Companion Planting Guide: Best Combinations for Every Garden

When researchers studied borage planted alongside strawberries, they found something that stopped the gardening community in its tracks: strawberry plants growing next to borage produced 35% more fruits and a 32% greater yield by weight than plants grown alone [1]. No new fertiliser, no special variety — just the right neighbour.

Companion planting is the practice of growing different plants in proximity to mutual benefit. It sounds almost too simple, but the mechanisms underneath it — chemistry, microbiology, insect ecology — are genuinely fascinating. The challenge is that the field is crowded with folklore. Some combinations that gardeners have repeated for decades turn out to be unsupported by research. Others have solid science behind them. This guide separates the two.

You’ll find the science explained in accessible terms, the best-evidenced vegetable, flower, and herb combinations, an honest look at what the research doesn’t support, the plants you should actively keep apart, a seasonal calendar for planning it all, and a section on companion planting in containers — because not every garden has ground space to spare.

What Is Companion Planting?

Companion planting is simply growing two or more plant species together because their proximity benefits at least one of them — and ideally both. The benefits vary: one plant might fix nitrogen that feeds its neighbour; another might release chemicals that deter pests; a third might attract the predatory insects that keep an aphid outbreak in check.

People have been doing this for a very long time. The best-known example — the Three Sisters method of corn, beans, and squash — was practised by the Ho-Chunk, Menominee, Ojibwe, Pueblo, and Haudenosaunee peoples of North America over thousands of years before it was ever written about in a gardening book. Modern science has since confirmed why it works, but the original observation was agricultural, not academic.

Today, companion planting sits at the intersection of traditional knowledge and contemporary ecology. A 2023 study found that companion planting significantly increased soil enzyme activity and microbial diversity compared to monoculture growing [11] — evidence that what benefits one plant often benefits the entire ecosystem beneath the soil surface.

The Science: Four Ways Plants Interact

Understanding how companion planting works makes it much easier to use well. There are four primary mechanisms [2].

1. Nitrogen Fixation

Legumes — peas, beans, clover — harbour Rhizobium bacteria in nodules on their roots. These bacteria fix atmospheric nitrogen into plant-available forms. What’s less well-known is that some of this fixed nitrogen is shared with neighbouring plants while the legume is still alive, transferred through mycorrhizal fungal networks beneath the soil in exchange for root exudates [2]. You don’t need to wait for the bean plant to die and decompose — the sharing is ongoing.

This is the engine behind the Three Sisters: beans fix the nitrogen that corn and squash need in large quantities throughout the growing season. UMN Extension’s companion planting research describes this as the mycorrhizal network in action — not a theoretical future benefit, but an ongoing one [2].

2. Allelopathy

Allelopathy is the influence one plant has on another through the chemical compounds it releases — allelochemicals — from roots, leaves, or decomposing plant material. These can suppress or stimulate neighbours depending on the compound and concentration [3].

The practical implications cut both ways. Some allelopathic effects are useful: mow-killed grain rye releases allelochemicals that can reduce weed biomass by approximately 70% while leaving transplanted vegetables unharmed [3]. Others are destructive: fennel releases anethole and fenchone into the surrounding soil, stunting the growth of most neighbouring vegetables. Black walnut produces juglone, which can kill tomatoes, potatoes, and other solanaceous crops planted within its root zone.

3. Chemical Masking and Pest Confusion

Strongly aromatic plants — basil, rosemary, thyme, sage — release volatile compounds that interfere with how flying insects locate their host plants. Pests navigating by scent are essentially confused: the aromatic background noise makes it harder for them to find their target. Iowa State University research confirmed this mechanism for basil grown with tomatoes, recording less insect damage when basil was present nearby [10].

The effect is real, but it scales with quantity: a single basil plant among twenty tomatoes provides far less masking than a generous interplanting throughout the bed.

