When to plant onion sets

Two seasons for onion sets

Onion sets are small, immature onion bulbs. You plant them and they grow on into full size onions. The RHS describes two kinds, and each has its own planting time.

Overwintering sets, often called Japanese onion sets, are planted in autumn. The RHS School Gardening crop sheet gives September to November. They sit through the winter and are ready early, in June and July, according to the RHS.

Spring sets are the ordinary kind. The RHS plants them in early to mid spring, for harvesting from July to September. BBC Gardeners' World says to plant them in March or April for a quick crop. The RHS School Gardening sheet starts earlier still, from January to early April.

So in November you are at the tail end of the autumn window. If you have overwintering sets, plant them now. If you missed it, spring sets are only a few months away.

Autumn sets or spring sets?

Autumn sets give you onions early in summer, when stored onions from last year are running out. The trade off is storage. The RHS says autumn planted onions keep only until early winter, while spring planted or seed raised onions can last until well into the following spring.

Many growers do both: autumn sets to fill the early summer gap and spring sets for onions to store through winter.

The RHS lists 'Marshalls Red Fen' among varieties whose sets can be planted in autumn. It also recommends several Award of Garden Merit varieties, including 'Centurion', 'Sturon', 'Red Baron', 'Rumba' and 'Hercules', which it says has good bolting resistance.

How deep and how far apart

Plant onion sets about 2 cm deep, so that the tip is just showing at the surface, the RHS says. BBC Gardeners' World also plants them with the tip protruding. The RHS School Gardening sheet makes a shallow drill about 2 cm deep and pushes each set in with the roots facing down.

Space them 10–15 cm apart. The RHS uses rows 20–30 cm apart and notes that wider spacing gives larger bulbs. The RHS School Gardening sheet leaves more room between rows, 30–40 cm.

The RHS firms the soil around the sets and waters them well. BBC Gardeners' World waters thoroughly after planting and then at regular intervals in dry weather.

Soil and site

Onions want sun and good drainage. The RHS advises a sunny site with free-draining soil, weeded thoroughly before planting. Dig in about a bucketful of homemade garden compost or well-rotted manure per square metre, but never fresh manure.

The RHS says the organic matter you add before planting should provide enough nutrients. On poor soil, it gives autumn planted onions an organic, nitrogen rich fertiliser in late winter.

No bed to spare? The RHS grows onions in containers at least 45 cm wide and deep, filled with peat-free soil-based compost.

Birds and bolting: the two classic problems

The RHS warns that birds will often peck newly planted sets out of the ground. Cover the row with biodegradable fleece or netting until the sets have rooted in. BBC Gardeners' World does the same with horticultural fleece and removes it once the sets are firmly rooted.

Bolting means the onion sends up a flower stem. The RHS says sets are more prone to it than seed raised onions, that low spring temperatures are the trigger, and that bolted bulbs will not store well. Using heat treated sets greatly reduces the risk. If a flower stem appears, the RHS says to remove it as soon as it forms.

The RHS lists allium leaf miner, leek moth, onion fly, slugs and snails, leek rust, onion downy mildew, onion neck rot and onion white rot among the common problems. If you ever reach for a treatment, use only products authorised in the UK, used as the label says.

Looking after the crop

Weeding. The RHS points out that onion foliage casts little shade, so weeds grow freely around them and can reduce bulb size. Keep the row clean.

Watering. Water during long dry spells, especially early on, the RHS says. Stop once the bulbs have swollen in mid summer, because watering after that can shorten their storage life. Avoid wetting the leaves.

Day length. The RHS explains that lengthening days trigger bulb formation. The plant needs plenty of leaves by late spring to make a good bulb, which is another reason to get the sets in on time.

Planting garlic this autumn too? See when to plant garlic.

When are onions ready to lift?

Autumn sets are ready in June and July. Spring sets and seed raised onions follow from July to October, the RHS says. The RHS School Gardening sheet says autumn sets are usually ready around June and spring sets from August.

The sign is in the leaves. The RHS looks for yellowing and toppling foliage and lifts the onions with a fork before the leaves have died down completely. BBC Gardeners' World draws the soil away from the bulbs in late summer to let the sun reach them, then lifts them after the leaves turn yellow and the stem bends over.

Drying and storing

Dry the bulbs for about two weeks in full sun on wire racks, the RHS says, or in a greenhouse or shed if the weather is damp. Then store them in a single layer in net bags or trays, or plait them.

Keep them somewhere light, cool, dry and well ventilated. The RHS warns against storing onions in the dark, as darkness encourages sprouting. BBC Gardeners' World stores only bulbs with narrow, well closed necks and uses any with wide necks first, because they do not keep well.