Direct sow onions outdoors in spring on any firm, well drained soil in full sun. Sow thinly at a depth of 1cm (½”) and a distance of 30cm (12″) between rows. Prepare the planting area several weeks in advance by incorporating some well rotted garden compost to improve drainage and soil fertility. Avoid planting onions on freshly manured soil. On particularly wet ground, try growing onions in raised beds. When large enough to handle, thin onion seedlings to 10cm (4″) apart for medium sized bulbs, or at a slightly wider spacing if larger bulbs are required.
Alternatively sow onion seed indoors in module trays at a depth of 1cm (½”), using free-draining, seed sowing compost. Place in a propagator or seal container inside a plastic bag at a temperature of 10-15C (50-59F) until after germination which usually takes 10-14 days. Once germinated, grow onions on in cool conditions until all risk of frost has passed and they are large enough to plant outdoors at a distance of 10cm (4″) apart. Cover with a protective netting or fleece to prevent attack from birds and insects. Water the plants thoroughly after planting.
Onions are undemanding and require only occasional watering during particularly dry periods. However, rows should be weeded regularly. Harvest onions from July onwards, a week or two after the leaves begin to turn yellow. Choose a dry day to loosen them from the ground with a fork. Leave them on the soil surface for a day or two until they have fully dried in the sun. Once dry, remove the top foliage and store them in a well ventilated, dry position. Bulbs can be stored for several months.
March, April, May