4. Trap Cropping and Beneficial Insect Habitat

Trap cropping works by deliberate sacrifice: you plant a crop that pests prefer even more than your primary vegetable, drawing them away from where you don’t want them. Nasturtiums draw aphids so effectively that their stems will often be covered while nearby lettuces remain clean. It’s important to understand the mechanism here — nasturtiums don’t repel aphids; they attract them. The nasturtium is the decoy, not the deterrent.

Companion plants can also provide habitat for the insects that do the pest management for you: parasitic wasps, hoverflies, and ground beetles. Borage, dill, sweet alyssum, and calendula all attract these beneficial insects in numbers. Research shows that beneficial insect populations concentrate more reliably around mixed flowering plantings than around monoculture vegetable beds [2].

The Three Sisters: The Original Companion Planting System

The Three Sisters — corn, climbing beans, and squash — is the most rigorously documented companion planting system in existence. It works on multiple levels simultaneously [6][7].

Corn grows tall quickly and provides the physical structure for climbing beans to ascend. Without it, beans sprawl across the ground and squash competes for space. With it, the vertical dimension of the bed is used efficiently.

Beans climb the corn and fix atmospheric nitrogen through root nodule bacteria. This nitrogen is partially shared with the heavy-feeding corn and squash while the bean is still growing — not only after harvest and decomposition. Beans also add organic matter and stimulate beneficial microbial activity in the soil.

Squash spreads its large leaves to cover the soil surface, acting as a living mulch that shades out weeds, retains moisture, and keeps the soil cool during summer heat. The rough texture of squash leaves is also thought to deter some crawling pests, though this is less well-studied than the shading effect.

Together, the three crops produce a combined yield per unit area that’s greater than any one of them grown as a monoculture, and the produce has near-complete nutritional value: carbohydrates from corn, protein from beans, and vitamins and minerals from squash [6]. Modern research confirms that Three Sisters plots show significantly less weed pressure and higher total yield per acre than monoculture comparisons [7].

For a practical Three Sisters planting: sow corn first, about 30 cm (12 inches) apart in a rough grid. When corn is knee-height, plant beans around the base of each corn stalk. Plant squash at the edges, where its leaves can spread without shading the corn. The timing matters — corn that isn’t established before beans are planted gets smothered rather than supported. For detailed guidance on growing the squash side of this combination, our complete zucchini growing guide covers sowing, spacing, and pollination in full.

Best Vegetable Companion Combinations

The table below covers the most useful and most reliable combinations for a kitchen garden, with an honest evidence rating for each.

Tomato and basil is probably the most famous companion pairing in the kitchen garden. The Iowa State research recorded less insect damage on tomatoes grown with basil — the volatile compounds from basil leaves interfere with the host-finding behaviour of thrips and whitefly. UMN Extension notes that “intercropping with basil may even help promote tomato growth” [2], though the mechanism for the growth effect is less clear. For everything about growing tomatoes successfully, our complete tomato plant care guide covers variety selection, feeding, pruning, and the most common problems.

Carrot and spring onion is a classic allotment combination. The theory is elegant: carrot fly navigates by smell, and the pungent onion volatile masks the carrot signal; onion fly does the same in reverse, with carrot scent confusing it. Alternating rows of carrots and spring onions is the traditional approach. The evidence here is largely experiential rather than trial-based, but the combination is low-cost and low-risk — and generations of British allotment growers can attest to its apparent usefulness.

Lettuce and radish is a space-efficiency pairing as much as a pest-management one. Radishes mature in 3–4 weeks and are harvested long before lettuce needs the full space. They’re also said to deter leaf miners. The timing works: sow both together, harvest radish first, let lettuce fill in.

Flower Companions

French Marigolds (Tagetes patula): The Nematode Specialist

Marigolds are the most scientifically validated flower companion for vegetable gardens, but the reason is more specific than most gardeners realise. The active compound is alpha-terthienyl — released by living marigold roots directly into the surrounding soil. In peer-reviewed research, alpha-terthienyl demonstrated an LC₅₀ of 0.84 μM against root-knot nematode second-stage juveniles, generating oxidative stress by producing reactive oxygen species within nematode tissue [5]. French marigolds suppress 14 genera of plant-parasitic nematodes [4].

