Onion in April: monthly care

Onions thrive in full sun and need well-drained soil to prevent bulb rot. Sandy soil and loam are ideal; heavy clay should be improved with grit and organic matter several weeks before planting. Choose an open site that hasn't grown onions, garlic, or leeks for at least three years to reduce the risk of disease. You can grow onions from sets (small immature bulbs) or from seed, though sets are far more reliable for most gardeners. Plant sets in March or April, once the soil has warmed slightly and is workable. Push each set gently into the soil so that the tip just shows above the surface, spacing them 10 cm apart in rows 25–30 cm apart. If birds pull them up in the first few days, replant immediately and consider covering the row with fleece or netting until the roots take hold. If sowing from seed, start indoors in modules in late winter or sow directly outdoors in March or April. Sow thinly, about 1 cm deep, and thin seedlings to 10 cm apart once they're large enough to handle. Transplant module-grown seedlings at the same spacing when they reach pencil thickness. After planting, water gently to settle the soil around the roots, but avoid waterlogging. Onions dislike competition, so keep the area weed-free from the start—their shallow roots make hoeing risky later on. A light mulch of well-rotted compost between rows helps suppress weeds, but keep it clear of the necks of the bulbs to prevent rot. Avoid planting in freshly manured ground, as this encourages lush foliage at the expense of bulb development.

Onions have a low water requirement once established, but consistent moisture is important during bulb formation in late spring and early summer. Water during dry spells, aiming for about 2–3 cm per week, but reduce watering as the foliage begins to yellow in July or August—wet conditions at this stage can encourage rot and reduce storage life. Avoid overhead watering if possible, as wet foliage increases the risk of downy mildew. Feed once in April with a balanced general-purpose fertiliser or a light dressing of blood, fish, and bone. Onions don't need heavy feeding—too much nitrogen produces soft, leafy growth and poor bulbs. If your soil is reasonably fertile, feeding may not be necessary at all. Weeding is the most important maintenance task. Onions have shallow roots and struggle to compete, so hand-weed carefully and regularly, especially in the first two months. Mulching lightly between rows helps, but keep mulch away from the bulb necks. Onion white rot is the most serious disease: it causes yellowing foliage and a white fungal growth on the bulb base. There is no cure, and the spores persist in soil for many years, so crop rotation is essential. Onion fly larvae tunnel into young bulbs, causing collapse; covering rows with insect mesh after planting offers good protection. Downy mildew appears as greyish fuzz on leaves in damp seasons—improve air circulation and avoid overhead watering. Onions are fully hardy and need no winter protection. Harvest in July, August, or September when the tops collapse, then cure and store in a cool, dry, frost-free place.