Step-by-Step Growing Guide
Onions are triggered to form bulbs by day length — the single most important factor in variety selection. Long-day varieties (14–16 hours) suit latitudes above 40°N; short-day varieties (10–12 hours) suit latitudes below 35°N; intermediate-day varieties bridge the middle. Plant in fertile, well-drained soil at pH 6.0–7.0. Sow seeds 6mm deep indoors 10–12 weeks before last frost or plant sets (small bulbs) 2.5cm deep and 10–15cm apart in rows 30–40cm apart. Sets are easier but have more bolting risk than transplants. Water consistently — 2.5–4cm per week — but reduce watering significantly as bulbs reach near-harvest size. Feed every 2–3 weeks with a nitrogen fertiliser (21-0-0) from transplanting until mid-bulbing, then switch to a potassium-rich feed (0-0-50) to firm up bulbs. Stop all feeding 4 weeks before expected harvest. Keep weeds scrupulously cleared — onions have shallow roots and poor competition ability.
