If you want to plant onions in fall and harvest bulbs early the following year, four varieties with useful trial support are TopKeeper, Hi-Keeper, Bridger, and T-420. All four produced nice bulbs in both years of a New Hampshire low-tunnel trial. That makes them a promising shortlist—not a universal ranking or a guarantee for every climate.
Here, “winter onions” means fall-planted bulb onions grown through winter, not perennial Egyptian walking onions, which are primarily grown for greens.
Four winter onion varieties to consider
A University of New Hampshire and UMass Extension trial evaluated eight onion varieties over the 2011–12 and 2012–13 winters. Its report identified five varieties—TopKeeper, Hi-Keeper, Keepsake, Bridger, and T-420—as producing nice bulbs in both years. The four below fit this article’s shortlist; the report does not establish that they outperform Keepsake or rank them as the top four by yield.
| Variety | What the sources report | What to keep in mind |
|---|---|---|
| TopKeeper | A yellow storage onion that produced nice bulbs in both trial years, according to the UMass Extension Vegetable Team and University of New Hampshire report. | Its evidence comes from the report’s New Hampshire low-tunnel system; results elsewhere may differ. |
| Hi-Keeper | A yellow storage onion that produced nice bulbs in both trial years. Penn State Extension also lists Hi-keeper among varieties for overwintering. | Match the variety and planting window to local conditions. |
| Bridger | A yellow storage onion that produced nice bulbs in both trial years. Penn State Extension lists Bridger for late-summer direct seeding. | It has support from both the trial and an extension recommendation. |
| T-420 | Produced nice bulbs in both trial years. Penn State Extension lists T-420 among overwintering varieties. | Keep the variety code as written when checking seed listings. |
Sources: Penn State Extension, “Onion Production”; UMass Extension Vegetable Team and University of New Hampshire, “Research Report: Overwintering Onions in Low Tunnels”.
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Choose the right onion for your location
Match day length, not just winter hardiness
Onions begin bulbing when day length reaches a threshold that varies by cultivar. NC State Extension groups short-day onions at about 10–12 hours, intermediate-day onions at 12–14 hours, and long-day onions at 14–16 hours. If an onion has not reached its critical day length, it continues making leaves rather than forming a bulb. The right type depends on where you garden and when you want to harvest. Consult regional extension recommendations rather than assuming a variety suited to one area will bulb well everywhere.
Sources: NC State Extension, “Bulb Onions”; UC Statewide IPM Program, “Cultural Tips for Growing Onions and Garlic”.
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Understand what “winter onion” means
Fall-planted bulb onions are different from Egyptian walking onions. Maryland Extension describes walking onions as hardy perennials grown mainly for greens; other multiplier onions produce clusters of small bulbs. Those are options for a different purpose, not substitutes for varieties selected for overwintering and forming typical bulbs.
Source: University of Maryland Extension, “Growing Onions in a Home Garden”.
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What the New Hampshire trial does—and does not—show
The trial used low tunnels and row-cover protection in New Hampshire, including sites in Durham (zone 5B) and North Haverhill (zone 4B) during the 2012–13 season. It supports trying these varieties in a protected system under conditions comparable to those sites; it does not demonstrate identical performance in every open garden, region, or winter.
In that trial, seed was sown in August and plants were transplanted in October. Those dates describe the researchers’ method, not a general planting calendar. The report found that later fall sowing appeared to reduce spring bolting, but resulted in smaller, later-maturing bulbs. Earlier sowing encouraged more vigorous growth while raising the chance of bolting. The report also notes that variable fall weather makes one ideal planting date difficult to prescribe.
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How to choose and plan your planting
- Check your region’s day-length recommendations. Choose a cultivar suited to your latitude and intended harvest, not simply one labeled “winter.” NC State’s variety recommendations are for eastern North Carolina; gardeners elsewhere should use their own regional extension guidance.
- Decide what you want to harvest. The four listed varieties are supported as bulb onions. “Winter White Scallion” in the New Hampshire trial produced leek-like shanks rather than typical bulbs, illustrating why a winter label alone does not establish bulb performance.
- Account for bolting risk. Fall planting that is too early can increase premature seedstalk formation. Local fall weather, planting timing, and growing setup all matter; use regional guidance rather than copying the New Hampshire dates.
- Consider protection suited to your site. Row cover and plastic low tunnels were part of the New Hampshire trial. They are an option for adapting that protected approach, not a requirement established for every climate.
- Verify the variety name on seed listings. Penn State lists Bridger, Hi-keeper, Toughball, and T-420 for late-summer direct seeding. The New Hampshire trial supports the four varieties in this article, but current inventory and retailer stock are not established here.
Sources: NC State Extension, “Bulb Onions”; Penn State Extension, “Onion Production”; UMass Extension Vegetable Team and University of New Hampshire, “Research Report: Overwintering Onions in Low Tunnels”.
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