Seed

Growing bulb onions from seed gives you a far greater choice of variety, but seed must be sown as early as possible in spring. If the ground is wet and cold you have to delay sowing, unless you can sow directly under polythene, or use modules under cover for later transplanting after hardening off.

Sets

Sets are small bulbs that have been grown and harvested early in the previous year ready for planting in late spring when the soil conditions should be good. Their shorter growing season and reduced susceptibility to dry conditions, weeds, pests and diseases make them popular with many kitchen gardeners as a reliable way of getting a good crop with little effort.

Bolting

Many varieties of onion are prone to bolting (ie producing a flower stem) if their growth is checked by cold weather. The plant registers the cold snap as winter and moves prematurely into its second phase of growth, which is to flower and die. Seed-sown onions are more vulnerable than sets because of their longer growing period, but late cold snaps can still affect sets. Heat-treated sets are much less prone to bolting and are supplied at the correct planting time (late March or April).

Winter hardy

Until relatively recently, overwintering bulb onions could only be grown from seed, but now sets are available from some suppliers such as Marshalls and Mr Fothergill's, which dispatch in autumn.