Leek, garlic, shallot and other species of the genus have more distinct growing seasons. Technically, onion, Allium cepa, is a cool season vegetable. It grows through winter to be ready for harvest before summer dormancy. Alternatively, it can grow through spring for a later summer harvest. In cool situations, it can grow through summer for autumn harvest.

Onions do not actually mature within a single season anyway. Most grow from onion sets which are simply small onions that grew from seed earlier. They simply finish growing for another season once plugged into a garden. Those that grow from seed in autumn might go dormant through the middle of winter. Spring is actually their second growing season.

The most popular onions are yellow or brown, red or purple, or white. Each type exhibits a distinct flavor. All develop a few hollow bluish green leaves that stand about a foot tall. Their foliage shrivels as they mature enough for harvest. They are useless if they bloom. Green onions are merely juvenile onions which have not yet developed distended bulbs.