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Onions: seed or set?

Traditions surrounding prize onions

world record onion grown by Tony Glover
The world record onion grown by Tony Glover, at 18lb 11.5oz, at Harrogate Autumn Show 2014. Picture; Sue Welford

There’s a lot of folklore surrounding sowing onion seeds, the most popular one being that the biggest bulbs are grown from those sown on Boxing Day – or even Christmas Day.

Of course, there’s a lot of truth in this, especially if you want onions for the exhibition bench in September – they need all that time to grow so big.

You can use this method for alliums that will end up in your kitchen. Bear in mind, though, this is for onions from seed only – sets are started off in spring.

If you really want to put your feet up, you can sow right through to February – but expect smaller bulbs. For best results, choose varieties such as Wellington, Red Sunrise, Kelsae, Robinson’s Mammoth, Rose de Roscoff or Red Pearl.

To sow, fill a seed tray or cell insert with good-quality seed compost and tamp down the surface, then water in. (Watering now means you won’t disturb the spacing of the seeds.) With cell trays, sow four or five per compartment, sow thinly in a standard tray, then lightly cover with 5mm (¼”) more compost.

Cover the tray with a propagator lid, or sheet of glass/polythene.

The ideal germination temperature is 15-20°C (60-68°F), but it should not be lower than 15°C (60°F).

After germination, remove the cover and move to an area with plenty of light. Keep the compost damp.

Maintain a temperature of about 18°C (65°F) until “crook stage” (when the seedling is still looped over like a shepherd’s crook, with the tip of the seed leaf still in the compost, usually two-three weeks after germination, just before the second leaf).

After this, place them somewhere cooler, reducing the temperature to 10°–13°C (50-55°F).

The seedlings need maximum light and ventilation until the pricking out stage but be careful of scorching from direct sunlight.

Transfer boxes and trays to a cold frame in March or April.


potted-guide-logoPotted guide: onions from seed

  • SOWING TIME: December-February under glass for large bulbs; March-April direct for smaller bulbs.
  • HARVESTING TIME: August/September.
  • PLANTING DISTANCE: 23cm-30cm (9″-1ft) apart.
  • ASPECT AND SOIL: Full sun, well-drained – a pH of 6.75-7.25 is ideal.
  • HARDINESS: Protect against frost, although perennial varieties are fully hardy.
  • DIFFICULTY: Growing exhibition-quality onions takes a great deal of skill. Beware of onion fly, stem and bulb eelworm, neck rot and white rot.

Onion sets: ideal for beginners

Onion Rumba sets
Onion Rumba sets, March 2013

After a few years of growing Hercules brown onions from sets, I tried Rumba in the raised beds. If you’ve never grown anything before, give one of the onion family a try.

They’re very easy and when cured and dried, keep through the winter (how they look in the supermarket, with papery skins).

The sets won’t be planted out until the soil warms up. Keep them in a cool, well-lit place, checking over for any mould or rot and they should be fine to plant out at the beginning of April.


St Thomas’s onions and planting folklore

The pagan midwinter festival was handily incorporated into the Christian church by creating the feast day of St Thomas on December 21.

Baby leeks and Welsh onion Ciboule
Baby leeks and Welsh onion Ciboule Red, October 2015

Various perennial onions are now called ‘St Thomas’s onion’, traditionally planted on the shortest day (December 21) and harvesting at the summer solstice (June 21). The name variously applies to the potato onion (Allium cepa); Egyptian onion (Allium cepa aggregatum) and the Welsh onion (A. fistulosum).

They’re all propagated by offsets, not seed, crop three months earlier than other varieties and are not attacked by onion fly.

I grow a heritage variety of the Welsh onion, Ciboule Red, which is handy, a bit like spring onions but stronger.


Autumn-planted onion sets

Keeping your garden productive during winter can be a challenge, but autumn-planted onion sets are easy to cultivate and give a good harvest, so why don’t more gardeners grow them?

It’s probably because many people shut up shop in October – that’s a shame because once planted, sets are easy to look after – leave them to it.

A set is an immature onion, easier to grow and less prone to disease, but more prone to bolting (setting seed if weather conditions are bad) – buy heat-treated varieties to prevent this.

If your soil is very heavy and prone to waterlogging, it’s best to plant onions in spring or grow in raised beds.

Choose an open, sunny site and well-drained, non-acidic soil (below pH6.5). If you have acid soil, apply lime in autumn and winter. Enrich soil with organic matter, but avoid using fresh manure.

When planting, apply 35g per sqm (1oz per sq yd) of Growmore or specialist onion fertiliser. If you don’t have any organic matter (compost, etc), double the amount.

Place each set with the growing tip peeking out – cover with netting if you have a problem with birds pulling them out of the ground. Firm in and water. Don’t plant if the ground is waterlogged or frozen.

Apply a nitrogen-rich fertiliser such as sulphate of ammonia or dry poultry manure at the rate of 35g per sqm (1oz per sq yd) in late winter to boost growth and suppress bolting. Plants must be growing strongly by late spring as the lengthening days trigger the formation of bulbs.

As the foliage casts little shade, onions can be overtaken by weeds. Don’t use a hoe, as this can damage the bulbs – you’ll have to hand weed, I’m afraid.

Water every 14 days during dry spells, but avoid overhead irrigation that encourages fungal diseases. Bulbs should be ready June-July.


potted-guide-logoPotted guide: autumn-planted onion sets

  • PLANTING TIME: September-November.
  • HARVESTING TIME: midsummer.
  • PLANTING DISTANCE: 5-10cm (2-4inches) apart.
  • ASPECT AND SOIL: Full sun, well-drained soil, avoid acid soils.
  • HARDINESS: Hardy.
  • DIFFICULTY: Easy.
  • RECOMMENDED VARIETIES: Red Cross, Senshyu, Shakespeare, Electric, T&M’s First Early, Tornado, Radar, Troy.