How to Sow and Grow Onions: A Complete Guide
Onions can be grown from seed, sets or young transplants. Seed offers the widest choice and lowest disease risk; sets are simple and quick; transplants shorten the season. Whichever method you choose, onions need an open sunny position, firm soil and little competition from weeds.
Choosing onions
Bulb formation is controlled by day length, so choose short-, intermediate- or long-day varieties suited to your latitude. Storage onions have firm skins and keep for months when cured; sweet onions are best used sooner. Red, white and yellow varieties share the same basic care.
Growing onions from seed
Sow indoors in late winter to early spring, 1 cm deep in modules or trays of peat-free seed compost. Keep at 15–20°C until germination, then provide bright light. Thin crowded seedlings and keep compost just moist. Harden off before planting when seedlings are sturdy and the soil workable.
Direct sow from early spring into drills 1 cm deep. Sow thinly, rows 25–30 cm apart, then thin in stages to 8–10 cm for full bulbs. Thinnings can be used as spring onions.
Planting onion sets
Plant sets in spring with the pointed tip just showing and the base 10 cm apart, rows 25–30 cm apart. Planting too shallowly invites birds to pull them out; firm the soil around each set. In mild areas, specially bred autumn sets can overwinter, but wet ground increases losses.
Soil and feeding
Choose fertile, well-drained soil in full sun. Add mature compost well before planting but avoid fresh manure, which can encourage soft growth and poor storage. A balanced organic fertiliser can be incorporated on poor ground. Stop nitrogen-rich feeding once bulbs begin to swell.
Watering and weeding
Onions have shallow roots. Water during prolonged dry periods, especially while bulbs expand, but reduce watering as foliage begins to yellow. Keep beds weed-free by hand or with shallow hoeing; deep cultivation damages bulbs and roots. A light mulch helps, provided it does not hold wet material against the necks.
Bolting and other problems
Bolting: temperature stress or oversized sets can trigger flower stems. Remove bolting onions for prompt use because they will not store well.
Onion fly: use fine insect-proof mesh and rotate alliums; remove badly damaged plants.
Allium leaf miner: cover during local adult flight periods and do not compost infested material.
Downy mildew: improve spacing and airflow, avoid overhead watering and rotate crops.
White rot: persistent soil-borne disease; remove plants and soil around roots and avoid growing alliums in that area for many years.
Harvesting and curing
Harvest when most tops naturally fall and begin to yellow. Lift bulbs carefully on a dry day. Cure in a warm, airy, rain-free place for two to three weeks until skins are papery and necks dry. Do not bend healthy tops over to force maturity; this can damage tissues.
Storing onions
Trim dry tops or plait them, then store sound bulbs in nets, trays or slatted boxes in a cool, dry, ventilated place. Check monthly and remove soft or sprouting bulbs. Thick-necked, damaged and bolted onions should be used first.
