Onions are Cool Season Vegetables, also known as scallions, bunching onions, green onions (Allium cepa var. cepa from Alliaceae Family. Onion is not native to North America. Unknown in wild. Probably originated in the Middle East or Asia.
What type of onion to select
Almost any onions sown from seed can be scallions (also called bunching onions, green onions, spring onions or green tails).
To have scallions in the summer, plant seeds of an onion variety such as 'Beltsville Bunching' or 'Evergreen Bunching' as early in the spring as you can.
Some varieties, such as 'Long White Bunching', don't ever form bulbs. 'White Portugal' is a bulbing onion grown all over the country and harvested young for sale as scallions.
How to Make Your Own Indestructible Onion Sets:
The sets were fed over the first 70% of the 4 months with a splash of liquid fertilizer.
Plant from seed
Bunching onion transplants for sale at the store are not worth buying. Bunching onions planted from seed will be at the eating stage in just a matter of weeks. Some survive winter temperatures as low as -30F.
Propagate by seed. To grow your own onion sets, sow seeds thickly in a block in midsummer. About 2 months after planting, roll down the tops, forcing the plants to form small bulbs. (Those about the size of a dime work best.)
Plant seed ¼ inch deep, ½ inch apart, in rows 12 to 18 inches apart. Thin to 4-inch spacings for large bulbs, 2-inch spacings for smaller bulbs but higher yields, or 1-inch spacings for scallions.
Days to emergence: 4 to 5.
Raised beds are ideal. Onions are good for intercropping with other garden plants, especially early-maturing spring greens.
Onions have shallow root systems and need consistent moisture
Spring-sown early varieties can be harvested just 60 to 80 days after planting.
From seed (early-maturing):
Early Yellow Globe
Green or Bunching:
Evergreen Hardy White
Long White Bunching
Southport White Bunching
Here are 2 varieties I am growing this year (summer 2012):
Tokyo White Bunching from Burpee:
Evergreen Seeds has a large selection of bunching onions: http://www.evergreenseeds.com/bunchingonion.html
Big Bunch from Burpee (from Home Depot).
Gardening Articles - National Gardening Association http://goo.gl/nO5yT
Explore Cornell - Home Gardening - Vegetable Growing Guides - Growing Guide http://goo.gl/xGERx
Watering Onions - Gardening Association - http://goo.gl/hfRsp
Growing Onions - Gardening Association - http://goo.gl/0P2Cu
Onion Varieties - Gardening Association - http://goo.gl/EHHkU
Planting Onions - Gardening Association - http://goo.gl/lkN1Q
Vegetables for a Fall Garden in the Midwest
Planting Indestructible Onion Bunches: Onion Sets (No) vs Onion Bunches (Yes)- TRG 2014 - YouTube http://buff.ly/1m4iPLN
I actually disagree with Gary (from the video above) - onion sets are just fine to grow green onions quickly. Also, you can harvest the leaves as "cut and come again" lettuce as the plant grows. You can use the same approach to grow green garlic from garlic gloves (Costco's garlic works the best).