I grew hundreds of onions every year – here’s what I learned about when to plant onion seeds

Onions are a fantastic and simple crop to grow, even for those who are new to growing vegetables. There may be different ways to grow these reliable and low-maintenance crops, but there are real advantages to discover from planting onion seeds. 

I grew onions for many years for chefs in kitchen gardens that I worked in, and continue to grow the crop on my home allotment plot. I always tended to grow my onion crop from seed, starting them off indoors in the early months of the year, though you can plant onion seeds directly into the ground too.

Onions may be a crop with a long growing season, but growing them from seed is not a difficult process. It is a bit more time-consuming than growing from sets, however, in my opinion it is well worth trying and seeing what results you can get.

Why grow onions from seed?

Onions can be grown from onion seeds and onion sets - and many growers have their preference of the two. 

One of the main benefits of growing onions from seed is that the resulting bulbs tend to be larger - that is definitely what I have found from growing onions in both ways over the years - and they are less likely to bolt in hot weather. 

There is also a wider range of types of onions that can be grown from seed, compared to the sets you see commonly available in garden centers and online. The downside is the time and effort that it takes growing from seed, compared to sets that can simply be put into the ground in fall or spring.

You can see the range of onion seeds available at Burpee

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When to plant onion seeds indoors

There is an old adage among onion growers that the best time to plant onion seeds is on Boxing Day. That may seem early, and indeed it is. Sowing onion seeds in December is normally only reserved for those who want huge bulbs, such as the types that are put on show benches. And you do need a special set up to sow any seeds that early, including a warm greenhouse and grow lights to supplement the low heat and light levels of early winter. 

Onion seeds can be sown indoors in mid-to-late winter, either January or February. I was always fortunate enough in my days as a professional gardener to have a heated greenhouse to start my onion seeds off in, and grow lights too. I didn’t sow as early as Boxing Day, but did start sowing onion seeds in January.

If you have the luxury of a warm environment, and grow lights that are on for 12 hours per day, then you can plant onion seeds in January, otherwise it is maybe best to wait until February when you can either use a propagator or a warm and sunny windowsill inside the house. Onion seedlings should germinate strongly if you can provide a consistent temperature of 50-60°F.

When to plant onion seed outdoors

Onion seeds can be sown directly outdoors into their growing space once the soil warms up and is workable in spring. This tends to be during the last weeks of March - however the exact time will depend on your US hardiness zone. 

The soil temperature ideally wants to be 50°F, and it is not recommended to sow onion seeds into cold or wet soil. The use of cloches or row covers, such as these Plant Covers at Amazon, can help to warm the soil prior to sowing, and also create a warmer environment for the seeds to germinate in. 

As part of a companion planting plan, avoid planting onion seeds in the same space in which you grew alliums in the last three years.

When to plant onions from seed

It is best to wait until the frosts have passed for your location to transplant onion seedlings out into the vegetable garden. The ideal time to plant out onions would be mid-to-late spring. The seedlings want to be around 6-8 inches tall to transplant and to be given time to harden off to get used to being outside. 

Some growers trim the roots and stem of their onion seedlings when transplanting. It is thought that trimming produces stronger seedlings and bigger bulbs. To be honest, this is something I have done when growing leeks, but not when growing onions - and I have managed to get strong harvests of big onions without trimming. However, it may be worth a trial with some onion seedlings to compare the end results. 

FAQs

Can you plant onion seeds in the fall? 

There are varieties of onions, known as Japanese onions, that can be grown from seed in late summer or early fall. They can either be sown indoors in August to be planted out into the garden come the end of September, or sown directly into the garden in August. For the seedlings to overwinter successfully, they want to be at least eight inches tall come the onset of winter. The alternative, and potentially less-risky, method is to plant sets of hardy onions in fall. 

When done correctly, you can store onions for up to eight months after harvesting. However, you could also consider leaving one or two of the biennials to go to flower in their second year, to then collect onion seeds from them to sow.