Onions are having a moment in the seed catalogs right now, and a lot of that buzz is about overwintering them, tucking small transplants into the bed in September, letting them sit through the cold, and pulling bigger, earlier bulbs the following June. It sounds like a clear win. But the biology underneath it is more particular than the marketing lets on, and the trade-offs are real.
In this episode, I'm breaking down the day-length science that controls when an onion decides to bulb, the separate cold-exposure trigger that can send it to flower instead, and why those two mechanisms together explain both the promise and the risk of overwintering. I'll walk through the specific varieties that university research has found reliable for this system versus the ones that bolted their way out of a harvest, and I'll lay out the honest cost side of the ledger: a bed occupied for the better part of a year, a real chance of premature flowering if timing or variety selection is off, and whether any of that beats simply direct-seeding or transplanting in spring.
Whether you've got seed garlic already planned for a bed and you're eyeing the space next to it for onions, or you're just trying to figure out which system fits your garden, this episode will give you the research to make that call yourself. Let’s dig in.
References and Resources:
Short Day Onions
• Latitude: ~25°–35° N
• Daylength trigger: 10–12 hours
• Climate: Mild winters (USDA Zone 7+), These are generally planted in fall, mature in late spring.
Intermediate Day Onions
• Latitude: ~32°–42° N
• Daylength trigger: 12–14 hours
• Climate: Central U.S. conditions; planted in spring (or early spring in cooler areas)
Long Day Onions
• Latitude: ~37°–47° N
• Daylength trigger: 14–16 hours
• Climate: Northern latitudes; often sweet, specialty, or long storage types. Planted in fall or spring depending on climate
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Episode 296 — Successions: Never an Empty Bed: https://justgrowsomethingpodcast.com/episode/succession-planting-101-how-to-keep-your-garden-producing-from-spring-through-frost-ep-296
- Oregon State University Extension — Winter Vegetable Production on Small Farms and Gardens West of the Cascades (PNW 548): oregonstate.edu/catalog/pnw-548-winter-vegetable-production-small-farms-gardens-west-cascades
- University of New Hampshire Extension / NH Agricultural Experiment Station — Research Report: Overwintering Onions, 2014–15 (Becky Sideman et al.): unh.edu/resources/files/Resource005477_Rep7652.pdf
- Iowa State University Extension and Outreach — What Is a Long-Day Type Onion Variety?: extension.iastate.edu/faq/what-long-day-type-onion-variety
- University of Minnesota Extension — Growing Onions in Home Gardens: umn.edu/vegetables/growing-onions
- NC State Extension Publications — Bulb Onions: ces.ncsu.edu/bulb-onions
- University of Missouri Extension — Planting, Growing and Harvesting Onions: missouri.edu/news/planting-growing-and-harvesting-onions
- Clemson Cooperative Extension, Home & Garden Information Center — Onion, Leek, Shallot & Garlic: clemson.edu/factsheet/onion-leek-shallot-garlic
- Oregon State University Extension — Plant Onions Early for Bigger Summer Bulbs: oregonstate.edu/news/plant-onions-early-bigger-summer-bulbs
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