Considering onions are a staple ingredient in kitchens around the world, I’m surprised that more of us aren’t planting onions in the fall. Onions are most often planted in spring, but planting onions in the fall gives you a jump on an onion harvest and keeps your garden productive throughout the year. Planting onions in the fall is easier in some regions than others, of course, but you can make a plan to put your seeds or sets in the ground as the weather cools, even as far north as New Hampshire and many parts of Canada. In this article, I’ll cover the details of fall-planting these alliums, from the basics of onion growth to the best types of onions, garden preparation, and what to expect come springtime.

Getting a head start on next year 

While I might like the idea of keeping my garden active year-round, selfishly, my top reason for planting onions in the fall is to get a jump on the growing season. When you plant onions in the spring, they’re ready to harvest at full size in late summer or fall. But planning ahead and planting onions in the fall offers a harvest of small onions come late spring and full-grown onions in mid-summer. I can have my onions earlier in the year, and my spring-planting workload is reduced. This is a win, all around.

The basics of how onions grow

Onions are biennials, meaning it takes two growing seasons for them to complete their lifecycle. They’re biologically programmed to survive a winter and then produce seeds the following year. Onions varieties are divided into three categories depending on the length of daylight they need to form bulbs: 

• Short day onions, 11 to 12 hours (examples: White Bermuda heirloom white onions or Southern Belle red onions)
• Intermediate day onions, also called day-neutral, 12 to 13 hours (examples: Candy yellow onions or Super Star white onions)
• Long day onions, 14 to 16 hours (examples: Red Zepplin red onions or the sweet, early maturing Walla Wallas)

Hours of daylight triggers onion plants to switch from forming leaves to forming bulbs. Bulb size depends on the green leafy growth at the time of bulb formation: One leaf equals one ring of onion. You don’t want your onions trying to bulb too early, as this will create very small bulbs and could lead to bolting—sending up a flower stalk that signals the end of the plant’s growth. Yet if your onions never get the daylength signal to form bulbs, you’ll have a lot of green onions on your hands (which are also delicious, but not the end goal).

Why plant onions in the fall 

Planting onions in the fall makes sense to me for several reasons:

• Why wouldn’t you want to eat fresh onions earlier in the season?
• Fall-planted onions take up space in the garden during the winter. When you harvest the onions in mid-summer, you open up garden space for late-summer and fall crops.
• Your fall garden workload is generally lighter than your springtime to-do list.
• It’s nice to see the green stalks growing in the garden when everything else is brown and gray (or white from snow).

In what zones can you fall-plant onions?

Onions are tolerant of light frosts and temperatures down to 20 degrees F (-6 degrees C) if they have an established root system. In USDA Zones 8b and lower, fall-planted onions are still possible with added protection from the weather. In fact, with the right season extension, gardeners with temperatures as cold as Zone 4 can try planting onions in the fall. The season-extending options you can use for growing onions will depend on your garden setup.

Season-extending options:

• Mulching. There are two ways to mulch fall-planted onions. The first is to cover the garden bed with a black plastic or paper mulch or landscape fabric to warm the soil. The second mulch option is once the onions become established, mulch them with leaves or straw. This is a winter-planting strategy as well as one that will reduce weeds and improve moisture retention year-round.
• Floating row cover. Row cover is a spun-plastic fabric that can be used as pest protection or season extension. It comes in various weights that offer a buffer of 2 to 8 degrees F, like a blanket. (Learn more about row cover here.)
• Low tunnels. These inexpensive structures are temporary. At 3 to 4 feet tall and 3 to 6 feet wide, low tunnels can be installed in most home gardens. PVC or wire hoops are covered with heavy-duty row cover or polyethylene plastic sheeting to create a greenhouse effect around the onions. Inside the low tunnel, you can create more layers of warmth using mulch and floating row cover. (Learn more about low tunnels here.)
• High tunnels. If you’re really serious about your gardening and have a high tunnel available to you, fall-planted onions can do well in these tall, unheated greenhouse structures

One or all of these season-extension methods can make planting onions in the fall a possibility. 

The best types of onions for fall planting

Some onion varieties are better than others for fall planting. The types of onions you plant will depend on your latitude and the length of daylight you get in the spring. Once the Spring Equinox hits, northern gardeners’ length of daylight is longer and increases faster than that of our friends in the South. We’re looking for intermediate or long day varieties. In most of the South, intermediate-day onions are best for fall planting. Fall plant onions of the short-day varieties in the warmest climates with the least and most consistent amount of daylight.

