How to grow onions, shallots and garlic relatives

Onions ask for very little once established — a sunny, well-drained spot, some basic weeding and careful drying before storage — but they reward that modest effort with a harvest that can last through many months.

Sets or seed: which to choose?

Onions can be grown from seed or from sets. Each has genuine advantages, and the right choice depends on your situation.

Onion sets are small, partially-grown bulbs (typically the size of a large marble) that have already passed through their most vulnerable early weeks. They establish quickly, are easier to handle and plant than tiny seeds, have a head start on the growing season, and are less prone to pest and disease problems in the early stages. The range of varieties available as sets is smaller than seed, but the reliable, well-proven choices cover most needs. For beginners, sets are strongly recommended.

Growing from seed gives you access to a far wider range of varieties, costs significantly less per plant, and allows you to start very early in the season (often under cover in modules). The trade-off is that seedlings are small and slow to establish, require more careful handling, and are more vulnerable to bolting if they experience cold followed by warmth in the early weeks. Sowing directly outside works in warmer climates; in cool temperate regions, starting seeds in modules under glass or on a windowsill gives better results.

Shallots are almost always grown from sets (though seed-grown shallots are available). Each set multiplies into a cluster of six to ten shallots, making them excellent value. Their flavour is more delicate and sweet than onions, and they store particularly well. 'Red Sun', 'Jermor' and 'Hative de Niort' are well-regarded varieties.

Garlic is planted differently — as individual cloves from a bulb, either bought specifically for planting or saved from your own crop. Garlic benefits from a cold period to develop its full flavour and bulb structure, so it is typically planted in autumn (in cool temperate climates) or late winter/early spring (in warmer regions). Split a bulb into cloves and plant each one individually, pointed end up, about twice the depth of the clove itself.

Soil, sun and drainage

Onions and their relatives need an open, sunny position. They dislike shade and do not thrive in waterlogged or compacted soil. Free drainage is one of the most important factors — bulbs sitting in wet soil will rot.

• Soil preparation: a fine, firm tilth (surface texture) is ideal. Rake the bed level and remove stones and large clumps. Onions do well in soil that was well-manured or composted for a previous crop — adding fresh manure at planting time is not recommended as it promotes leafy growth and disease at the expense of bulb formation.
• pH: onions prefer a slightly alkaline to neutral soil (around pH 6.5–7.0). If your soil is on the acidic side, add garden lime in the season before planting and work it in well. Avoid adding lime and manure at the same time as they react and lose effectiveness.
• Raised beds: these are particularly good for onions as drainage is naturally superior. See our raised bed gardening guide for how to set one up.
• Rotation: avoid growing alliums in the same spot year after year. Moving them around the garden each season reduces the build-up of soil-borne diseases, especially white rot (see Common problems).

Planting: spacing, depth and timing

Correct planting depth and spacing matters more with onions than many other crops — plant too deep and bulb formation is restricted; too shallow and roots don't anchor properly.

1. Prepare the bed. Rake to a fine, level surface. Firm lightly by pressing with the back of the rake or walking gently over the bed — onions like firm, not fluffy, soil.
2. Plant sets at the right depth. Push each set into the soil so only the very tip is showing above the surface. The tip (the pointed end, where the dried foliage was attached) should be just barely proud of the soil.
3. Observe correct spacing. Space maincrop onion sets 10 cm apart in rows 25–30 cm apart. Shallots can be spaced 15–18 cm apart, as each set multiplies into a cluster. Garlic cloves want 10–15 cm between them in rows 30 cm apart.
4. Net against birds. Birds — particularly pigeons and rooks — are drawn to the tips of sets and will pull them out of the ground. Lay fleece or fine netting over the bed until the roots are firmly established, usually after a few weeks.
5. Label your rows. Different varieties look identical until harvest. Mark what you've planted and where, especially if you're trying multiple types side by side.
6. Check for heaving after frosts. Cold weather can push sets upwards. Walk along the rows after cold spells and push any displaced sets gently back in.

Weeding — the one thing onions really need

Of all the things onions need, consistent weed control is the most important. Onions are exceptionally poor competitors with weeds — their upright, narrow leaves cast almost no shade on the soil, and their root system is shallow and not aggressive. Weeds that grow up between and around the plants compete directly for water, nutrients and light, and a neglected onion patch can see dramatically reduced yields.

• Hoe between rows shallowly and regularly, particularly while the plants are small. A sharp Dutch hoe used in dry conditions is highly effective — the cut weeds wither on the surface rather than re-rooting.
• Hand-weed close to the bulbs themselves to avoid accidentally hoeing off foliage or disturbing bulbs.
• A fine mulch of compost or well-rotted material around plants (not over the top of the bulbs) can help suppress weed seedlings between hoeings.
• As onions mature and the bulbs swell, they push up above the soil surface — this is normal and desirable; do not cover them with soil.

Watering and when to stop

Onions need water, but not in the same way as most other crops. During the growing season, water during dry spells to keep the soil from drying out completely — particularly during the period when bulbs are actively swelling. However, overwatering is almost as harmful as underwatering: wet conditions promote neck rot and other fungal diseases.

The critical point about watering comes towards the end of the season: stop watering completely once the foliage begins to lean and yellow naturally. This signals that the plant is moving towards ripening, and the bulbs need dry conditions at this stage to cure on the plant. Continued watering once the foliage has started to go over leads to soft, poorly stored bulbs prone to rotting in storage. If rain is forecast at this point, do not panic — natural rainfall is usually fine — but avoid adding supplemental water.

