Onions Growing Guide
Onions are a popular, easy-to-grow vegetable known for taking up relatively little space and providing reliable harvests. They are versatile in cooking and can be stored for extended periods, preventing sudden gluts. Onions are typically grown from small sets planted outdoors in autumn or spring, or from seed, though this requires more care.
This guidance applies to the crop in general. Individual varieties and local conditions can differ.
Planting information
Onions can be grown from seed, sown either indoors or outdoors. For indoor sowing: Sow in mid- to late winter (or late summer for overwintering varieties) into modular trays filled with peat-free seed compost. Sow 2 seeds per module for single plants, or 5-6 seeds per module if growing clumps of 3-4 plants (thin later if necessary). Place trays in a heated propagator or on a warm, sunny windowsill at 10-16°C. Seedlings typically appear within a couple of weeks. Once emerged, remove from propagator, place in bright light, and keep compost moist. Harden off indoor-sown plants in spring (or early autumn for overwintering onions) before transplanting. For outdoor sowing: Sow from late winter until mid-spring, once soil is drying and warming. Sow seeds 1.3cm deep, in rows 20cm apart. Thin seedlings first to 5cm apart, then later to 10cm apart. Closer spacing results in more but smaller individual bulbs. Onion sets are usually planted in spring (mid-March to mid-April), with some varieties available for autumn planting (overwintering/Japanese onions). Plant sets 2cm deep in drills or gently push into loose soil so the tip is just showing. Space sets/plants 5-10cm apart, in rows 25-30cm apart. Firm the soil around them and water well. Cover with fleece to protect newly planted sets from birds until rooted. Alternatively, plant through slits in black weed-suppressing membrane to eliminate weeding. Onions can also be grown in large containers (at least 45cm wide and deep) using peat-free soil-based compost. If soil is very wet/cold, plant onion sets indoors in modular trays (one set per module) and keep in an unheated greenhouse or on a sunny windowsill, then transplant outdoors when conditions improve.
Harvesting information
Autumn-planted onions are ready to harvest by early to mid-summer, while spring-planted sets and seed-raised onions are ready from late summer to early autumn. A key sign of maturity is the yellowing and toppling of foliage. Harvest before the foliage dies down completely. Carefully lift the bulbs with a fork, being careful not to damage or bruise them, as this can lead to rot in storage. Use any damaged onions immediately. Bending foliage or gently lifting bulbs to break roots is no longer recommended.
Storage information
Dry harvested onions for about two weeks before storing. Place them in full sun outdoors on wire racks or similar structures to allow air circulation. If the weather is damp, dry them in a sunny greenhouse or a well-ventilated shed. Once all foliage is papery and dry, store onions in net bags, trays (in a single layer), or tied into plaits and hung up. Keep them in a light, cool, dry, and well-ventilated place; avoid dark storage as it encourages sprouting. Autumn-planted onions typically store until early winter, while spring-planted or seed-raised onions can last until well into the following spring.