Companion planting is one of the most important strategies in an organic garden. Carefully choosing where to grow onion family crops, and what to grow alongside them, you can ensure greater yields and greater successes in your garden.
In nature, plants don’t live or work alone – they live alongside other plants, animals and fungi, creating ecosystems that are stable and resilient because of how the elements within them work together.
By mimicking nature and creating polycultures of compatible and mutually beneficial plants, we can help our gardens to thrive, and make sure that they are more resilient whatever the seasons may bring.
When it comes to onion companion planting, their relatives such as garlic and chives are especially useful. Their strong scent can help to keep other plants safe. Other plants can also benefit onions in return.
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KEY TAKEAWAYS
- Onions are powerful companion plants thanks to their strong scent, which deters pests such as aphids, carrot fly, and cabbage worms.
- Best vegetable companions include carrots, cabbages (and other brassicas), tomatoes, and peppers.
- Helpful herbs for onions include parsley, mint, and chamomile — each attracting beneficial insects or deterring pests.
- Flowering companions such as roses and chrysanthemums can protect onions and beautify your garden.
- Avoid planting onions with peas, beans, sage, or asparagus as they compete or interfere with beneficial soil relationships.
- Crop rotation is essential in annual beds— don’t plant onions in the same place each year to prevent disease and pest buildup.
- In a polytunnel, onions thrive when interplanted with quick crops or aromatic herbs, taking advantage of stable conditions.
- Healthy soil, biodiversity, and thoughtful planting are key to productive, resilient onion crops.
Why Onions Are Excellent Companion Plants
Members of the onion family (Allium species) including onions, garlic, leeks, shallots, and chives play a particularly valuable role in companion planting. Their strong scent and sulfur-containing compounds make them natural pest deterrents.
Onions help repel a range of common garden pests such as:
- Aphids
- Carrot fly
- Cabbage worms
- Slugs and snails
- Japanese beetles
- Spider mites
These protective properties make onions superb companions for a wide range of vegetables, herbs, and flowers. At the same time, onions themselves can benefit from other plants that improve soil structure, attract helpful insects, or create the right microclimate around their shallow roots.
Let’s look more closely at some of the best onion companion plants and those to avoid.
Companion Plants for Onions
Vegetables: Carrots, Cabbage, Tomatoes, Peppers
Carrots
One of the best-known combinations in companion planting is onions and carrots. This pairing is more than traditional gardening wisdom. It is backed by biology.
The scent of onions confuses and deters the carrot fly, a persistent pest whose larvae can tunnel through carrot roots. Meanwhile, the smell of carrots helps repel the onion fly, which lays its eggs at the base of onion bulbs.
Together, these two crops protect each other from their respective enemies. They also share similar growing needs. Both prefer loose, well-drained soil and benefit from consistent moisture without waterlogging. When interplanted in alternating rows, onions and carrots can form a natural pest barrier within your polytunnel or in another growing area in your garden.
Cabbage
Members of the Brassicaceae family such as cabbage, broccoli, kale, and Brussels sprouts — are another excellent match for onions. The sulfurous aroma of onions masks the scent of brassicas, confusing pests like cabbage moths, loopers, and cabbage maggots.
Onions’ narrow upright form also makes efficient use of space around the edge of a Brassica bed, or beneath sprawling brassica leaves, allowing you to maximise your growing area without overcrowding.
When planting onions with brassicas, make sure to maintain the nutrient balance in the soil: both cabbage and onion family plants are heavy feeders, so they’ll benefit from a fertile, compost-rich growing area.
Tomatoes
Onions and tomatoes complement each other beautifully both in the kitchen and in the garden. Onions can deter spider mites and aphids, which are frequent tomato pests, while tomatoes’ leafy canopy helps shade onion bulbs, keeping the soil moist during hot spells.
In a polytunnel, this combination can be particularly effective. Tomatoes benefit from the stable warmth of tunnel conditions, while onions prefer the drier soil near the tunnel edges. Interplanting them can create a balanced microclimate, with tomatoes shading the onions and onions protecting the tomatoes from pest outbreaks.
Peppers
Like tomatoes, peppers thrive alongside onions. The onion’s scent wards off pests such as aphids and thrips, which can otherwise spread viral diseases to pepper plants. Peppers, in turn, offer modest shade and help stabilise the soil temperature around onions.
In a tunnel or outdoor bed, this partnership supports both crops through the growing season, especially if you also integrate flowering herbs to attract pollinators for the peppers.
Herbs: Parsley, Mint, Chamomile
Parsley
Parsley is an underrated companion for onions. It attracts hoverflies, whose larvae feed on aphids and thrips – two pests that commonly target onion foliage. The deep taproot of parsley also helps to break up compacted soil, improving aeration and drainage around onion roots.
If you are growing onions in containers or raised beds, a border of parsley can serve as a living mulch, shading the soil and reducing moisture loss.
Mint
Mint is another aromatic herb that pairs well with onions, as its scent deters onion flies and aphids. However, mint’s vigorous growth can easily take over a bed, so it’s best confined to pots or containers. Positioning these near your onion rows allows you to benefit from mint’s protective properties without risking it spreading uncontrollably.
Chamomile
This herb is often described as a “plant doctor” — it improves the growth and flavour of nearby crops, onions included. Chamomile attracts beneficial insects such as lacewings and parasitic wasps, both of which prey on onion pests. It also contributes organic matter when chopped and dropped at the end of the season.
