Bulb-onions are cool-season, frost-tolerant crop, growing best at 13-24 Celsius degrees (55-75 F). They grow well in an open site, in fertile, light and well-drained soil. Prepare the planting area in autumn by digging in a lot of well-rotted manure.
Usually the onions are grown from sets, but they can also be grown from seeds. Growing them from seeds is cheaper but slower to develop. Sets are easier to grow but you might find only certain cultivars. Either way, bulb onions require a long growing period, especially if they are grown for storage.
Sets are usually planted in early spring, at the same distances as you would keep the seedlings after thinning. Plant them in shallow furrows so that the tips protrude just above the soil. There are also autumn planting sets available.
Sow the onions in spring, when the soil is workable. Choose a firm seedbed, sow seeds very thinly 1 cm deep, in rows of 23-30 cm apart. Thin out the seedlings in stages and use them in the kitchen. For medium size onions keep a distance between them of about 4 cm and for larger onions thin seedlings to 5-10 cm apart. In colder area you can start seeds under cover starting from late winter to early spring. Sow them in seed trays or modules at 10-15 Celsius degrees (50-59 F) and harden them off when the seedlings are at the two-leaf stage and then plant them out. Also there are some cultivars that may be sown in summer or autumn to overwinter for earlier crops the next year.
Keep their beds weed-free. Having fairly shallow-roots they only need little water once established, but they will need some watering in very dry conditions.
Spring-sown bulb onions will need 12-18 weeks to mature and summer-sown onions up to 42 weeks. Consume them as needed to use fresh. For storage you should wait until the leaves have died back naturally and then uproot all the onions. Leave them in the sun to dry for about 10 days or if the weather is wet, hung them in nets to allow maximum ventilation and place them in a greenhouse. Before putting them to storage, make sure the outer skin and leaves are completely dry. Keep them at 0-7 Celsius degrees (32-45 F) and in low humidity. Never store bull-necked onions, they are the ones that need to be consumed first.