Onion sets can be planted to produce a quick crop of green onions, but they will not produce a good crop of mild flavored bulbs. The onions sold as sets are usually pungent varieties and they often go to seed before they produce a bulb in the San Diego area.
The best way to grow a crop of mild flavored onions in southern California, is by planting seed of short or intermediate day length varieties at the correct time. Short day varieties like Grano and Granex, which require 12 hours of daylight to form a bulb, should be planted from early to mid November. Intermediate day length varieties like Sweet Spanish and Italian Red, which require 14 hours of daylight to form a bulb, should be planted in early February. Mature bulbs will be ready to harvest in mid to late summer.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension,