CORVALLIS, Ore. – Get onions in the ground in spring and avoid heartbreak when it comes time to harvest big, beautiful bulbs this summer.
Plant as soon as the soil is dry enough to work, said Jim Myers, a plant breeder at Oregon State University. March and April are prime times.
Most onions grown in Oregon are long-day onions. They make top, green growth until a critical day length is reached, which triggers bulbing. That generally begins at about 14 hours of light per day.
If you plant onions in early spring, they’ll grow to fairly large plants by the time daylight reaches 14 hours. Large bulbs result. However, if you wait to plant until the end of April when days are already 14 hours long, bulbing will begin immediately and you’ll have small pearl onions.
"The size of the bulb is dependent on the size of the plant when bulbing begins," Myers said. "This is why early planting is critical if you want large onions, as well as plenty of water and fertilizer."
Buy onion sets while firm and dormant. Harvest early in the season to eat as green onions, or in late summer when they’ve formed bulbs. When purchasing transplants, look for plants that have not been damaged by decay or excess drying.
After getting your onion sets home, sort into two groups by size – those smaller than a dime and those larger. Use the larger size for green onions. The smaller ones will produce large dry bulbs.
Onions can be grown in almost any type of soil as long as it has good fertility and drainage. Before planting, amend soil with compost or aged manure. A handful of complete fertilizer, such as 10-10-10, 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 green onions, plant the sets 1½ to 2 inches deep and 1 inch apart. They’ll be ready for eating in about a month when the tops reach 8 to 10 inches tall. For larger onions, plant the sets about ½ inch deep and 4 inches apart. Plant transplants 3 to 4 inches apart in rows 12 to 15 inches apart.
Fertilize every two to three weeks until onions start to form bulbs and make sure to keep soil consistently moist.
"Onions are shallow rooted," Myers said. "If allowed to dry out, they bulb early and small size is the result."
When tops begin to dry out and are falling over, stop watering so the bulbs mature in dry soil. After about half the tops have fallen over, wait about a week and harvest the bulbs. Cure them for a week or so by spreading in the sun and covering with a sheet or tarp at night.
The OSU Extension Service recommends the following varieties as performing well in Oregon:
About the OSU Extension Service: The Oregon State University Extension Service shares research-based knowledge with people and communities in Oregon’s 36 counties. OSU Extension addresses issues that matter to urban and rural Oregonians. OSU Extension’s partnerships and programs contribute to a healthy, prosperous and sustainable future for Oregon.