There are three different types of onions and picking the right type
is key to growing a great crop:
Short-day onions are varieties that form bulbs as soon the days reach 10 to 12 hours in length. They’re perfect for southern gardeners below the 35th parallel whose days are slightly shorter throughout the growing season. If you grow short-day onions in the north, you’ll end up with tiny bulbs that go to flower early in the season because the bulbs stop growing as the days lengthen.
Long-day onions are varieties that form bulbs when the days reach about 14 hours in length. They’re best for gardeners in the northern tier of the U.S. and Canada. Long-day onions won’t form bulbs south of the 35th parallel because the days aren’t long enough to trigger bulb formation.
If you live somewhere across the mid-section of the U.S., grow day-neutral onion varieties (also called intermediate day). Varieties like these begin to set bulbs when days range from 12 to 14 hours in length.