The Long and the Short of Onions
Have you ever tried to grow onions and had them fail to form bulbs, even when the growing conditions seemed right? It may be that you were growing varieties that weren't suitable for the length of days in your area.
To understand the effect of day length on onions, we first need to recall a little bit from school science class. Remember that the earth is tilted on its axis -- 23.4 degrees from perpendicular to be precise -- and this tilt is maintained as the earth makes its way around the sun over the course of the year. The day length changes as the orientation of the tilted earth relative to the sun changes as our planet completes its yearly orbit. For half the year, the northern hemisphere is tilted toward the sun, resulting in our summer. The farther north on the globe, the more hours each day that part of the globe remains illuminated by sunlight. In winter, the reverse is true -- the northern half of the globe is tilted away from the sun, and the farther north on the globe, the shorter the days. The spring and fall equinoxes are the midpoints in this yearly cycle, when the length of day and night is equal.
Long Day, Short Day
So what does all this have to do with onions? Well, day length is the signal that tells most onions that it's time to stop growing vegetatively -- putting growth into forming leaves -- and time to start forming a bulb.
Some onions are called long day varieties. They need exposure to 14 to 16 hours of daylight to bulb up. These onions grow well north of 35 degrees latitude, or approximately above a line drawn through northern North Carolina, Oklahoma, Arizona to central California. These are the onions that are grown for summer harvest in the northern half of the country. They are planted in early spring, putting on vegetative growth until the lengthening days of early summer trigger bulb formation. Long day onions generally have a pungent flavor and store well.
South of 35 degrees latitude, with its shorter summer day lengths, gardeners need to grow short day onions, ones that form bulbs when the days are 10 to 12 hours long. Short day onions are planted in the fall in the south and grown through the winter for spring harvest or sown in very early spring. Some of the well-known sweet onions are short day varieties. Because of their higher water content, most short day onions do not store well and are best for fresh eating.
What happens if you get your day lengths mixed up? If a northern gardener planted a short day variety, the onion plants would be exposed to enough daylight hours to initiate bulb formation so early in the season that big bulbs would never have the chance to form. And if a southern gardener planted a long-day variety, the onions would never be exposed to sufficiently long days to cause bulbs to form.
Modern plant breeding has helped out with what are known as intermediate day, or day neutral, onions. These varieties aren't as sensitive to day length and bulb up well in response to 12 to 14 hour days. They grow well across a broad range of the country. Day neutral onions are usually planted in the spring.
Growing the Best Onions
Scallions, also called green or bunching onions, don't form bulbs. Their green tops and blanched below-ground sections are used fresh, both raw and cooked. They may be bulbing onion varieties that are simply harvested when young or they may be different species that never produce bulbs. 'Evergreen White Bunching' is a very hardy, non-bulbing scallion variety that can be planted in early spring for summer harvest or in late summer or early fall to overwinter; protect with mulch in northern areas.
Sow seeds, transplants, or sets for scallions as you would for bulbing onions, but space or thin to only an inch apart. Begin harvesting as soon as the tops are 6-8 inches tall. The more mature the plants, the stronger their flavor. Clumps of the winter hardy varieties can be divided their second summer to produce a new crop.