UF/IFAS Extension’s nematode management guide identifies French marigolds (T. patula) as the most effective species against the widest range of nematode genera, with suppression documented across 14 genera [4]. Three practical points the garden centre information never mentions:

• The compound is only active from living roots — marigold extracts, dried plants, and mulched leaves provide no benefit. The plants must be growing in the soil.
• You need a two-month lead time: plant marigolds at least eight weeks before you want nematode suppression to be in effect [4].
• Plant them no more than 18 cm (7 inches) apart for effective soil coverage [4]. Sparse planting at bed edges doesn’t provide the root density needed.

One honest caveat from UMN Extension: marigolds do not deter Colorado potato beetles from potatoes, despite the widespread claim [2]. Evidence for that specific combination doesn’t hold up. Our marigold care guide covers the full range of varieties, growing requirements, and where to use them to best effect.

Nasturtiums: The Misunderstood Trap Crop

Almost every gardening article describes nasturtiums as aphid repellents. They’re not. Nasturtiums actively attract aphids — blackfly, greenfly, and whitefly will colonise nasturtium stems rapidly. This is precisely what makes them valuable: they’re acting as a trap crop, not a deterrent.

I realised this properly after growing nasturtiums as a border plant for a couple of seasons without quite joining the dots — one July the stems were so coated in blackfly you could barely see green, while the lettuces thirty centimetres behind them were completely untouched. That’s the mechanism: not repellence, deliberate attraction.

Flying aphids navigating toward brassicas, lettuces, or roses encounter the nasturtium first and settle on it. The aphids are now concentrated on a plant you’re not trying to harvest, where you can either spray them directly or leave them for predatory insects and birds. Washington State University’s integrated pest management materials list nasturtiums as a trap crop for both aphids and flea beetles [2].

Position nasturtiums approximately 1–1.5 metres from the crops you want to protect — close enough to intercept pest movement, but not so close that aphids hop directly between plants. As a bonus, nasturtium flowers and leaves are edible (peppery, good in salads) and the unripe seed pods can be pickled as a caper substitute.

Borage: The Pollinator Powerhouse

The Griffiths-Lee 2020 study published in Ecological Entomology is one of the most important companion planting papers of recent years [1]. In both researcher-led experiments and citizen science data, strawberry plants adjacent to borage produced 35% more fruits and a 32% greater yield by weight. Pollinator visits to strawberry flowers were estimated 25% higher when borage was nearby.

The mechanism is simple: borage flowers are exceptionally productive for bees. A Polish three-year study found that a single borage plant averages 953 individual flowers over its season, with a square metre of borage supplying approximately 5.2 grams of nectar sugar. For the strawberry growing guide, borage as a companion is now a first-line recommendation.

The benefit extends beyond strawberries. Borage is widely grown alongside cucurbits — cucumbers, courgettes, pumpkins, melons — where pollination is essential for fruit set. If you’re already following our advice to hand-pollinate when bees are scarce, adding borage to your bed will make hand-pollination less necessary. Borage self-seeds freely once established, so plant it once and it tends to return every season.

Calendula: The All-Round Companion

Calendula (pot marigold — different from Tagetes) deserves its place in every vegetable garden. Its long flowering season from spring until frost provides a sustained nectar source for hoverflies and bumblebees. Hoverfly larvae are voracious aphid predators — a single larva can consume 400 aphids during its development — and they need adult nectar from open-faced flowers like calendula to sustain their population. Plant calendula throughout vegetable beds rather than just at the edges: biological control works best when beneficial insects are distributed across the whole growing area [8].

Herbs as Companion Plants

Basil

Covered above as a tomato companion, basil deserves a broader mention: its volatile compounds (linalool and eugenol, primarily) deter a range of soft-bodied insects across the whole garden. Plant generously rather than sparingly for best effect. Our basil growing guide covers the best varieties for both culinary use and companion planting — some are bred for high volatile output.

