Plant onions in early spring for biggest bulbs

Picture of a whole yellow onion.
Picture of a whole yellow onion.
Last Updated: 
April 12, 2010

CORVALLIS - The earlier you get your onions in the ground in the spring, the better the chance they will have to grow nice big bulbs, Oregon State University researchers say.

OSU horticulturalists Deborah Kean and Jim Myers test many of the vegetable varieties for home gardeners at the OSU Vegetable Research Farm in Corvallis. Onions, they say, can be grown three ways: from bulbs, also known as "sets;" from smaller transplants (young, non-bulbing onion starts); or from seed. Plant any of these as soon as the soil is dry enough to work.

March through April are good times to plant summer harvested onions in most areas in Oregon. And in Oregon's milder areas, overwintering onion seed can be planted in August to mid-September for harvest next June or July. These can be harvested as scallions all season long.

Onions are biennial plants, meaning they take two years to reach the flowering and seed setting stage. Onions store food the first year in the bulb, and if left unharvested, will bloom and set seed the second year of growth.

The onion lifecycle is sensitive to day length. Vegetable researchers call this characteristic "photoperiodic."

"Most onions grown in Oregon are long-day onions," Kean said. "They will make top growth (the green part above the ground) until a critical day length is reached, triggering bulbing."

Bulbing generally begins in long-day onions when there is about 14 hours of light per day, said Myers. If you plant your onions in the early spring, they will be fairly large plants when the days get 14 hours long. Large bulbs will result.

If you wait to plant your onions until the end of April, when days reach 14 hours, bulbing will begin immediately and small pearl onions will result, explained Myers.

"The size of the bulb is dependent on the size of the plant when bulbing begins," added Kean. "This is why early planting is critical, as well as plenty of water and fertilizer."

Buy and plant onion sets early while they are firm and dormant. Onions can be harvested early for use as green onions, or they can be left to grow until late summer for mature onions. Plant transplants as soon as you get them, and look for plants that have not been damaged by decay or excess drying.

After purchasing onion sets, sort them by size into two groups - those smaller than a dime and those larger. Use the larger sets for green onions because they may flower early and not produce a good dry bulb. The smaller sets will produce large dry bulbs, since there's little chance they'll go to seed early.

For scallions or small green onions, plant the sets 1 1/2 to 2 inches deep and an inch apart. They will be ready for eating in about a month when the tops reach eight to 10 inches. For larger onions, plant the sets about a half-inch deep and four inches apart. Plant transplants three to four inches apart in rows 12 to 15 inches apart.

Planting onions from seed can be a bit more challenging, as germination is sometimes tricky. Keep the seedbed moist, but water lightly, as soil must not crust over.

"Onions are shallow rooted," explained Kean. "If allowed to dry out, they bulb early and small size is the result."

Stop watering your onions when they reach the desired size and the tops have begun to fall over. Harvest them when most of the tops are down. Sun cure them for at least a week before storing to make them last longer.

With all this fuss, there is a payoff - there are more varieties available from seed, often of better "keeping" varieties.

The OSU Extension Service recommends the following varieties as performing well in Oregon:

  • Yellow: Copra, Prince, First Edition, Millennium, Fiesta, Frontier, New York Early;
  • Overwintering: Buffalo, Walla Walla Sweet;
  • Red: Red Wing, Benny's Red;
  • White: White Sweet Spanish, Superstar, Blanco Duro;
  • Green bunching: Ishikura, Tokyo Long White, Hishiko.

To help eliminate difficulties with onion seed germination, or to avoid waiting for soil to dry out or warm up in the spring, start onion seeds in flowerpots indoors. Transplant these when the tops are two to three inches tall.

Use good potting soil, a container with a drain hole, and provide plenty of light. Seed-planted onions need a longer period of development than with onion sets.

Plant onion seeds a half inch deep at a rate of one to five seeds per inch. Thin seedlings after they are established. For large dry onions, thin seedlings to two to three inches apart; for medium-sized onions, one to two inches; and for boilers and green onions, a half-inch to an inch. The key to getting good seed establishment is to keep soil moist so it doesn't form a hard crust.

Onions can be grown in almost any type of soil as long as it has good fertility, drainage and tilth. Onions respond to both compost and commercial fertilizers.

A handful of complete fertilizer, such as 16-16-16, applied along the row at planting time will get the plants off to a good start. A good compost or organic fertilizer will also supply the needed nutrients for onions.

For more information, see "Grow Your Own Beets, Carrots, Radishes, Onions, and Similar Crops," EC 1231.

Author: Carol Savonen
Source: Deborah Kean, Jim Myers