Onion sets go in from March, or in autumn for an earlier crop

Plant onion sets in the UK from early March to mid-April. Those give you onions to lift from late summer that will store into winter. You can also plant hardy overwintering sets from mid-September to mid-October for a crop in early summer, weeks before the spring-planted ones.

Shallot sets have a slightly different window: early October to late November in autumn, or early February to late March in spring.

Autumn or spring sets

Autumn onion sets are sold as overwintering or Japanese types. They make roots and a little leaf before winter, then grow away fast in spring. The trade-off is storage: autumn onions have softer skins and are best eaten in summer and early autumn, while spring-planted onions keep much longer.

	Autumn planting	Spring planting
Onion sets	Mid-September to mid-October	Early March to mid-April
Shallot sets	Early October to late November	Early February to late March
Varieties	Sold as overwintering, autumn or Japanese types	Most standard sets
Ready to lift	From early summer	Late summer
Storage	Short, use first	Several months

Many growers plant both, so there’s a gap-free run of onions from June onwards.

These dates are for a typical lowland garden in England or Wales, with a last frost around 20 April and a first frost around the end of October. In Scotland, the north of England and frost pockets, plant autumn sets at the early end so they root before the cold, and wait until the soil has dried out in spring. In the south-west, near the coast and in cities, you can push both windows a little later in autumn and a little earlier in spring.

Sets or seed

Sets are small onions grown from seed the year before and stopped part way. They’re quicker, easier and more forgiving than seed, cope better with poor soil and are less bothered by onion fly. Seed is cheaper and gives you far more choice of variety.

If you grow from seed, sow indoors from late January to late February, or outdoors from early March to early April, 1 cm deep. Onions from seed take around 120 to 160 days to reach harvest, against around 90 to 120 days from planting a set.

How to plant onion sets

1. Pick a sunny bed with soil that drains well and was manured for a previous crop. Fresh manure gives soft onions that rot in store.
2. Rake the soil to a fine, firm surface.
3. Push each set in pointed end up so just the tip shows. Don’t screw them in hard, as that damages the root plate.
4. Space onions 10 cm apart in rows 30 cm apart. For bigger bulbs, go to 15 cm.
5. Space shallots wider, at 20 cm apart in rows 30 cm apart, because each one splits into a cluster of eight to twelve bulbs.
6. Cover with netting or fleece until they root. Birds pull them up otherwise, and frost can lift them too. Push any that come up back in.

Keep onions and shallots away from peas and beans. They do well next to carrots, beetroot, lettuce and strawberries.

Bolting

A bolting onion sends up a flower stalk. Once that happens the bulb stops swelling and won’t store. The main causes are a cold snap after a spell of growth, which fools the plant into thinking it has been through a winter, and dry soil in summer.

To cut the risk:

• Buy heat-treated sets for spring planting. The treatment kills the flower bud and many suppliers label them clearly.
• Don’t plant spring sets into cold, wet soil too early. Mid-March onwards is usually safer than early March in colder areas.
• Water in long dry spells while the bulbs are swelling.

If one bolts, snap off the stalk and use that onion first.

Looking after them

Onions have shallow roots and thin leaves, so weeds smother them easily. Hoe carefully between the rows or hand weed close to the bulbs. Water in dry spells in late spring and early summer, then stop once the bulbs are a good size so the skins can firm up. There’s no need to bend the leaves over by hand.

Lifting and drying

Onions are ready from early June to early September, depending on type and planting date. Shallots are ready from mid-June to mid-August. The sign is the leaves turning yellow and flopping over at the neck.

1. Wait a week or so after the tops fall, then ease the bulbs out with a fork.
2. Lay them out to dry in the sun on a rack or a path, or under cover if it’s wet, for two to three weeks.
3. They’re ready to store when the skins rustle and the necks are thin and dry. Any with thick, soft necks won’t keep, so use those first.
4. Store in nets, trays or plaits somewhere cool, dry and airy.

Shallots come up as a clump. Separate the bulbs once dry and keep a few firm ones back for planting if you like, though bought sets are less likely to carry disease.

Common problems

Onion white rot. Yellowing leaves and a white, fluffy mould at the base. It lives in soil for years, so move onions, shallots, garlic and leeks to a different bed.

Downy mildew. Grey, furry patches on the leaves in damp weather. Give plants their full spacing and don’t water over the leaves.

Onion fly. Maggots tunnel into the bulb in early summer. Sets are less affected than seed. Fine mesh over the crop keeps the flies off.

If you’d like these windows set to your own town’s frost dates, VegLog works them out and adds the jobs to your weekly list.