When to Plant Onions in the UK
Unusually for a vegetable, onions give you two quite separate planting seasons. Most UK gardeners plant sets in spring, but hardy overwintering types can be planted as the leaves start to fall, and they reward you with fresh onions weeks ahead of the spring-planted crop. Picking the right window, and the right conditions on the day, makes a real difference to how big and how reliable your bulbs turn out.
Quick answer
Spring onion sets go in from March to April, once the soil has dried out enough to work and is beginning to warm. Hardy overwintering (Japanese) sets are planted in September or October and are usually ready by early summer. Seed can be started under cover in the depths of winter, or sown straight into the ground once spring is under way.
Onion planting times at a glance
Autumn, September to October: plant overwintering (Japanese) sets. Harvest June to July.
January to February: sow onion seed under cover. Harvest from late summer.
March to April: plant spring onion sets, the main season for most gardeners. Harvest July to September.
Early to mid-spring: sow onion seed outdoors. Harvest from late summer into autumn.
Planting onion sets in spring
Spring is the easiest and most reliable time for most people. Wait until the ground is no longer cold and claggy. A good test is to squeeze a handful of soil: if it crumbles apart, you're ready. If it stays in a sticky ball, give it another week or two.
Planting into cold, wet ground in late winter doesn't give you a head start. Sets sit and sulk, may rot, and a sharp cold spell after planting can trigger them to flower early rather than bulk up. Heat-treated sets are much less likely to do this, so they're worth looking out for if you want to plant at the early end of the window.
Raised beds and light, sandy soils warm and dry out faster in spring, so you can often plant a week or two earlier there than on heavy clay.
Planting overwintering onions in autumn
Overwintering onions have been bred to tolerate frost and short days. Planted in early autumn, they put down roots before winter, sit fairly quietly through the coldest months, then grow away strongly in spring. Come early summer, they're ready to lift just as last year's stored onions are running low.
Autumn planting suits gardens with soil that drains well over winter. On heavy ground that stays soggy, there's a real chance the sets will rot away before spring. If that sounds like your plot, pop each set into a cell of compost instead and keep the tray somewhere cold but sheltered, like a greenhouse or cold frame, until you can plant them out in spring.
Birds love tweaking freshly planted sets out of the soil, so throw some fleece or a net over the row for the first few weeks. September and October are the ideal window. Miss that window and it's still worth planting any leftover sets straight away, before the ground turns cold and heavy.
For this month's other jobs, see what to plant in October
Sowing onions from seed
Seed takes longer and needs more care than sets, but it's cheaper and gives you far more varieties to choose from. For the longest possible season, sow into modules under cover during January and February. Alternatively, wait until the soil has warmed in spring and sow straight into the bed.
Buy fresh seed each year, as onion seed loses its ability to germinate quickly.
Planting onions in containers
Onions in containers can be planted at the same times as in the ground. Pots are particularly handy for autumn planting on heavy soil, as the compost drains far better than wet clay over winter.
For container advice, see growing onions in containers
Planning your onion year
Onions planted in autumn are generally ready in early summer, and they're best used up before winter, as they don't keep as well. Spring-planted sets and seed-grown onions follow from July to October, and these are the ones that keep through winter and into the next spring. Planting a few of each is a simple way to have homegrown onions for most of the year.
For full growing advice, see how to grow onions in the UK
For lifting and drying, see when to harvest onions
Common questions
Is it too late to plant onion sets in November?
It's getting late. September and October are the best months for overwintering sets. If you still have sets in November, plant them straight away, as long as the ground isn't waterlogged. On heavy, wet ground, you'll have more luck starting them in a module tray somewhere sheltered.
Can I plant onion sets in February?
Usually it's better to wait until March. Cold, wet soil in late winter can make sets rot or bolt. If you do plant early, choose heat-treated sets and make sure the soil isn't frozen or sodden.
When should I sow onion seeds?
Under cover, sow during January and February. Outdoors, wait until the soil has warmed up in spring before sowing. Seed gives you more varieties to choose from but takes longer than sets.
Why did my onions bolt after planting?
Early flowering, or bolting, is usually triggered by a cold spell after planting. Sets planted into cold soil too early are most at risk. Heat-treated sets are much less likely to bolt.