Short-Day Onions is a term used by people who grow onions. It refers to onions that need fewer daylight hours to start plumping up below ground.Onions will form a bulb when the temperature and the number of daylight hours hit the right combination for them which triggers their clock. Short-Day onions will start bulbing when the day is 11 to 13 hours long (Long-Day onions want 14 to 16 hours.)
Onions use daylight to produce a good deal of top growth before they form bulbs (and the more top growth, the bigger the bulb.) When the day reaches the right number of hours for that variety of onion, the onion will stop forming top growth, and form a bulb instead.
In southern areas, Short-Day Onions are even started in the fall, and grown over the mild winters, when the days are short. This allows for maximum top growth, as the bulbs won’t start forming until the spring when the magic number of daylight hours happens.
In North America, the limit is roughly 32 degrees north, a latitude line that runs from central Georgia through to southern California. Short-Day Onions would start bulbing too soon in the north, before a good deal of top growth had happened, and so the bulbs would be small, about the size of a pearl onion. In very northerly places, such as Canada or Scotland, which can have 14 to 20 hour days, Short-Day Onions might never form a bulb.
Temperature wise, Short-Day Onions are generally only hardy to about 20 F / -7 C. Most sweet onions are Short-Day varieties.