Overwintering Onions for Spring Harvest
Recommendations from the Onion Overwintering Trials
Note
These recommendations come from recent Hensley's team overwintering trials in hardiness zone 5a.
Harvest timing will vary. For the best quality and yield, factor in your latitude and growing zone and adjust dates and materials to match.
For more, see our latitude and day length map for long-, intermediate- and short-day onions. Your local Cooperative Extension office can give advice for your area.
At our Zone 5a trial site, summer-planted trials ripen fast as fall arrives, and even while we evaluate and enjoy the harvest, we're getting ready for the first frost just weeks away.
Every year we adjust and run new trials of crops to harvest through winter and into early spring. One recent overwinter trial produced lots of data and plenty of fresh, tasty, full-size onions by late May. That trial was in a long Gothic-style high tunnel, though we often overwinter onions in low tunnels too. Here's how we take an onion trial through a northern winter to a good spring harvest.
Sowing & Transplanting Goal: Onion Seedlings with 4–5 Leaves
As usual, we sowed this trial in flats in the greenhouse in late August and transplanted the seedlings into the high tunnel about 5 weeks later, in early October. For us and most growers in long-day onion latitudes (37°–47°N), sowing in late August to early September and transplanting from late September to mid-October works well for overwintered onions.
As the Winter Growing Guide section on Recommended Crops & Varietiesexplains, the aim is for seedlings to have about 4–5 leaves before the hard freeze. (Some go by eye, aiming for pencil thickness, but we find leaf count more reliable.) Bigger plants bolt more easily in spring; smaller ones struggle through the coldest part of winter and may be slow to bulb.
As with other market crops, diversifying lowers risk. Where you can, spread your bets by sowing and planting on several dates and growing more than one variety.
Falling Temperatures & Decreasing Day Length
When nights started dropping below freezing in late October and November, we covered the plants with row cover. We suggest a heavyweight cover (such as AG-50 or AG-70).
On mild days we pulled the cover off so the plants could adjust gradually to winter's coldest temperatures without shock. It's like hardening off seedlings before spring planting, except the plants are adapting to falling rather than rising day length and temperature.
Air and soil kept getting colder into winter, but the average daily soil temperature seldom dropped below 30°F (-1°C), and the lowest soil readings were about 27°F (-3°C). Through January and February the row cover stayed on full time.
Lengthening Days + Rising Temperatures = Results!
In spring we removed the covers whenever daytime air rose above freezing, and took them off for good once temperatures evened out.
The plants started growing again and began bulbing in early to mid-May.
Here's a combined summary of this trial and several later overwintered onion trials:
Full-size White
- Although 'Bridger' won our early overwinter trials, we've since found
'T-448' about a week earlier than 'Bridger' when overwintered, with bigger, rounder bulbs and excellent bolt resistance at our latitude. For earliness, size, shape and bolt resistance, 'T-448' is our top full-size white onion for overwintering in the North. - For an early spring-sown onion, though, 'Bridger' is better; spring-sown 'T-448' isn't as early or as big.
Full-size Red
- 'Red Spring' has vigorous plants that overwinter well and form even, round, deep red bulbs. Growers who liked 'Desert Sunrise' will find 'Red Spring' a good substitute; it matures a few days later, is a little more prone to bolting and has deeper red bulbs. (Being long-day, 'Red Spring' also works as an early spring-sown red onion in the North, which 'Desert Sunrise' doesn't.)
Bunching Onions/Scallions
- Red: 'Deep Purple' shows good bolt resistance when overwintered.
Full-size Yellow Onions from Sets
- Onion sets are another way to get great spring onions, either alongside overwintering from seed or instead of it if you lack time or labor for raising seedlings. At latitudes of 37° and above, we suggest 'Shakespeare' sets.
- Fall timing for sets is different from seed. Plant sets before the ground freezes, about when you plant garlic or fall bulbs. In zone 5a we plant ours from mid-October to late November.
- A big plus of sets is that planting time is more flexible. From seed, you have to time sowing just right, and weather in fall, winter or spring can upset things. Sets can simply go in the ground and come up reliably in spring, usually giving onions about 2 weeks after overwintered seed-grown plants.
- Since set-grown onions come in a little after seed-grown overwintered onions, you can use both and stagger the harvest. By the time the set onions are finished, your earliest spring-sown onions may be ready.
Common Problems with Overwintered Onions
Overwintering isn't simple, but if you have a market for these crops it can pay off. Here are common problems with onions overwintered in a cold climate.
Bolting
Bolting is probably the most common problem with overwintered onions, and one of the traits we watch most closely in trials.
Onions are biennials, so a spring cold snap can tell an overwintered plant it's time to finish its life cycle. It responds by sending up a flower stalk to make seed, drawing sugars and nutrients from the bulb to do it, so a bolted plant has a tough center and off color. The bulb also stops growing, which can leave you with bulbs too small to sell.
Every onion variety will bolt eventually after overwintering. The varieties we recommend for overwintering stand out because they form a saleable bulb before they bolt. Besides variety choice, success depends a lot on testing sowing dates where you are. Plants that go into winter too big or mature bolt sooner in spring. Good ventilation reduces bolting by preventing big temperature swings. Finally, harvest as soon as bulbs reach saleable size.
Botrytis Leaf Blight
Some varieties are very susceptible to botrytis leaf blight, caused by the fungus Botrytis squamosa. It likes moist conditions and first shows as small white spots or flecks on the leaves. As it spreads, the spots grow, the leaf turns sunken and brown and finally dies (see inset).
Since botrytis thrives in humidity, good tunnel ventilation and lowering humidity around the plants by uncovering as often as possible will help prevent it.
Doubling or Splitting
If a plant splits into two stalks, you have a double. Doubles usually aren't saleable but are fine to eat, just a little more work to prep.
Many things can cause doubling, from genetics to uneven watering, but environmentally it often comes from temperature swings, another reason for good ventilation.
Small Bulbs
Onions need full sun; less means small plants and small bulbs. One corner of our trial tunnel sits near an apple tree that, lovely as it is in bloom and fruit, shades that corner a little. Plants there were noticeably smaller than those in full sun, so even slight shade matters.
If you plant while leaves are falling, remember they'll be back in spring and may shade your crop. Uncovering as much as conditions allow also gives plants more light.
Death
As you'd expect, not every variety or plant can handle the extra stress of winter, even with the shelter a tunnel provides.
It may have been too cold, rodents may have damaged bulbs looking for a winter meal, or plants may not have been well rooted when temperatures fell.
Onions can fail to overwinter for many reasons, some in our control and some not. Either way, we look forward to another early spring harvest of delicious onions, so we'll take notes, adjust and try again next year.
Learn More
To read more on overwintering onions in a cold climate, see these reports from trials in New Hampshire by Dr. Becky Sideman and colleagues.
- Overwintering bulb onions under low tunnels in New Hampshire • HortTechnology • open-access article
- Research report: overwintering onions UNH Cooperative Extension • PDF
