Onions (Allium cepa) are cool season biennials, members of the Amaryllidaceae family, which includes garlic, leeks, chives and scallions. Onions started from seed store better than sets, but mature later. Bulbing onions are dependent on day length for bulb production; short-day onions produce bulbs when they receive 11-12 hours of daylight, long-day onions need 14-16 hours (latitudes north of 35º), and moderate day onions like Walla Walla and Gladstone fall in between these.
Soil Nutrients and Requirements
Onions prefer soils rich in organic matter that are well-drained. Optimal pH is 6.2-6.8. They cannot tolerate acid soils, especially in early stages. 80 lbs/A nitrogen is recommended. Sidedress 4-5 weeks after planting. High levels of sulfur in the soil will increase pungency. Best results come from selecting a bed with the least weed pressure possible.
Medium size onions 3-4”, for large onions 4-6”, for sweet onions 4”
When to Sow
Days to maturity are from direct seeding, subtract 1-2 weeks if transplanting. Direct seed onions as soon as soil can be worked or start transplants 10-12 weeks before planting date. Sow thickly in flats or 1” cells, in singles, doubles or triples. When seedlings reach 5” trim to 1” to increase girth. Transplanting is recommended for short growing seasons and sweet onions. Optimal soil temperature for germination is 75-85°F.
Onions compete poorly with weeds. Cultivate regularly to control weed pressure.
For fresh eating, onions can be harvested whenever they reach desired size. Storage onions can be harvested when tops dry up and flop over. Pull bulbs from ground and cure for 3-5 days in the field or bring into barn or greenhouse and cure for two weeks at 75-80°F and 80% relative humidity.
Cool slowly, and store at steady temperatures. Rapid cooling followed by a sudden warm period might break dormancy and cause onions to sprout. Optimal storage is at near freezing temperatures at 65-70% relative humidity.