Bulb onions are particular about where they grow. Onion bulb formation is triggered by the number of summer daylight hours. Bulb-forming onions can be divided into three types.
Three types of bulbing onions
- Long-day onions require about 15 hours of summer daylight.
- Short-day onions require about 12 hours of summer daylight.
- Day-neutral (also called intermediate-day) onions that are not affected by the number of daylight hours. (Day-neutral onions are modern hybrids, bred to not be sensitive to day length.)
Which type of onion to grow in your garden
It’s relatively easy to know which type of onion you should grow in your garden. It depends upon where you live.
Draw an imaginary line across the country from San Francisco to the north of the Carolinas. Do you live north or south of this line?
- Summer daylight north of this line is 14 to 16 hours long or longer: grow long-day onions.
- Summer daylight south of this line is about 10 to 12 hours: grow short-day onions.
- Not sure, too-much trouble to figure this out: grow day-neutral onions.
In short: long-day onions grow the best north of the 40th parallel; short-day onions grow the best south of the 28th parallel; day-neutral onions are the best bet between the 28th and 40th parallel.
What happens if you make a mistake? Long-day varieties grown in the South will never form bulbs because the days are not long enough. Short-day varieties grown in the North will form bulbs early but the bulbs will be small and weak because the plant has not grown strong enough to grow large, mature bulbs.
Onion bulb formation explained
An onion bulb is a short underground stem. From the base of the stem, roots grow down and leaves grow up. The onion plant has a fan of hollow, bluish-green leaves. Each leaf is connected to its own underground stem; the larger the leaf the larger its underground stem. The greater the number of large leaves, the greater the size of the bulb beneath the ground; each leaf corresponds to a ring of the onion bulb; the larger the leaf the larger the ring. (Like all plant stems, the onion stems or leaf bases store nutrient reserves for growing the plant.)
Onions are photothermoperiodic; which means they are sensitive to temperature and also to daylight. Onions quit forming leafy tops and begin to enlarge their underground stems (bulbs) when the daylight each day reaches a certain length. The amount of daylight needed for an onion plant to begin forming a bulb varies by variety; each variety has its own genetically determined bulb formation trigger.
For more detail on this process read: Bulb Onion Growing: Day Length and Temperature.
When to plant bulbing onions in your garden
To grow an onion with a mature bulb:
- Long-day varieties (growing in northern regions) should be started 12-10 weeks before the last frost in spring: direct-sow seed in a plastic tunnel or cold frame, or sow seed indoors for transplanting into the garden 5 to 4 weeks before the last frost in spring. Harvest will come in late summer or early fall depending upon the variety.
- Short-day varieties (growing in southern regions) should be planted mid-fall to mid-winter either directly sown or started in the garden with onion sets or transplants. Harvest will come in late summer or early fall depending upon the variety.
- Day-neutral varieties (planted either in northern or southern regions) can be planted in mid-fall to early spring in mild-winter regions and in early spring in cold-winter regions. Harvest will come in late summer or early fall depending upon the variety.
Onion varieties to grow
- Long-day onion varieties include Walla Walla Sweet, White Sweet Spanish, and Yellow Sweet Spanish.
- Short-day onion varieties include Georgia Sweet, Sweet Red, Texas Super Sweet, Texas Sweet White, Yellow Granex (Vidalia), White Granex, and White Bermuda.
- Day-neutral onion varieties include Red Candy Apple, Candy, and Superstar. (All of these are hybrids.)
More on onion growing read: