I grow a lot of overwintering onions as well as spring planted. I also live just outside Portland so we're in the same region.
For starters, spring planted onions are WAY easier to grow than overwintering ones. I can't stress this enough. They have a much shorter growing season, more varieties are available, watering is mostly under your control, less bolting, less disease, less weeding, better dry down timing.....On and on.
If you have issues growing spring planted onions you may want to look at that before taking on overwintering ones, but that's just my opinion....Lets move on.

"Keepsake" is about as good as any of the japanese bred overwintering yellow onions. "Topkeeper" is the industry standard. For small quantities, Territorial can't be beat.

Most commercial growers direct seed onions, I start them in flats. The hot weather makes germination difficult for me and I have limited water in my overwintering field. I also grow by organic standards so the weeds are difficult to control in a slow germinating, slow growing crop like onions.
I start them in open flats in the middle of August and transplant out (bare root) with the fall rains in mid-October. I use lots of composted manure at planting time and side-dress in late winter. Weeding is constant.
No you won't need any sort of tunnel or row cover for protection unless we have a freak cold snap. I've grown them successfully when winter temps dropped to 15 F.

Let me know if you have any more Q's, I could go on all day.
(Plus I should let someone else chime in.)

-Mark

Madroneb: When do you harvest them? How well do they keep? I live just north of Boston, 6a, and I am wondering about growing them for DH in the greenhouse, unheated, or double covered tunnel. They way my guys eat I figure I need about 100 lbs of onions a year, and a spring harvest would really help me find enough storage space.

Plant now...harvest in spring/early summer
I will be planting "keepsake"...it's all in the name, very good keepers.
Here, we don't get too many deep freezes and tunnel is not required. Not sure about your area(?)
I'm trying it as an experiment

This year we harvested somewhere around 1500 lbs. the second week of June. That was after selling fresh onions for about a month before that. Some years it's earlier, some later, depending on the weather.
The downfall to OW onions is that they only keep till mid November. After that they start to sprout.

I'd think near Boston, the protection you suggest should work. Just watch out for overheating on sunny days (which could cause early bolting) and fungal issues.

Madroneb:

What varieties do you recommend for overwintering? I am in western Oregon in the Willamette valley.

There doesn't seem to be a clear favorite in any catalog I have read.

I am growing for just me and would like a long harvest (some diversity in maturity would be good) and hope to get very sweet juicy onions like I get at the farmers market.

I am also in the PNW( Seattle area).
So I would appreciate your suggestions on the type of seeds.
One thing I know about the onions is that they are very DAY LIGHT sensative. We have almost 16 hours of day light in the beginning of summer. TOOO long .
So , obviously, need to grow LONG DAY type. Where , and WHAT can I buy locally for spring planting ?

Thanks.

So I'm hearing two different questions. One for suggestions of overwintering onions (which are short day) and suggestions for varieties for spring planting.

As far as the overwintering, I planted Hi-keeper, Bridger, T-420, Walla Walla and Desert sunrise this last season. Next week i'm probably doing the same as I still have seed left. I honestly can't recommend one yellow over the other as they all did very well. Desert Sunrise is red and makes the prettiest spring onion and Walla Walla is a sweet onion.

Nancy, if you're looking for sweet, i'd say the walla walla is the best for you. You can start eating them at any size but they mostly finish up all at the same time and keep till November.

Seysonn, I only grow onion from seed so I don't really know where to get onion starts locally.

-Mark

I wonder if it's too late to direct sow cipollini type onions. I was late preparing my onion beds, am almost ready now...

In the past, sowing at the halfway mark of August, I end up with a lot of bolting in the spring. Might later sowing increase or decrease that tendency?

Thanks, for anyone who can shed some light there for me.

J.

I don't know any cipollini that will overwinter without bolting. As far as I know, all of them are long day.

To overwinter onions, usually you want short day or at least indeterminate. If you choose the right variety, you should still be able to direct seed now.

That said, if you have had success with a certain variety, a little later planting date might keep the bolting down. Keep in mind, there are many other factors, like winter temps and soil fertility/moisture that trigger flowering.
-Mark