When to plant onions – for a healthy harvest

It's important to know when to plant onions, because if they are planted at the wrong time, results are likely to be disappointing. You may get lots of leafy green growth above ground, but when you come to dig up your harvest, the bulbs will potentially be very small, or may not even have formed at all. 

So what's the secret? To learn how to grow onions you first need to know when to plant them – and that depends on the variety you choose, and where you live. 

When to plant onions

There are three kinds of onions – long-day onions, short-day onions and day-neutral onions. The key difference between the three is the number of hours of daylight they need in order for the bulbs to form. 

Long-day varieties need around 15 hours of daylight in a 24-hour period, while short-day onions only require around 10 hours. By contrast, day-neutral onions – also known as intermediate onions – will form bulbs regardless of day length. 

So knowing which of the three you’ve chosen is essential when it comes to deciding when to plant onions as part of your vegetable garden ideas.

Which variety of onions to plant

In areas where summer days are long (US zones 6 and colder), long-day onions are your best choice. In zones 7 and warmer, day length doesn’t vary so much between summer and winter so short-day onions are your best bet. Day-neutral onions will happily grow anywhere, but are an especially good choice in zones 5 and 6. 

For the English climate, James Wong, an ambassador for the Royal Horticultural Society and author of Grow For Flavour, is a fan of long-day varieties. ‘For me, the sweetest of all is Walla Walla, from the valley of the same name in America’s Washington State,’ he says. 

What is the best time of year to plant onions?

You can plant onion seeds, and these are usually sown during cooler weather, from December to January. 

However, ‘although you can grow onions from seed, it’s easier to start off with sets
– small bulblets – which establish quickly and grow on to form full-sized onions,' explains TV presenter and garden designer, Mark Lane.

The best time for planting onions differs depending on the variety:

• Plant long-day onions in late winter or early spring for your kitchen garden ideas, and they will mature 90 to 110 days later, in mid to late summer
• Plant short-day onions in fall, ready to harvest them in spring
• In mild winter climates, plant day-neutral onions in fall, but in colder, northern regions, wait until early spring – either way, you’ll get a harvest around 110 days later

Along with planting onions at the correct time, consider onion companion planting to help further boost crop yields.

What month do you plant onion sets?

The best month to plant onion sets, as above, depends on the variety you choose and also where you live. 

Short day onions can be planted in warmer regions in September to November. In cooler areas plant long-day onions between February and April depending on the conditions, whereas day-neutral onions can be planted in October to November in milder climates, but are better planted between March and April in colder areas.

In norther states, onions are usually planted from mid-March to mid-April, although look out for varieties that are also suitable for planting in October. Ensure you know when to harvest onions when you are planning the planting.

If you don't have a dedicated vegetable plot, then onions are still a versatile option to grow as vegetable garden container ideas.

When can I plant onions outside?

If you’re planting onion sets (bulblets) in cold regions, get long-day and day-neutral onions into the ground as soon as the soil is easily workable. Short-day onion bulblets can go into the ground in fall.

Can you grow onions in winter?

Short-day onions are suitable for growing over the winter – plant them outside in fall, and they will form lots of good top growth during the colder months. Then, as soon as day length reaches 10 hours, plump, flavorsome bulbs will start to form below ground, ready to be harvested in spring.

Are onions easy to grow?

If you know when to plant onions, they are super-easy to grow, according to Jo-Ellen Meyers Sharp, president of GardenComm in Indianapolis. 

‘I recommend starting with onion sets – these look like tiny onions and are nearly a sure thing. But remember that choosing what kind of onion to grow depends on where you live.’ 

She also recommends growing shallots. ‘They, too, are super easy and can be grown from either seeds or bulblets. I’ve planted them in both fall and spring with success.’ 

If onions are planted at the right time you are almost guaranteed a good crop, so they are a good choice for a small vegetable garden where if you are short on space, you need to get the best yield from crops you grow.