Growing Bulb Onions: Pick the Right Variety for Your Garden


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.

Long-day White Sweet onion
You must pick the right type of onion for your region. Pick the wrong type and the onions will not form 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:

How to Grow Onions

Onion Seed Starting Tips.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.


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  1. Hi im rocky from philippines i really wanted to try planting onion upon i read one farmers harvest more than million in planting onions in the area of 1 hectare only can please give me some ideas how could i learn the process of planting onions..?please can you give me a guide step by step hope in tagalog version so that i can fully understand thanks and more power..

    • Check out the Onion articles at this site; go to the Index, then click on Onions. You will find many articles there to help you grow.

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