Growing onions and garlic

Plant garlic cloves from mid-autumn to early winter, and onion sets in early to mid-spring (or overwintering sets in autumn). Give both a sunny, free-draining bed, keep them weeded, and lift them when the leaves turn yellow. Timings are ranges: wet winters and late frosts vary a lot across the UK, so adjust to your plot.

At a glance

	Garlic	Onions (sets)
Difficulty	Low maintenance	Sets are very easy
Plant	Mid-autumn to early winter (some types late winter to early spring)	Early to mid-spring, or overwintering sets in autumn
Depth	Tip about 2.5cm below the surface	About 2cm deep, tip just showing
Spacing	15cm apart, rows 30cm apart	10–15cm apart, rows 20–30cm apart
Harvest	Late spring to early autumn, depending on planting	June to September, depending on type
Protection and support	Start in modules on wet soil	Cover new sets against birds
Rotation group	Peas, beans and onions	Peas, beans and onions

Figures from the RHS. Check the sowing calendar for the months in your area.

Overview and difficulty

Both crops take up little room and store for months once dried. The RHS calls garlic a low-maintenance crop. Onion sets (small, part-grown onions) are very easy and suit beginners. Seed is cheaper and gives more choice, but it's slower and doesn't keep well.

Conditions

Both like a sunny site with free-draining soil. Wet soil makes garlic more prone to disease, especially when it's planted in autumn. Overwintering onion sets may rot in heavy soil that gets waterlogged.

Space and kit

• A sunny bed, or part of one, kept for the onion family this year
• Module trays if your soil is heavy or wet over winter
• Fleece or netting to keep birds off new onion sets
• A hoe for weeding between rows

Sowing and planting

Garlic. Most varieties need one to two months at 0–10°C to form good bulbs, which is why autumn planting suits them. Push individual cloves in 15cm apart, with the tip about 2.5cm below the surface, in rows 30cm apart. Don't plant deeply in heavy soil. If your soil stays wet over winter, start the cloves in modules in autumn and plant them out in spring.

Onion sets. Plant spring sets in early to mid-spring, and overwintering sets in autumn. Set them about 2cm deep so the tip just shows, 10–15cm apart, in rows 20–30cm apart. Birds often peck new sets out of the ground, so cover them until they've rooted.

Onion seed. Sow indoors from mid-winter to early spring, or outdoors from early to mid-spring once the soil starts to warm.

Routine care

• Weed regularly. Onions don't grow well when competing with other plants.
• Water onions in dry spells, but stop once the bulbs have swollen in mid-summer, or spring-planted onions may store less well.

Common problems

White rot is the one to fear. It's a soil-borne fungus that attacks onions, garlic, leeks and other alliums: leaves yellow and wilt, and the roots and bulb rot. Its resting bodies survive in soil for at least 15 years, so rotation isn't practical once it's in, and there's no chemical treatment for UK gardeners. It spreads in soil on tools and muddy boots, so the main defence is keeping it out. See clubroot, carrot fly and onion white rot.

Bolting (running to flower) is more common with sets than seed. Heat-treated sets greatly cut the risk.

Rust is a common fungal disease of leeks and garlic, and it also affects onions.

Harvest and storage

Garlic. Autumn-planted garlic is ready from late spring to early summer, spring-planted from mid-summer to early autumn. Lift the bulbs once the leaves turn yellow. Dry them in a single layer in the sun for two to four weeks, then store them somewhere cool.

Onions. When the leaves yellow and topple, the crop is nearly ready: lift before the foliage dies right down. Dry for about two weeks, then store somewhere light, cool, dry and airy. The RHS says not to keep them in the dark.

Smaller spaces

Garlic grows in a pot at least 25cm deep, filled with peat-free multi-purpose compost. Onions need a bigger container, at least 45cm wide and deep, in a sunny spot. Pots dry out faster than open ground, so check them often.

FAQs

When should I plant garlic in the UK?

Mid-autumn to early winter for most varieties, so it gets the cold spell it needs.

Why have my onions gone to seed?

That's bolting. Sets are more prone to it than seed-grown onions, and heat-treated sets greatly reduce the risk.

How do I know garlic is ready?

The leaves turn yellow. Lift the bulbs then and dry them before storing.

Related guides and your next step

Next step: if it's autumn, pick a sunny, well-drained bed and get your garlic in. If your soil is heavy, start the cloves in modules instead. Onions and garlic belong with peas and beans in a simple rotation, so check where that group goes this year.

• Clubroot, carrot fly and onion white rot
• Crop rotation: a simple 4-year plan
• What to do on the allotment in November