When to Plant Onion Sets in England

Onions are a reliable, easy-to-grow crop in the UK and suit most garden spaces, from large vegetable beds to compact raised beds and containers. They grow best in fertile, free-draining soil in full sun. Whether grown from seed or sets, onions need consistent care in the early stages but are relatively low-maintenance as they mature. Good soil preparation and choosing the right planting time make all the difference.

How Long Do Onions Take to Grow?

Onions typically take between 20 to 30 weeks to reach maturity, depending on the variety and growing method. Sets planted in spring are usually ready by mid to late summer, while autumn-planted sets are harvested the following late spring to early summer. Seed-grown onions take longer, often needing to be started indoors or under cover from January or February to ensure a full growing season.

How to Sow Onion Seed

Sowing onions from seed requires more time and patience than planting sets, but it allows for more variety and can be more cost-effective. Start seeds indoors or under cover between January and March. Sow in trays or modules using a fine seed compost, keeping them in a bright, cool spot. Once seedlings are established and strong enough to handle, they can be hardened off and planted out from April onwards. Consistent moisture and weed control are key during early growth.

What Are Onion Sets?

Onion sets are small, immature onions grown the previous year and dried for replanting. They’re a popular choice in UK gardens because they establish quickly, have a higher success rate than seed, and are less vulnerable to early pests. Sets also reduce the growing time significantly and are perfect for gardeners who want to grow onions without the need for a heated space or long indoor propagation.

How to Plant Onion Sets

Plant onion sets by gently pressing them into well-prepared soil so just the tip is showing above the surface. Space them about 10cm apart in rows 25–30cm apart. Water them in after planting, and avoid overly wet conditions that could cause rot. In heavy soils, adding grit or planting in raised beds helps improve drainage. Birds sometimes pull up freshly planted sets, so fleece or mesh protection may be needed until they root.

When to Plant Onion Sets

In England, onion sets are typically planted in two main windows: spring and autumn. Spring planting happens from mid-March through to April once the soil has begun to warm. These onions are harvested in July and August. Autumn planting (mainly using overwintering varieties) takes place from September through early November. These overwintering onions establish before the cold sets in, then resume growth in early spring and are harvested earlier in the season, usually from late May to early July. Choose the right variety for your timing — not all onions are suited to autumn sowing.

How to Care for Onion Crops

Onions need consistent watering, especially during dry periods in spring and early summer, but avoid overwatering. They dislike sitting in wet soil. Weeding is important, as onions don’t compete well with weeds. As the bulbs swell, stop watering to help the skins form and prepare the crop for storage. Onions don’t usually need feeding in rich soil, but a light general fertiliser in early growth can help boost development. Avoid nitrogen-heavy feeds late in the season, as these encourage soft growth and reduce storage life.

Common Issues and How to Resolve Them

The most common problems with onions in England are poor drainage, birds pulling sets, bolting, and fungal diseases like downy mildew or white rot. Improve drainage by raising beds or incorporating organic matter. To prevent bolting (flowering early), plant at the right time and choose bolt-resistant varieties. If birds are pulling up sets, cover the bed with netting for the first few weeks. Rotting issues are harder to fix once present, so rotate crops and avoid planting onions in the same spot more than once every three years. Inspect plants regularly for signs of disease and remove any affected bulbs promptly.

Timing Should Match the Variety

Not all onion sets are suited to both spring and autumn planting. Spring sets are often fast-maturing and bred to handle a short, intense growing season. Autumn sets are typically hardier, bred to withstand cold, and selected for their ability to overwinter without bolting. Trying to plant spring varieties in autumn often leads to poor growth or early flowering, which ruins the crop. When buying sets, always check the label — it will usually state whether they’re intended for autumn or spring planting.

Soil Temperature Makes a Difference

In spring, don’t be tempted to plant too early into cold, wet soil. Onion sets need the soil temperature to reach at least 7–10°C to establish well. If planted too early, growth can stall or sets may rot. In very wet areas or clay soils, waiting until late March or early April can give better results than rushing into soggy beds in February.

Autumn Sets Give You a Head Start — But With Some Risk

Autumn planting gives you onions earlier in the year — often ready by late May or June — and can help spread your harvest window. But there’s a trade-off. Very cold winters or prolonged waterlogging can damage or kill overwintered sets. To reduce the risk, plant in raised beds or lighter soil and choose known hardy varieties like ‘Radar’ or ‘Electric’. In mild winters, these sets will thrive. In colder ones, some loss should be expected.

Light Levels and Spacing Matter

Onions are light feeders but heavy light users. They need full sun to grow strong, healthy bulbs. Shade or overcrowding can lead to small, underdeveloped onions. Spacing is also important — if planted too close together, bulbs compete for space and moisture, reducing size and increasing the chance of disease. Stick to at least 10cm between sets for good airflow and growth.

Succession Planting Isn’t Common — But Can Work

Although not typically done with onions, succession planting can extend the harvest. You could stagger spring planting over a few weeks or combine spring and autumn plantings with different varieties to cover more of the growing season. This works well in larger beds or kitchen gardens where consistent harvest timing matters.