Sow
Although usually grown from sets, onions can be grown from seed, sown either indoors or outside. Seed is cheaper to buy, but slower to grow and the seedlings need more careful attention, however seed-grown plants can be less susceptible to bolting (flowering). To ensure a good crop, seed-raised plants must be growing strongly by late spring, as the lengthening days trigger the formation of bulbs – the more leaves plants have at this time, the better the bulb will be.
Sowing Indoors
Sow onion seeds in modules in mid- to late winter and keep in a greenhouse at 10–16°C (50–60°F).
Although one plant per module is effective, growing three to four per module saves space. Sow five or six seeds per module, then thin out if necessary to three or four plants. Harden off indoor-sown plants in spring, before transplanting into the ground. When multi-seeded modules are planted out, the onions form a clump of bulbs.
Sowing outdoors
You can sow onions direct outdoors from late winter until mid-spring, once your soil is drying out and beginning to warm up. Sow seeds 1.3cm (½in) deep in rows 20cm (8in) apart. Thin out seedlings first to 5cm (2in) apart, and later to 10cm (4in). Closer planting will result in more bulbs and a larger overall crop, but smaller individual bulbs.
