How to Overwinter Onions

On a hot summer day in August you will find us in a greenhouse starting onion seeds for overwintering. You might wonder what is the benefit to overwintering onions? If you're an onion fan like we are, it will provide you with a continuous supply of onions all year round. Right now, we are harvesting main-season onions that we planted in the spring and will last through the winter into next spring. The seeds that we start now for overwintering will be ready for harvest in late spring or early summer of next year--just as the main-season harvest is wrapping up.

Overwintering onions can be a little tricky depending on your location. They are worth growing and doing a little trial and error will help you see what the best planting dates are for your area. Here in Cottage Grove, Oregon we are in zone 8b; our average minimum temperature is between 15 and 20°F, and bulb onions are generally cold hardy down to about 5 or 10°F. That means most years, on average, we can overwinter onions outdoors without protection from the weather. If it’s an option for you, one of the best ways to grow overwintered onions, especially if you’re in an area colder than ours, is to grow them in a cold frame or in low tunnels.

Over the last several years of trialing overwintered onions, we've determined that the best time for sowing the seed in our area is the first half of August. We are trying to achieve an onion that is about the width of a pencil at the onset of winter. If it’s a little too small going into winter, it can either be frozen out or you’ll end up with a small bulb in the spring. If your onion is a little too big going into winter, then that’s going to cause it to bolt prematurely when spring rolls around. We recommend trying different sow dates to see what works best for your area. Try Late July, Early August, and Late August to start, and see which yields the best results.

Also, we recommend starting more onions than you hope to end up with and expect a little bit of loss. When we overwinter onions, depending on the variety, we can expect a certain percentage of them to bolt, or if it’s a particularly cold winter some may freeze out.

Sowing Tips

• Fill your tray about ¼” down from the top with Organic seed starting mix

• Sow 3-5 seeds per cell in your tray

Note: if it’s especially hot where you are, there may be some seeds that don’t germinate, so you may want to sow a little heavier.

• Top the seeds with about ¼” of vermiculite

Vermiculite is great because it keeps the seed moist and helps germination. It also keeps bugs down that might live on the soil. If you don’t have vermiculite, no problem! You can just use a little bit more of your seed starting mix or even some sifted compost if it’s well broken down.

• Once you’re done sowing, water well and continue to keep evenly moist

• Trim the tops to about 3-4” if they reach over that before transplant (you may have to trim them more than once)

Transplanting Tips

We transplant our onion starts in mid to late October when they are about 8 weeks old. Typically, when transplanting your onions, you space them about 6 inches apart, but for overwintering we recommend spacing them 3” apart (because of the possibility of loss discussed before). Wait until the spring, and if they all pulled through, then you can thin them out. Use those as green onions and let the rest bulb up.

Harvesting & Storage Tips

• Our overwintered onions are usually ready to harvest here in June (it can vary depending upon the variety)

• As bulbs approach maturity withhold water so protective paper can form

• After about half the tops have fallen, push over the remainder; wait about 1 week then harvest

• Once harvested, cure in a warm (75-80°F), shaded, well-ventilated location until outer skin and necks are dry

• Once cured, give the root hairs a trim (to about 3/8 -1/4 of an inch) and trim the top off (don’t cut it too close—leave about 1” above the bulb)

• Remove any excess dirt and place in an onion bag

• Store in cool, dry, well-ventilated area

• Check occasionally and immediately remove any sprouting or rotting onions

Note: If any of your onions have thick, green necks that didn’t dry down well, or if they have any soft spots on them don’t put them in the bag with the rest. They’re not going to store well and they will just spoil the rest, so go ahead and eat those first! Walla Walla is an example of an onion that is great for overwintering but doesn’t dry down and store well. So, eat those within 2-4 weeks of harvest.

We hope you’ll give overwintering onions a try. It is a very rewarding experience!

Our recommended varieties for overwintering can be found here.
If you’d like to see our video about overwintering onions, check it out here.

Happy planting!