Plant them around your garden perimeter or near aphid-susceptible crops like brassicas, beans, and cucumbers. When aphids colonize your nasturtiums, you can cut and compost that stem, or leave it as a feeding station for predatory ladybugs.

How to plant: Direct sow nasturtium seeds at the edges of your beds after last frost. They're fast-growing and will bloom within 6–8 weeks. Both climbing and dwarf varieties work. Bonus: the flowers and leaves are edible, with a peppery flavour excellent in salads — and they're beautiful.

Combinations to Avoid

Not all plant neighbours are beneficial. These combinations actively inhibit growth, share disease, or compete intensely enough to reduce yields for both plants:

🍅 Tomatoes + Potatoes — Both are in the nightshade family and share the same blight diseases. Growing them near each other allows disease to spread rapidly between them. Keep them in separate parts of your garden.
🫘 Beans + Onions or Garlic — Alliums (onions, garlic, leeks) inhibit the growth of beans and peas. The mechanism isn't fully understood, but the effect is consistent and well-documented. Keep these plant families separated.
🌿 Fennel + Almost Everything — Fennel produces allelopathic chemicals from its roots that inhibit the germination and growth of most vegetables. Grow fennel in its own pot or a separate area of the garden, never in a raised bed with other vegetables.
🥬 Brassicas + Tomatoes or Strawberries — These compete intensely for similar nutrients and root space, and brassicas can inhibit the growth of tomatoes in particular. Keep your broccoli, cabbage, and kale bed separate from your tomato bed.
🥒 Cucumbers + Potatoes — Potatoes attract pests like Colorado potato beetle that will also attack cucumber plants. They also compete for similar nutrients and both suffer when crowded together.

Why Companion Planting Works — The Science

Companion planting benefits fall into four main categories, each with its own mechanism:

🧪 Chemical Repulsion

Plants release volatile organic compounds that confuse pest insects' ability to locate host plants. Basil, marigolds, and alliums all work this way. The scent of one plant masks the chemical signals the pest uses to find another.

🌱 Nitrogen Fixation

Legumes (beans, peas) host Rhizobium bacteria in root nodules that convert atmospheric nitrogen into plant-available ammonium. When legume roots decompose, they release this nitrogen into the soil for neighbouring plants.

🎯 Trap Cropping

Some plants are so attractive to pests that they draw them away from crops you want to protect. Nasturtiums attract aphids; mustard attracts flea beetles. Pests concentrate on the trap crop rather than spreading through the whole garden.

🐝 Beneficial Attraction

Flowers attract beneficial insects — hoverflies, parasitic wasps, and lacewings — that prey on garden pests. Dill, fennel, coriander, and marigolds are especially effective. A diverse, flowering garden is a healthier garden.

Frequently Asked Questions

Does companion planting actually work, or is it a myth?

Some companion planting claims are well-supported by research; others are folklore. The combinations with the strongest scientific evidence are: marigolds suppressing nematodes (confirmed in multiple studies), basil repelling insects near tomatoes (volatile compounds confirmed effective), the Three Sisters mutual benefit system (hundreds of years of validated practice), carrot-onion fly repulsion (confirmed by Scandinavian research), and nitrogen fixation by legumes (completely confirmed biochemistry). Treat companion planting as one helpful tool among many rather than a complete substitute for other pest management.

What is the best companion plant for tomatoes?

Basil is the most popular and well-supported companion for tomatoes — plant one basil plant per tomato and let it grow in the shade beneath the tomato canopy. French marigolds planted around the bed perimeter suppress nematodes and deter aphids. Carrots can be planted between tomatoes to loosen soil around shallow tomato roots. Borage (an edible herb with blue flowers) attracts beneficial pollinators and is believed to repel tomato hornworms. Avoid planting tomatoes near potatoes, fennel, corn, or brassicas.

Can I plant tomatoes and peppers together?

Yes — tomatoes and peppers are compatible companions. They're both in the nightshade family, share similar growing conditions (full sun, warm soil, regular watering), and don't compete intensely. They can share a raised bed, though they'll both need adequate spacing for airflow and to prevent disease spread. The risk is that being in the same plant family, they're susceptible to some of the same diseases. Don't plant them in the same location two years in a row — rotate crops so neither spends consecutive seasons in the same spot.

How far apart should companion plants be?

For pest repulsion through scent, companions work best within 60–90 cm of each other — close enough that the aromatic compounds reach the crop you're protecting. For trap cropping, companions can be placed at the perimeter of a bed. For nitrogen fixing, legume roots need to be close enough to the benefiting plant that the soil nitrogen is accessible — within the same bed. Use our Plant Spacing Calculator to figure out how many plants fit together at proper spacing.

What should I not plant next to squash?

Avoid planting squash near potatoes — they attract similar pests and potatoes can spread blight to squash. Don't plant squash near other squash family members (cucumbers, melons, pumpkins) in a small garden if you're concerned about cross-pollination — though for eating rather than seed saving, this doesn't matter. Fennel should always be kept away from squash. Good companions for squash include beans (nitrogen), corn (structure), nasturtiums (trap crop for squash bugs), and marigolds (general pest deterrence).

Plan Your Garden Layout

Use these tools to calculate spacing, soil, and timing

🥕 Plant Spacing 🪵 Raised Bed 🌍 Soil Volume 🧪 Fertilizer