Some multiplier onions are very high yielding. Walking onions (A. cepa, same species as bulbing onions) & bunching onions (A. fistulosum) are probably your best bets. Both are perennials, so as long as you care for them, you'll (almost) never need to buy scallions. Only during their dormant phase in mid-Summer.

You have probably already seen walking onions; "Egyptian" and "Catawissa" are the most popular. They form bulb clusters on the end of their "seed stalk". They can be harvested twice a year, in early Spring, and again in late Summer/Fall. The plants multiply both from dividing at the base, and from planting the bulb clusters. They are exceptionally winter hardy. These are my favorite scallions, very tasty.

Bunching onions can be harvested at roughly the same times. They get flowers on their stalks the second year after planting, and are less winter hardy than the walking onions... but probably more than hardy enough for your zone. The degree to which they multiply varies; those with the larger stalks tend to divide less, perhaps into 3-4 vs. 10 or more for the smaller ones. Did I say larger stalks??? Some varieties, well grown, can have stalks over 1" wide. I described several varieties in the "Unusual Vegetables" thread (on the Veggie Forum).

You could also grow shallots or potato onions, which would then double as bulbs when mature... but IMO, eating them as scallions would be a waste. They also would not multiply as rapidly as the walking onions, or the smaller bunching onions.

However, you might consider growing garlic chives. Even garlic bulbs, if left unharvested, will form large clusters of greens which can be harvested as scallions... you might even find that you have a taste for them.

"Both are perennials, so as long as you care for them, you'll (almost) never need to buy scallions. Only during their dormant phase in mid-Summer. "

Does this mean that if I grow bunching onions that multiply, I can harvest some of the new stems, and leave others in the ground, where they will divide again once their cycle reaches that point?

Thanks for the information, this is very useful to us! I will probably grow some multi-stem bunching onions such as the siji ("four seasons") variety this spring or summer.

"Does this mean that if I grow bunching onions that multiply, I can harvest some of the new stems, and leave others in the ground, where they will divide again once their cycle reaches that point?"

Yes. If you carefully harvest the stalks on the outside of the clumps, the remaining stalks will multiply. I use a thin, sharp knife, and cut away from the center, to avoid damaging the roots of the remaining plants.

It is not necessary to dig up the plants each year; I have a plot of "Catawissa" onions that I have not disturbed for 10 years. I just lay a thick layer of compost over them each year, during their dormant phase.

But if you divide & replant the onions each year, you will get larger stalks.

I grew "Four Seasons" as part of a bunching onion trial in 2006-7. It was the smallest, at about 15" high... but it was also the heaviest multiplier. I started them from seed indoors, and planted however many germinated (2-3 per pot). The clusters each formed a clump about 2" wide by the end of the first year, and quickly grew to 3" wide the following Spring! It's like chives on steroids. Flooding destroyed many of my onions this year, but I still have a few clumps of these.

With "Four Seasons", dividing will probably be necessary - unless you harvest enough to overcome their rapid multiplication. If you keep pinching off the flower stalks (which will appear the second year) the dormancy will be reduced, and you will have scallions most of the year.

I've always yanked the whole bunch and, stuck one back into the ground. It usually shocks the replanted onion badly. I've always harvested greens by whacking the outer leaves. Just never thought of doing it with my bunching onions.

I'd love to try "Catawissa" or "Egyptian" onions if someone is interested in a trade. I have a lot of vegetable seeds. Please email if interested.

One negative, to me, with the Egyptian Walking Onion, is that the taste can become strong later in season or during times of heat. If you are accustomed to the standard mild, crisp, green scallions in the store, we find these quite different that way. Even so, they are good to keep around for their tastiest times.

I'm going to give Evergreen Hardy White Scallion (from Fedco) a try. It sounds like what we are aiming for...

http://www.fedcoseeds.com/seeds/SeedsOrderItem.php?id=2439&listname=Onion

Hopefully it divides profusely like I want it to. :)