Overwintering Onion Sets for a Bigger Summer Harvest

I planted onion sets every spring for years and wondered why my onions always came out the size of golf balls. Not terrible. But not the big gorgeous bulbs I kept seeing at the farmers market either. Turns out overwintering onion sets is the move I wish someone had told me about a long time ago. Planting in fall gives them a head start that spring planting simply can’t match.

Why Fall Beats Spring for Onion Size

Onions need a long growing season to bulk up. Plant in spring and you’re racing against the calendar. Fall-planted sets get a head start, establishing roots before winter, then taking off the second the days start lengthening in February. By the time your spring-planted sets are just getting going, your overwintered ones are already sizing up.

The difference in bulb size is not subtle. We’re talking baseball versus golf ball. And you get them weeks earlier in summer, which is a nice bonus when you’re waiting on everything else to catch up.

Which Onion Sets Actually Overwinter

This part matters. Not all sets are equal for fall planting. You want varieties labeled as overwintering or hardy onion sets. Japanese overwintering types (like Senshyu Yellow or Radar) are bred specifically for this. They handle hard frosts without turning to mush.

Regular yellow or red storage onion sets planted in fall will often bolt straight to seed in spring instead of making a bulb. That’s frustrating to discover in June. Ask at your local garden center or check the seed catalog description before you buy. If it says “overwintering” on the label, you’re good. If it just says “yellow onion set,” skip it for fall planting.

Spacing and Planting Depth

Plant the sets about 1 inch deep, pointy end up. Spacing is 4 inches apart in rows, with rows about 10 inches apart. You can go tighter if you want to harvest some as green onions early and let the rest bulk up. That’s actually a decent strategy if you’re tight on space.

I always plant a few extra in a clump somewhere random and then forget about them entirely. Those ones somehow always do fine. Onions are stubborn like that.

Early to mid-fall is the sweet spot for planting. You want them to put out some root growth before the cold really sets in, but you don’t want them sending up big leafy tops that’ll get beaten down by winter weather. A few inches of green showing is fine. A foot of growth is too much, which usually means you planted too early.

Mulching Through a Wet Winter

Here in the Pacific Northwest, the bigger threat isn’t hard freezes, it’s months of cold wet soil sitting on young sets. Rot is the enemy. A few things help a lot.

First, plant in well-drained soil. If your bed holds standing water in January, that’s a problem no amount of mulching fixes. Raised beds are great for this reason. Even a slightly mounded bed helps.

Second, mulch lightly. Two to three inches of straw works well. It keeps the soil from getting waterlogged right at the crown and buffers any hard frost that does come through. Don’t go thick with it though. Heavy mulch traps moisture and that’s exactly what you’re trying to avoid. I use straw, not hay (hay has seeds, and I’ve made that mistake, and it was exactly as annoying as you’d expect). Pull the mulch back a bit in late winter once the sets start pushing up new growth.

Third, don’t panic when the tops die back in December. That’s normal. The roots are still doing their thing underground. You’ll see new green coming up in late winter and that’s when it gets exciting.

What to Expect Come Summer

By June or July, your overwintered onion sets should be well ahead of anything you’d have planted in spring. The tops will start to fall over when the bulbs are ready, same as always. Let them cure in a dry spot for a few weeks before storing.

If you’re already thinking about what else to get in the ground this time of year, it pairs well with all the other fall planting you should be doing anyway. Fall seed starting is one of those things that consistently pays off more than people expect, and onion sets fit right into that same mindset.

The sets themselves are cheap, usually just a few dollars for a bag of fifty or more. Overwintering onion sets can sometimes be harder to find locally, so ordering from a seed catalog in late summer is worth doing before they sell out. They always sell out.

Anyway. Fall onion planting is one of those things that feels slightly counterintuitive right up until you’re pulling out proper onions in June while your neighbor is still waiting on their spring-planted sets to do anything. I’m not competitive about gardening.

Okay, maybe a little. You could say I really know how to turnip the pressure come harvest time.

Photo by Bartolomeus Rumahorbo on Unsplash