Rosemary

Rosemary’s woody stems and strong camphor-and-cineole scent make it one of the better-regarded herbal companions for brassicas and root vegetables. The volatile compounds are thought to deter cabbage white butterflies, carrot fly, and bean beetle. The evidence is mostly observational rather than trial-based, but rosemary is a permanent garden plant that provides decades of companion planting benefit alongside its culinary use. Site it where it will stay — in the ground near the brassica bed, or in a large container positioned seasonally.

Sage

UMN Extension field research specifically identified sage as reducing diamondback moth pressure on Brussels sprouts [2]. The diamondback moth (Plutella xylostella) is one of the most economically significant brassica pests worldwide and has developed resistance to many insecticides, making cultural controls like companion planting particularly valuable. Sage grows well alongside brassicas, rosemary, and carrots, and the dried leaves have culinary uses throughout winter.

Chives

Chives are one of the most practical allium companions. They’re perennial, low-maintenance, and their volatile sulphur compounds are thought to deter carrot fly, aphids, and Japanese beetles. Traditional pairing with carrots mirrors the carrot-spring onion combination: the allium scent disrupts pest host-location. Chives also attract early pollinators with their globe-shaped flowers, bridging the period before borage and calendula are in full bloom.

Plants to Keep Apart: Bad Companions

Companion planting is as much about avoiding harmful combinations as creating beneficial ones. These are the most important separations to make.

Fennel and Almost Everything

Fennel (Foeniculum vulgare) is one of the few plants in the home garden with comprehensively documented negative allelopathic effects on its neighbours. It releases anethole and fenchone into the surrounding soil — compounds that suppress seed germination and stunt the growth of most neighbouring plants. Carrots grown near fennel show 25–40% yield losses. Tomatoes, beans, peas, and peppers are also significantly affected.

The soil effect persists for two to three growing seasons after fennel is removed. If you’ve grown fennel in a bed, avoid planting sensitive crops there for the following two years.

Fennel isn’t worthless as a garden plant — it attracts hoverflies, parasitic wasps, and is the primary host plant for swallowtail butterfly caterpillars in the UK. The solution is isolation rather than exclusion: grow fennel in its own container. In a pot, its allelopathic compounds stay within the root zone and won’t affect ground-planted neighbours.

Alliums and Legumes

Onions, garlic, and leeks are generally poor companions for beans and peas. Allium root secretions are thought to inhibit the nitrogen-fixing Rhizobium bacteria in legume root nodules, reducing the nitrogen-fixation benefit that makes beans valuable companions in the first place. Whether the effect is large enough to matter in a home garden is debated, but it’s a straightforward combination to avoid — separate your onion bed from your bean rows.

Walnut Trees: The Juglone Zone

Black walnut (Juglans nigra) produces juglone — a compound that persists in soil and is toxic to tomatoes, potatoes, peppers, aubergines, rhododendrons, and several other common garden plants [12]. The zone of impact extends to the full drip line of the tree’s canopy, and beyond: PSU Extension recommends siting gardens 15–18 metres (50–60 feet) from a mature black walnut [9].

A practical note for UK gardeners: black walnut is not common in British gardens. The English walnut (Juglans regia — the walnut sold in UK nurseries and garden centres) produces far less juglone than black walnut and poses a significantly lower risk to neighbouring plants. If you have a walnut tree and aren’t sure of the species, the most distinctive difference is fruit size: black walnuts have larger, rounder fruits with deeply ridged shells; English walnuts have the smooth, thinner-shelled nuts sold in shops.

Dill and Carrots

Dill and carrots are in the same plant family (Apiaceae) and cross-pollinate readily if both are allowed to flower simultaneously. The resulting seed is a genetic cross that produces poor versions of both crops. Additionally, some research suggests dill may inhibit carrot growth when in close proximity, though the evidence is largely anecdotal. Keep dill away from carrots — and away from parsley and other Apiaceae family members for the same cross-pollination reason.