Bolting: causes and what to do

Bolting means the plant sends up a flower stalk instead of — or before — forming a proper bulb. This is the most common disappointment with onions, and understanding why it happens makes it much easier to prevent.

The main trigger for bolting is a vernalisation response: the plant experiences a period of cold, interprets this as winter, and then interprets subsequent warming as a signal to complete its life cycle by flowering and setting seed. For established plants, this can happen when a cold spring spell is followed by warmer weather. Planting very early into cold soil also increases the risk.

• Use small or medium sets (not large ones), which are less likely to bolt.
• Don't plant into very cold, wet soil — wait until conditions have settled and warmed slightly.
• Look for bolt-resistant varieties, which are bred with reduced sensitivity to temperature fluctuations.
• If a plant bolts: snap the flower stalk off near the base. The plant won't produce a good storage bulb, but it will continue to develop a useable onion. Use bolted onions fresh rather than trying to store them — the thick flower stalk at the centre makes the neck weak and prone to rot in storage.

Ripening, lifting and curing for storage

Home-grown onions stored correctly will keep for many months, providing a supply of homegrown flavour well into winter and sometimes beyond. Getting the harvest and curing right is the key to long storage life.

Onions are ready to harvest when the foliage has naturally fallen over and turned yellow and papery. In some areas and seasons, the bulbs will ripen without any intervention; in others you may need to fold the tops over gently to help them along. Avoid forcing the foliage flat too early — the leaves are still supplying nutrients to the developing bulb.

Once the foliage is down and dry, loosen bulbs carefully with a fork (pushing it in well to the side to avoid spearing them) and lift. If the weather is dry and warm, leave them on the surface of the bed for a week or two to dry in the sun. In wet or unreliable weather, move them to a greenhouse staging, a slatted shed shelf or a raised rack somewhere dry and well-ventilated.

Curing is complete when the outer skin is completely dry and papery, the neck (where the leaves attached) is thin and hard, and the bulb feels firm. This may take two to four weeks. Once cured, store in nets, old tights, or plaited strings hung in a cool, dry, frost-free place with good air circulation. A garage, shed or cool pantry is ideal. Check regularly and remove any that show signs of softness or rot before they spread to neighbours.

For much more detail on preserving and storing your harvest, see our food preservation guide.

Spring onions for quick crops and small spaces

If you're short on space — or just want a quick return from a small patch — spring onions (also called scallions, green onions or bunching onions depending on where you are) are one of the best choices in the vegetable garden. Unlike maincrop onions, they're harvested young before any bulb forms, so they're ready in as little as eight weeks from sowing and take up almost no space.

• Sow direct: scatter seeds thinly in a drill 1–2 cm deep, or sow in bands across a raised bed. There's no need to thin them — they can grow in a dense row and be pulled individually as needed.
• Successional sowing: sow small batches every three to four weeks through the growing season for a continuous harvest rather than a single glut.
• Containers and windowsills: spring onions are among the best crops for growing in pots, troughs and window boxes. They're shallow-rooted and produce well in a modest volume of compost. Our guide to growing food in small spaces has more detail on container approaches.
• Varieties: 'White Lisbon' is the classic for quick harvesting; 'Performer' is more bolt-resistant; 'Red Baron' gives a mild, red-skinned crop; 'Ishikura' and similar bunching types are used in Asian cooking and grow quite tall.

White rot and other common problems

• White rot (Stromatinia cepivora): a serious soil-borne fungal disease that causes the foliage to yellow and wilt, and leaves a fluffy white mould at the base of the bulb. The devastating thing about white rot is that the fungus persists in the soil for many years in structures called sclerotia — essentially dormant spores that can survive for decades. There is no organic cure once your soil is infected. Strict rotation (not growing alliums in infected ground) is the main prevention; infected soil should be kept isolated from other areas. Never move soil or compost from infected beds. Some gardeners find that garlic extracts applied to the soil can stimulate sclerotia to germinate without a host plant, reducing the reservoir over time.
• Onion fly: the larvae (small white maggots) tunnel into developing bulbs, causing plants to yellow and collapse. Protect plants with fine insect-proof mesh from planting time — this physical barrier prevents the adult fly from laying eggs near the plants.
• Downy mildew: a grey or purple fungal coating on leaves, especially in wet, cool seasons. Improve airflow, avoid overhead watering, and remove and dispose of (not compost) affected material. Resistant varieties are available.
• Neck rot (Botrytis allii): a storage rot that often develops during curing or storage, causing the neck of the bulb to soften and turn grey and mouldy. It typically starts in damaged or incompletely cured bulbs. Ensure necks are fully dry before storing, and check stored crops regularly.
• Thrips: tiny insects that rasp the surface of the leaves, causing a silvery mottled appearance. Generally not serious enough to affect yield significantly unless numbers are very high.

Onion-growing checklist

• Start with sets for the easiest results; choose small to medium-sized ones to reduce bolting risk.
• Prepare a fine, firm, well-drained bed in a sunny position.
• Plant sets with only the tip showing — not buried deep.
• Protect from birds with netting for the first few weeks after planting.
• Weed frequently and thoroughly — onions are poor competitors.
• Water during dry spells but stop completely once foliage starts to fall and yellow.
• Snap off flower stalks if plants bolt and use those onions fresh.
• Lift and cure thoroughly before storing — papery skins and dry necks are the target.
• Store in a cool, dry, airy place and check regularly for rot.
• Rotate alliums to a new bed each year to avoid white rot build-up.
• Sow spring onions in pots or in small batches for a quick, space-efficient crop.