March and April are good times to plant onions in most areas in Oregon, according to Deborah Kean, Oregon State University horticulture researcher. Plant any of these as soon as the soil is dry enough to work. 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 daylength.
Most onions grown in Oregon are long-day onions.
Bulbing generally begins in long-day onions when there is about 14 hours of light per day, said Jim Myers, OSU vegetable breeder. 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 late spring, when days reach 14 hours, bulbing will begin immediately and small pearl onions will result, explained Myers. This is why early planting is critical, as well as plenty of water and fertilizer.