Sunflowers: Worth Knowing

Sunflowers produce allelopathic compounds — primarily from roots, hulls, and decaying leaves — that inhibit germination and growth of some neighbouring plants. Direct seed contact with sunflower hull mulch is particularly problematic. That said, sunflowers attract pollinators powerfully throughout the summer and are excellent at the edges of vegetable plots, away from direct root competition with sensitive crops. Our sunflower growing guide covers both the benefits and the considerations for growing them in productive gardens.

Seasonal Companion Planting Calendar

Container Companion Planting

Container gardening doesn’t preclude companion planting — it just requires tighter planning. The core principle remains the same: pair plants with complementary needs in terms of light, water, and soil, then select for combinations that offer a functional benefit.

The most practical container pairings:

• Tomato and basil — natural partners in a large pot (40 litres minimum). Basil benefits from the light shade that a tomato plant casts during the hottest part of the afternoon, and the aromatic chemistry works just as well in a container as in the ground. Plant basil around the base of the tomato stake.
• Lettuce and spring onions — spring onions grow vertically while lettuce occupies the horizontal space; onions deter some pests from lettuce; both tolerate partial shade. A 20–30 litre pot accommodates both comfortably.
• Strawberry and borage — the peer-reviewed evidence for this pairing [1] applies just as much to a container as to a bed. A deep planter with strawberries and one or two borage plants produces a self-sustaining pollinator magnet through summer. For more on container strawberry growing, see our complete strawberry guide.
• Cucumber and marigolds — marigolds at the edges of a large cucumber container provide the aesthetic benefit and some pest deterrence; the nematode-suppression benefit is less relevant in container compost than in garden soil, but marigolds still attract beneficial insects.

One specific advantage of containers: fennel, normally a garden pariah due to its allelopathic effects, is perfectly safe in a container. The allelochemicals remain within the pot’s root zone. If you want fennel for cooking or to attract swallowtail caterpillars, a dedicated container away from your vegetable beds is the right answer.

Container companions dry out faster than ground-planted combinations — choose pairings where both plants have similar water requirements. Avoid pairing drought-tolerant herbs (rosemary, thyme) with moisture-demanding vegetables (cucumbers, courgettes) in the same pot.

Frequently Asked Questions

Does companion planting actually work?

Some combinations have strong peer-reviewed evidence — borage with strawberries (35% yield increase [1]), French marigolds for nematode suppression [4], and the Three Sisters system [6]. Others are well-supported by university extension research: basil with tomatoes, sage with brassicas, nasturtiums as aphid trap crops. A significant number of popular recommendations are anecdotal and have not been tested in controlled trials. Illinois Extension puts it bluntly: “much of the recommended companions that we see are not always tested out in a research study” [10]. Use the evidence ratings in this guide to prioritise your planting decisions.

How far apart should companion plants be?

It depends on the mechanism. For aromatic chemical masking (basil, sage, rosemary), the companion plant needs to be close enough for volatile compounds to reach the target crop — within 1–2 metres. For trap crops like nasturtiums, the traditional guidance is 1–1.5 metres from the protected crop. For French marigolds suppressing soil nematodes, the plants themselves need to be no more than 18 cm apart within the bed for effective soil coverage [4]. For beneficial insect attraction (borage, calendula, dill), within 3–4 metres is generally sufficient to keep predatory insects present in useful numbers.

When should I plant French marigolds for nematode control?

Plant marigolds at least 8 weeks before you want nematode suppression to take effect [4]. In practice, this means planting in early spring for summer vegetable crops — late March or early April in the UK for beds where tomatoes and peppers will go in May or June. The alpha-terthienyl only suppresses nematodes while it’s actively being released from living roots, so early planting and dense spacing are both essential.

Can I grow fennel in my vegetable garden?

In the ground alongside other vegetables, fennel is problematic. Its allelopathic compounds persist in soil for 2–3 growing seasons and significantly reduce yields of carrots, tomatoes, beans, and most other crops. The practical solution is to grow fennel in its own container, which limits the allelopathic effect to the pot’s root zone. Fennel attracts beneficial insects and swallowtail butterfly caterpillars, so it’s worth keeping — just isolated.

