After I had posted about starting onion seeds inside, it got me thinking about the different kinds of onion varieties. If you look in most seed catalogs, you’ll see that they have three different kinds of onion seeds – long-day, short-day and day-neutral (sometimes called intermediate-day). Some of the catalogs do a good job of explaining what this means; others leave you guessing which variety is right for you.
At the heart of all of this is the phenomenon of photoperiodism. Some plants have within them a photoreceptor protein that reacts to the ratio of daylight and darkness. I won’t get into the whole process here (that’s for another post) but in photo-sensitive plants this protein allows the plant to know if the days are long or short and this regulates a lot of processes within the plant.
I usually think of photoperiodism as relating to flowering. Long-day plants blossom in the summer; short-day in the early spring or fall. But in the case of onions, photoperiodism regulates when the plant stops producing new leaves and begins to form a bulb. Long-day onions produce bulbs when the days are long; short-day when the days are short and day-neutral tend to have less of a photoperiodic response.
An issue that factors into where you can grow certain onions is that onions prefer cool weather. Here in the north they’re one of the earliest crops that you can plant in the spring. In the south, onions are planted in the fall so that they can grow during the cool weather of winter.
And that’s why there are different onions for different regions. The dividing line that separates what kind of onion you can grow in North America is the 36° latitude, approximately the border between Kansas and Oklahoma. North of this line, long-day onions are planted in the spring, produce bulbs when the days are long and are harvested in the summer; south of it, short-day onions are planted in the fall, bulb during the shortest days of the year and are harvested in the spring.
If I tried growing a short-day onion here in the north, the plant would never form a bulb because the days are too long to trigger the bulbing response. In the south, a long-day onion planted in the fall would just keep growing and growing and when the days were long enough to produce bulbing, the southern heat would stress the plant.
Where you live does limit your choices of onions. For example I can’t grow Vidalia or Bermuda onions because they’re short-day. But there are lots for different long-day and day-neutral ones from which I can choose.
I always have to stop and think for a minute to remind myself whether I need long-day or short day onions. Most catalogs and seed packets will give you this information. Just make sure the variety is right for your area. Or look for day-neutral onions. They’ll grow in any location.