Look for onion varieties that are bolt resistant. As the climate changes and our weather patterns become less reliable, our plants don’t always know how to react. Fall-planted onions are no exception. As biennials, onions can bolt in the spring if they have a warmer-than-usual fall, which encourages vigorous growth, followed by a cold winter, which means slower growth. An early spring warmup means the onion plant may be more likely to want to produce seeds. Carefully read the description for the variety of onion you’re considering. Some seed and set providers will even state whether the variety is suitable for fall planting and at what latitude.

Perennial onions like Egyptian onions, Welsh onions, and potato onions, as well as chives, can also be planted in the fall. They’ll set roots before the ground freezes to give them a strong start the following spring.

Preparing the site

Choose a garden space in full sun for planting onions in the fall. Onions are root crops that do best in deep, well-drained soils. Work compost into the soil 8 to 10 inches deep to build organic matter. Be sure the area is free of weeds to start out, as onions are shallow rooted and compete poorly with weeds.

There are three ways to plant onions in the garden:

• Direct seeded. Note that onion seeds are only viable for about a year.
• As transplants, started indoors as seeds either by you or by an onion grower
• As sets, which are small onion bulbs that were grown from seed the previous season. 

Planting onions in the fall from sets

When planting onion sets in the fall, look for those with bulbs smaller than a dime. You want your fall onions to have a head start but not too much of a head start. Larger onion sets are more prone to bolting. Plant sets 3/4 inch deep every 3 inches. Aim to plant your fall onion sets at the same time you would typically plant garlic. You want four or five leaves on the onions before the first hard freeze.

Planting onions in the fall from seeds

You can start your own onion seeds indoors to transplant later or by direct-seeding them into the garden. Direct-seed onions 1/4-inch deep in dense rows. Let them germinate and grow for a few weeks before you thin them to 3 to 4 inches apart. Eat these thinnings as green onions, or transplant them elsewhere in the garden. Direct-seed onions about 6 weeks before your expected first frost.

To grow your own transplants, start onions indoors 8 to 10 weeks before transplant time. Sow them 1/4-inch deep, and keep them warm and well-watered. These seedlings should be 6 inches tall and less than the diameter of a pencil come transplant time. Plant fall transplants around the time of your first frost, giving the seedlings a chance to root before a freeze.

Helping the plants through the winter

After planting onions in the fall, the best way you can help them through the winter is to provide them with the right climate conditions, which might include the season-extension options I mentioned above. Onions still need regular water, but they don’t want to be in saturated soils. Irrigate them, if necessary, but don’t overdo it.

What happens in spring?

In late March or April, as the soil warms and you start coming out of your winter hibernation, your fall-planted onions will do the same. The plants won’t have grown much over the winter but will resume their growth and start forming bulbs soon enough. Side dress the onions with a high-nitrogen fertilizer once in the spring, before the onion neck starts to soften and fall.  

If your onions are bolting, you can’t reverse course, but you can still eat them. Bolting onions will not cure and keep in storage. You’ll know an onion is ready to harvest when its top yellows and falls over. Cure storage onions, and then keep them in a dry, cool, well-ventilated place. If you’re not curing onions, keep them in a refrigerator.

How to winter-sow onions

One more option for growing onions outside of springtime is winter sowing. Using this method, you’ll seed onions in a covered container in mid- to late winter, even into February. Poke some holes in the lid and bottom of the container for ventilation and water movement, set the container outside, and wait for the onions to germinate. When they reach a transplantable size, you put them in the garden. You could look at this as a form of spring planting, but you’re relieving yourself of the springtime mental work by getting the seeding out of the way in the winter. Jessica breaks down all you need to know about winter-sowing onions, which is her favorite way to grow onions.

If onions are on your garden-crop list, you have plenty of ways to plant them. When I consider my spring garden tasks, I welcome any way to lighten the workload at that time of year. By planting onions in the fall, you not only reduce your spring to-dos but also ensure a supply of onions starts coming in sooner. 

If I may offer one more tip: Just because you plant fall onions doesn’t mean you can’t plant spring onions, too. You can keep yourself in fresh onions by keeping this crop growing all year long! Be choosy in your onion varieties, pick out your seeds or sets, be prepared with the right season-extending tools for your climate, and get planting. 

For more information on growing onions, be sure to read these in-depth articles:

• When to harvest red onions
• Onion spacing: How far apart to plant onions
• How to grow green onions
• A guide to curing onions