What is the best companion plant for tomatoes?

Basil and French marigolds are the two most evidence-supported companions for tomatoes. Basil provides aromatic pest masking against thrips and whitefly [10]. French marigolds address soil nematodes that can significantly reduce tomato yields in affected beds [4]. Borage and calendula nearby attract the pollinators and beneficial predatory insects that support a healthy tomato crop through the season. Nasturtiums as a perimeter trap crop can intercept aphids before they reach your tomato plants.

Are sunflowers good companion plants?

Sunflowers attract pollinators and beneficial insects powerfully, and they’re excellent planted at the edges of vegetable plots. Their allelopathic compounds from roots and hulls can inhibit the germination of some neighbouring seeds, so avoid placing sunflowers directly within vegetable beds or using sunflower hull mulch around sensitive crops. For more on growing sunflowers in and around the garden, our sunflower growing guide covers siting and management in detail.

Final Thoughts

The best companion planting strategies are built on a few well-understood mechanisms — nitrogen fixation, allelopathy, chemical masking, and trap cropping — applied deliberately rather than by rote. Use the Three Sisters when you have the space. Plant French marigolds two months early for nematode control. Add borage near your strawberries and cucurbits and let the evidence speak for itself. Be sceptical of combinations that lack any mechanistic explanation.

And keep fennel in a pot.

Sources

1. Griffiths-Lee, J. et al. (2020). “Companion planting to attract pollinators increases the yield and quality of strawberry fruit.” Ecological Entomology. Wiley Online Library
2. University of Minnesota Extension. “Companion Planting in Home Gardens.” UMN Extension
3. Research Progress on Plant Allelopathy in Agriculture. PMC4647110. NCBI PMC
4. University of Florida IFAS Extension. “Marigolds for Nematode Management.” EDIS NG045. UF/IFAS
5. Nematicidal Actions of Alpha-Terthienyl. PMC6504006. NCBI PMC
6. UW-Madison GROW Magazine. “Five Fascinating Facets of the Indigenous Three Sisters Cropping Method.” GROW
7. West Virginia University Extension. “Three Sisters Gardening Method.” WVU Extension
8. Clemson HGIC. “Plant Partners: 5 Benefits of Companion Planting.” Clemson
9. Penn State Extension. “Allelopathy in the Home Garden.” Penn State Extension
10. Illinois Extension. “Companion Planting: Anecdotal or Tried and Tested?” Illinois Extension
11. UC Master Gardeners. “Better Together: New Science of Companion Planting.” UC ANR
12. Morton Arboretum. “Black Walnut Toxicity.” Morton Arboretum

References

1. Griffiths-Lee, J. et al. (2020). “Companion planting to attract pollinators increases the yield and quality of strawberry fruit.” Ecological Entomology, Wiley.
2. University of Minnesota Extension. “Companion Planting in Home Gardens.” UMN Extension (Natalie Hoidal, 2021).
3. Research Progress on Plant Allelopathy in Agriculture. PMC4647110, National Center for Biotechnology Information.
4. University of Florida IFAS Extension. “Marigolds for Nematode Management.” EDIS Publication NG045.
5. Nematicidal Actions of Alpha-Terthienyl Against Root-Knot Nematodes. PMC6504006, National Center for Biotechnology Information.
6. UW-Madison GROW Magazine. “Five Fascinating Facets of the Indigenous Three Sisters Cropping Method.”
7. West Virginia University Extension. “Three Sisters Gardening Method.” (2022).
8. Clemson University Home & Garden Information Center. “Plant Partners: 5 Benefits of Companion Planting.”
9. Penn State Extension. “Allelopathy in the Home Garden.”
10. Illinois Extension. “Companion Planting: Anecdotal or Tried and Tested?” (Grant McCarty, 2015).
11. UC Master Gardeners, San Mateo & San Francisco. “Better Together: New Science of Companion Planting.” UC ANR.
12. Morton Arboretum. “Black Walnut Toxicity.”