Onion bulb formation is strongly linked with day length

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Last Updated: 
July 13, 2006

CORVALLIS - Ever have trouble getting the onions in your garden to cooperate and form nice bulbs? Do they "bolt" or set flower instead?

Bulb onions are highly influenced by the length of days and nights, explained Jim Myers, vegetable breeder with the Oregon State University Department of Horticulture. Different types of onions have different light (and dark) requirements.

Regardless of when an onion is planted, the amount of dark and light that a bulbing onion is exposed to strongly influences when and if they will bulb, flower and set seed, said Myers.

The varieties of onions that require a shorter period (11 to 13 hours) of daylight to bulb are termed "short day" onions. Those that require the longest period of daylight (14 hours per day or more) to form bulbs are known as "long day" onions. Those with intermediate requirements (from 13 to 14 hours of light per day to bulb) are called, logically, "intermediate" onions.

Short-day onions include: Yellow Bermuda, White Creole and Eclipse onions (12 hours daylight to begin bulb formation). California Early Red, Ebenezer, Early Strasburg (13 hours).

Long day onions include: Yellow Globe Danvers (14 1/4 hours) Sweet Spanish, Yellow Flat Grant (14.9 hours) and Yellow Rynsburg, Zittan Yellow (16 hours).

Intermediate-day onions include: Early Yellow Globe, Australian Brown, White Portugal and Southport Yellow Globe (13.5 hours) Red Wethersfield, Southport Red Globe, Italian Red and Flat Madiera (14 hours).

The OSU Extension Service recommends the following varieties as performing well in most areas of Oregon. All are long or intermediate day, except Walla Walla, which was originally a short day onion, now modified through years of selection by growers.

Yellow Onions
Buffalo, Simcoe, Copra, Capable, Prince, First Edition, Millennium, Fiesta, Frontier, New York Early.

Buffalo, Walla Walla Sweet.

Red Onions
Benny's Red, Redwing.

White Onions
White Sweet Spanish, Blanco Duro.

Because location (latitude, or distance north or south of the equator) determines day length, some kinds of onions are not as suited for some locations.

Why? If you grow short-day onions at northern latitudes such as Oregon, when the days reach short-day onions' critical photoperiod (11-13 hours of daylight) it is only March. The soil is still cold and the onion plant will still be tiny, forming only a tiny bulb. Pacific Northwest growers work around this.

"In general, if our climate allows a variety to overwinter, a fall planting with some vegetative growth followed by the short day trigger in the spring can allow a short day onion such as Walla Walla to be grown in the Pacific Northwest," said Myers. "Growers also exploit the short-day habit to get cocktail onions or produce onion sets [bulbs for planting]."

For more information about growing onions, beets, carrots, radishes and other root crops, visit our on-line publications and video catalog at: https://catalog.extension.oregonstate.edu/

Author: Carol Savonen
Source: Jim Myers