Onion from seed

Allium cepa

Onions from seed keep through winter. They cost less than sets (the small bulbs you plant instead) and come in more varieties. They are also less likely to bolt, or flower too early. Sow in early spring, indoors or in the row. Thin the seedlings in two steps so each bulb has room.

Looking after it

Water: Onions need normal watering. Keep the seedlings moist. Later on, water only in long dry spells. Stop once the bulbs have swollen in midsummer, so they keep longer. Feeding: Average feeding is fine. Compost dug in before sowing is usually enough. Extra nitrogen late on gives soft bulbs that keep badly.

Harvest and keeping

From July to October the leaves turn yellow and fall over. Then lift the bulbs, before the leaves die off completely. Dry them in the sun for about two weeks. Onions from seed keep until the next spring. Store them in nets or slatted trays somewhere cool, dry, airy and light. In the dark they sprout.

What can go wrong

Downy mildew. In cool, wet weather the leaves get pale patches with a grey-purple mould, then collapse. Remove the affected leaves and give the plants room. Grow your onions in a different bed next year.
Onion fly. Seedlings turn yellow, wilt and die because maggots are eating the roots and bulb. Dig up the affected plants and destroy them. Cover the crop with fine insect mesh.
Weeds. Onion seedlings are thin and slow to grow, so weeds soon smother them. Weed by hand often while they are small.

Growing facts: RHS, UC ANR and seed-company sowing guides. Onion care: RHS. How many to grow: Kansas State extension. pH: University of Connecticut extension.