The Long and Short (Days) of Onions

When you plant onions (Allium cepa), you expect them to provide you with nice fat bulbs at the end of the season. And as long as you buy your seeds, onion sets or plants locally, that’s just what they’ll do. But did you know that onions only produce a bulb worthy of that name when they receive the right day length? In fact, if you grow the wrong kind of onion, there’ll be no bulb at all!

Here’s an explanation.

Three Categories

There are actually three categories of onions: long-day onions, short-day onions and day-neutral onions.

In the Northern Hemisphere, long-day onions are grown in the North, above the 35th parallel. That means throughout Canada and the northern states of the USA and through most of Europe and Northern Asia as well. They need 14 to 16 hours of sunlight per day, otherwise no bulb is formed. They are usually planted early in the spring for harvest in late summer. Seed companies from northern areas usually only offer long-day varieties.

In the Southern Hemisphere, long-day onions are rarely grown at all, except in New Zealand, Chile and Argentina, as otherwise southern land masses really don’t really extend far into areas with long summer days.

Long-day onions tend to have a strong taste and store well.

Short-day onions are grown between the 35th parallel and the equator, since they don’t produce bulbs under long days. They form bulbs when the days reach from 10 to 12 hours. Given the hot summers nearer the equator (and onions like things on the cool side), they are usually sown in the fall for a spring crop, quite doable in areas with a mild climate.

Short-day onions tend to have a milder taste than long-day onions, but don’t store as well.

Day-neutral onions produce bulbs under the effect of 12- to 14-hour days. They are planted in the spring in cooler climates for harvest in late summer and in the fall in the hotter ones for a spring harvest. They usually give their best performance on either side of the 35th parallel, so are popular in the south of Europe and in the southern United States (except Florida, where short-day onions reign) and are also widely grown in Australia and South Africa.

Day-neutral onions tend to be intermediate in taste and shelf life compared to long- and short-day onions.

If you grow an onion in the wrong area and no bulb forms, all is not lost: you can still use it as a green onion.

How Pertinent Is This Information?

As long as you grow onions from locally produced seeds, plants or sets, the information above will not likely change the way you garden. But if you order seeds by mail, it is important to consider where they came from. The best catalogs will mention the category their various onion varieties belong to.