How to grow onions

Video by GrowVeg. This guide draws on “How to Grow Onions (UK) From Sets to Harvest” from the GrowVeg YouTube channel. The GrowVeg team — one of the most-watched UK vegetable gardening resources on YouTube — cover everything from choosing sets to storing the finished crop, with specific advice for UK growing conditions.

1. Choose sets rather than seed for easier growing

Onion seeds work, but they require starting indoors in January and involve a fair bit of faffing with pricking out. Onion sets — small, partially-grown bulblets — bypass all of that. You push them into the ground in spring and a bulb develops over the following months. Much more straightforward, and the results are just as good.

Popular UK varieties from sets include ‘Sturon’ (reliable, good for storing), ‘Red Baron’ (the red-skinned type, slightly milder), and ‘Centurion’ (large bulbs, bolt-resistant). Heat-treated sets are worth buying — they have been treated to reduce bolting, which is when the plant sends up a flower stalk instead of forming a bulb.

2. Prepare a firm, weed-free bed in a sunny spot

Onions like a firm, fine-textured seedbed in full sun. They do not need particularly rich soil — too much nitrogen produces lush foliage at the expense of the bulb. If you have added compost or manure to the bed recently for another crop, the residual fertility is usually enough.

Rake the bed level and firm it gently by shuffling over it in your boots. A well-firmed bed helps onions anchor properly. Loose, fluffy soil leads to sets that sit on top or get dislodged by birds — which is a common annoyance. Robins in particular seem to find freshly planted sets irresistible.

3. Plant sets in March or April

The main UK planting time for onion sets is March to April, once the soil has begun to warm up. Plant each set with the pointed tip just showing above the surface, about 10 cm apart in rows 25–30 cm apart. The tip should be visible — burying them too deep is a common mistake that can cause rotting.

After planting, birds may pull them out — they seem to mistake the papery tips for something worth investigating. Check after the first few days and push any dislodged sets back in. Once the sets have developed roots and are anchored in the soil, the problem usually stops on its own.

4. Keep the bed weed-free throughout the growing season

Onions are poor competitors with weeds. A weedy bed early in the season can significantly reduce the final bulb size. Hoe between the rows every week or two, being careful not to damage the shallow roots. Hand-weed close to the plants where the hoe cannot reach without cutting into the bulb.

Mulching between rows with fine compost can help suppress weeds without covering the developing bulbs. Avoid mulching too close to the bulb itself — onions need the tops of the bulbs exposed to the sun and air as they ripen, and smothering them with material causes problems.

5. Water in dry spells, but stop as the season ends

Onions need regular moisture through spring and early summer while the bulbs are forming. In a dry spring — not uncommon in east Kent — water once a week with a good soak. This is more important than many gardeners realise; dry conditions during bulb development produce smaller onions with thicker necks that do not store as well.

Once the foliage starts to turn yellow and flop over in late summer, stop watering entirely. The bulbs need to dry and ripen properly in the soil. Continuing to water at this stage encourages soft necks and rots in storage, which is deeply disappointing after months of careful growing.

6. Harvest when the tops have fallen over

The traditional sign that onions are ready is when the green tops collapse and begin to dry. This usually happens from July into August depending on the variety and the season. Do not rush it — the longer the bulbs stay in the ground to ripen, the longer they will store.

Once the foliage has browned off, ease the bulbs out with a fork on a dry day. Lay them on the surface in the sunshine for a few hours to begin the curing process, then move them somewhere airy and sheltered — a greenhouse staging, an open shed, or a warm porch all work well. Leave them to dry for three to four weeks before storing.

7. Dry and store for months of use

Properly dried onions store remarkably well. Once the outer skins are papery and the necks are completely dry and withered, you can trim the roots, remove the outer loose skin, and store them in a net bag, a slatted tray, or plaited into ropes if you are feeling traditional.

Keep them somewhere cool, dry, and with reasonable airflow — not a damp garage or a warm kitchen. Check regularly and remove any that show signs of soft rot before they spread to the others. Good varieties stored correctly should last four to six months. That is a lot of cooking from one modest bed.

When to call a handyman

Onions are one of the simplest vegetables to grow from scratch. If the garden needs preparing first — a new bed digging out, an old overgrown patch clearing, or a raised bed building for better drainage — Richard can help with the groundwork before you get planting.