Onions are planted in the garden from seeds, transplants or sets. Onion sets are actually immature bulbs that grow into spring onions or bulbs. Spring onions are the foliage of the onion sets, while bulbs are formed once the sets mature. Spring sets are grown mostly for spring onions, as warmer weather causes the onion plant to "bolt" and flower before the bulb is mature. To get mature bulbs from sets, plant them in fall so the bulbs have time to mature before bolting.
Prepare a well-drained garden bed in full sun in early fall, approximately 12 weeks before the typical first frost date in your area. Lay a 2-inch layer of compost over the bed. Till in the compost to a 10-inch depth to improve drainage.
Plant winter onion sets 1 1/2 inches deep in the soil. Space each set three inches apart to give the bulbs room to develop.
Water the bed thoroughly after planting so that the soil compacts slightly and there are no air pockets. Keep the soil evenly moist at all times, but not soggy.
Allow the tops to yellow and fall over naturally, before or just after the first fall frost. Cut off the tops to within 1 inch of the soil surface.
Pull up each onion and brush off the excess soil. Spread the onions out in a single layer in a dry, shaded area to cure for one week.
Store the onions in a cool, well-ventilated room at about 7.22 degrees Cor three or more months. Check stored onions regularly for soft spots; immediately use any that have developed soft spots.
Plant intermediate or long-day varieties of onions if you require a longer storage life. Plant short-day varieties in areas with less autumn sunlight. If growing sets for spring onions, space the bulbs only one to two inches apart. An application of balanced fertiliser before planting is beneficial.
Only use fully mature compost. Remove any dead plants from the onion bed to prevent onion maggots from being drawn to the onion bed. Winter sets are often smaller than spring-planted onion sets.