Shape and size: Some onions produce very small, nearly insignificant bulbs; these are called bunching onions, green onions, and scallions.
Some onions produce small bulbs round or spindle in shade; these are called picking onions.
Some onions produce medium-size globe-shaped bulbs; these tend to be sharp-flavored storage onions.
Some onions produce large, round, mild-flavored bulbs for fresh use; Spanish and Bermuda onions are large, mild, and sometimes sweet.
Color and flavor: Onion colors range from white or yellow to red or purple. Yellow onions are all-purpose onions; they have a balance of astringency and sweet in their flavor.
White onions have a sharper more pungent flavor; they are also more tender and have a thinner, papery skin.
Red onions have red flesh and purple skin; the flavor of a red onion is similar to a yellow onion but the layers of a red onion are less tender and less meaty than a yellow onion. Generally, the strongest-tasting onions are the best choice for storing into the cold; they have the toughest skins.
Daylight needed: Bulb onion varieties differ according to the amount of daylight needed for bulb formation. Some varieties require 12 hours of light each day to form bulbs (called short day); some require 13 to 16 hours of daylight to form bulbs (called long day). .Grow short-day varieties where the cold is mild. Grow-long day onions in the cold. Onions are not sensitive to mild frost in either spring or autumn. It may be worth noting, long-day onions tend to be round and globe-shaped and short-day onions tend to be flatter in shape.
Seeds, seedlings, sets: Onions can be grown from seeds, seedlings, or sets (sets are small bulbs grown the previous year). Obviously, sets have a head start; they will mature more quickly. Seedlings will need more time to produce bulbs than sets. Seeds will need a long growing season to produce bulbs; sometimes they produce a bulb the second season they are in the ground. Sets are only available in spring. There will be a greater selection of varieties if you shop for seeds than in sets or seedlings. If you live in a short growing season region, choose sets to ensure you have enough time to grow bulbs.