Early in the season, onions send up tubular, hollow leaves, before beginning to form bulbs. Most onions are biennial, so you will seldom see an onion flower. Multiplier onions or Egyptian walking onions are a perennial variety that does indeed send up a flower stalk with a clump of tiny bulbs or bulbils on top. When the top becomes to heavy to stand, it falls over. The bulbils take root, forming new plants and giving the perennial onion its description of walking.
Mature Size
Both eh size and shape of onions varies by type and growing season. Bulbing onions are greatly affected by day length, as described below. The bulb size is related to the size and number of the leaves. Each leaf translates to a ring of onion. Larger leaves make larger rings. So choosing the right type of onion for your day length will give your onion tops time to form before the onion bulb begins developing: more leaves, more bulb.
Exposure
Onions need a spot in full sun, to develop properly.
The time required for the bulbs to mature depends on variety and whether they were started from seed (110 - 125 days), transplants (70-90 days) or sets (50-60 days).
Harvesting
You can harvest onions at any stage. The plants you thin from a row can be used as green onions. However, onion bulbs are ready when about ½ the tops have fallen over and the bulbs’ skins have a papery feel. Bulbs allowed to remain in the ground until 50% or more of the green tops have fallen over will store longer.
Once you see ½ the tops are down, very gently coax the remaining leaves down, without breaking them off the bulb. Then allow the bulbs to sit in the ground and cure for a couple of days before you lift them. You’ll have better luck digging the onion bulbs, rather than pulling. You don’t have to go deep, just enough to loosen the remaining roots.
Shake and brush off any loose soil and let the bulbs finish curing in a warm, dry place with good air circulation. Leave the leaves on. You can use fresh onions at any time now.
For storing onions, wait until the outside onion skins dry and the neck - where the leaves meet the bulbs, starts to shrivel. Then you can store them in a cool, dry location, like your basement. Onions keep longer in cool temperatures (35 - 40 degrees F.) but should not be allowed to freeze. Store onions in mesh type bags or by braiding the tops together and hanging. Just make sure they are not piled on top of each other and not getting any air.
Suggested Varieties
If you are buying transplants or sets, you will find types suitable to your area in any good nursery, although they will sometimes only be labeled as yellow or red.
Onions are categorized according to day length; how much day light there is when onions stop forming tops, or new green leaves, and start making bulbs.
1. SHORT DAY ONIONS: Begin forming bulbs when the day length is 10 to 12 hours. (Tend to do better in the south and west.)
'Granex' - Large, globe-shaped, sweet onions sometimes called the Vidalia onion. Early producer. (Red or White)
'Texas Grano 1015Y' - An improved strain of 'Yellow Grano' with treat disease resistance and good storing ability.
'Cipollini' - Flat, doughnut shaped onions. Store up to 5 months.
2. LONG DAY ONIONS: Begin forming bulbs when the day length is 14 to 16 hours. (Tend to do better in the North.)
'Italian Red Torpedo' - Heirloom onion. Reddish-purple with an elongated in shape. Can also be grown as a short day onion.
'Redwing' (F1) - Red color holds up well. Nice solid bulb. Late season onion, so best from transplants.
'White' or 'Yellow Sweet Spanish' and other Spanish types - Long season varieties (120-130 days from transplant). Includes ‘Walla Walla’ and the earlier Olympic (F1).
See more here