Flowers: Roses, Chrysanthemums
Roses
It might surprise some gardeners to learn that onion family plants (and particularly chives or other perennial onion varieties) make excellent companions for roses. Chives’ strong scent helps deter aphids and black spot, common problems in rose beds.
Planting chives or ornamental alliums at the base of rose bushes not only adds visual appeal but also encourages healthier, pest-resistant roses. The same principle applies within a polytunnel, where chives can act as living pest barriers around ornamental or edible flowering plants.
Chrysanthemums
Chrysanthemums produce natural pyrethrins – compounds that have insecticidal properties. When planted near onions, they help repel harmful insects while attracting pollinators and predatory wasps. Their bright blooms also contribute to a more diverse and visually appealing growing space, embodying the permaculture idea of combining beauty and function.
What Not To Plant With Onions
While onions are versatile companions, some plants simply don’t get along with them. Avoid the following:
Legumes (Peas and Beans)
Onions and other alliums can inhibit the growth of legumes by disrupting the beneficial bacteria that fix nitrogen in their root nodules. Legumes prefer a different soil microbiome from onions, so they should be kept in separate beds within your rotation plan.
Asparagus
Both onions and asparagus are long-term crops that compete for similar soil nutrients. Growing them together can stunt the development of both. It’s best to give each their own space and nutrient regime.
Sage and Other Strong Herbs
Some herbs, particularly sage, can inhibit onion growth. Their competing root systems and differing moisture preferences make them poor companions.
Keeping Onions Flies Away: How To
The onion fly (Delia antiqua) is one of the most troublesome pests for onion growers. Its larvae tunnel into bulbs, causing rot and weakening plants. Here are some permaculture-based methods for prevention and control:
- Use Companion Plants – Interplant onions with carrots, chamomile, or mint to mask their scent and confuse onion flies.
- Crop Rotation – Avoid growing onions or related alliums in the same place year after year; rotate them with leafy greens or legumes instead.
- Physical Barriers – Fine mesh netting or fleece can prevent flies from laying eggs at the base of plants.
- Healthy Soil and Compost – Robust plants resist pests better. Feed your soil with organic matter, and ensure good drainage to prevent bulb rot.
- Trap Crops and Timing – In some cases, planting early or late can avoid peak onion fly activity. Using sacrificial rows (trap crops) can draw pests away from your main crop.
Can You Plant Onions In The Same Place Every Year?
Growing onions in the same bed continuously can lead to a buildup of pests and diseases, particularly onion white rot, which can persist in the soil for years. To keep your soil healthy:
- Rotate onions with leafy crops, legumes, or root vegetables that don’t host the same pests.
- Add organic matter between rotations to restore fertility.
- Keep the soil surface covered with green manures or mulch to support soil organisms and reduce erosion.
A three- or four-year crop rotation cycle is ideal for most home gardens.
The exception to this general rule is if you have created mixed polycultures (such as fruit tree guilds in a forest garden, for example) using perennial types of plant in the onion family. Where perennials are in place, these can grow in the same location over multiple years.
Growing Onions in a Polytunnel
A First Tunnels polytunnel offers many advantages for onion growers. It provides shelter from excessive rain and harsh winds, maintains steady soil temperatures, and allows for early planting or overwintering of certain onion varieties.
Planting and Timing
You can sow onion seeds in modules in late winter or early spring inside your polytunnel, then transplant them once they’re large enough to handle. Alternatively, overwintering varieties like ‘Radar’ can be planted in autumn for an early summer harvest.
Soil and Spacing
Onions prefer well-drained, fertile soil with a pH around 6.5–7.0. Mix in compost before planting, but avoid fresh manure as it can promote disease. Space bulbs about 10–15 cm apart, with 25–30 cm between rows.
Polyculture Planting in a Tunnel
In a polytunnel, interplant onions with fast-growing crops like radishes, lettuce, or spinach. These early crops can be harvested before the onions need the space, maximising productivity in your tunnel.
You might also combine onions with parsley or chamomile to help repel pests naturally while maintaining a balanced microclimate.
Maintenance
Keep soil evenly moist, especially during bulb formation. Ventilate the tunnel regularly to reduce humidity, which can encourage fungal issues. Mulching between rows with straw or leaf mould will conserve water and reduce weeds.
Harvest and Storage
When onion tops begin to yellow and fall, it’s time to harvest. Lift the bulbs gently and allow them to cure in a warm, dry space — the tunnel itself works perfectly if well ventilated. Once dry, store them in mesh bags in a cool, dark place for long-term use.
Companion planting is not just about preventing pests or improving yields — it’s about building relationships in your garden. When we plant onions among a diverse range of allies — carrots, herbs, flowers, and leafy greens — we invite nature to work alongside us.
What should I not plant next to onions?
Avoid peas, beans, asparagus, and sage, they either compete with onions or disrupt soil relationships.
What can I plant with onions in the UK?
Carrots, cabbages, tomatoes, peppers, parsley, chamomile, and mint are excellent choices. Many of these also thrive under polytunnel protection.
Can you grow onions in the same bed every year?
With typical bulb onions or spring onions in vegetable beds, no. No. Rotate your crops every three to four years to prevent disease and pest buildup. Perennial members of the family can however remain in place over multiple years.
What plants should you keep away from onions?
Keep legumes and deep-rooted perennials like asparagus away from onions to maintain balance and soil health.
