Onions are one of the most widely grown vegetables, forming the backbone of countless dishes. They’re hardy, relatively easy to grow, and store well for use throughout the year. There are two main ways to grow onions in the UK – from seed or from sets – and each has its benefits.
When to Sow
- From Seed: Sow indoors January–March, or outdoors March–April. Growing from seed offers more variety and can reduce the risk of bolting.
- From Sets: Plant sets March–April for maincrop onions, or choose autumn sets in September–October for overwintering. Sets are the easiest way to start onions, with fewer issues around germination.
How to Grow
- Transplant or plant 10cm apart in rows 30cm apart.
- Firm sets lightly into the soil with tips showing, while seedlings should be planted just below the surface.
- Keep the bed weed-free, as onions dislike competition.
- Water during dry spells, but avoid waterlogging.
Harvesting
- Harvest from July–September once the foliage turns yellow and flops over.
- Lift bulbs carefully and dry thoroughly in the sun or a well-ventilated space before storing.
- Well-cured onions can last for months in storage.
Polytunnel Growing Tips
- A polytunnel allows earlier sowings from seed with better germination rates.
- Protects overwintering sets from frost and rot.
- Creates opportunities for earlier harvests, particularly from autumn-planted sets.
- Keep the tunnel ventilated to reduce humidity and fungal risk.
Quick Tips Recap
- Sow/Plant: Jan–Mar (seed indoors), Mar–Apr (seed or sets), Sep–Oct (autumn sets)
- Spacing: 10cm apart, 30cm between rows
- Harvest: Jul–Sep
- Polytunnel advantage: Earlier starts, better germination, frost protection, earlier harvests
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