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May 11, 2010 / Malcolm Dalebö

Growing Onions

Onions are one of the most popular vegetables for growing on allotments and are one of the first crops of spring. They can be stored over winter thus making a versatile crop.
Types of Onions
Onions should be treated differently according to their type. Each of the three types is described below:

  • Maincrop onion seed and sets (small, part grown onion bulbs) should be sown outside starting March. If they are sown under cloche protection, they can be sown four weeks earlier.
  • Japanese onion seed should be sown outside in mid-August.
  • Scallions (spring or salad) onion seed should be sown at three week intervals from early April to early June.
  • Onions will grow in most climates and are frost resistant.
  • If adding manure or composted organic matter then add a few weeks before sowing / planting out.
  • You can tread the soil gently to firm it up a bit prior to sowing.
  • Onions will grow in almost any soil from sandy loams to heavy clay. The soil should be firm. 
  • If your soil is heavy then you can introduce some organic compost or manure into the soil to help its moisture retaining properties.
  • Onions prefer a slightly acidic soil – PH 5.5-6.5 is a good PH for growing onions.

Onions can be planted from seed or from sets (small partly grown onion bulbs). Sets are more expensive but they tend to be more reliable in their results and also require less work – no thinning and reduced onion fly risk.

  • If sowing from seed then sow in drills about 2cm deep with about 1 inch between seeds. 
  • If sowing in rows then space the rows about 30cm apart. 
  • The soil should be moist before sowing so check the soil the day before sowing and water if the soil is dry.
  • If planting onion sets then they can be planted around Mid to Late March (earlier if a cloche / polytunnel is used). Again space rows about 30cm apart. Sow sets around 10cm apart as they shouldn’t require any thinning. Dig a small hole for each set and place the set in neck upwards. When covered back up with soil the tip of the neck should just show through the soil surface.
  • Spring onions (scallion) can be sown from April and planting should be staggered every few weeks to ensure a continuous crop throughout the growing season.
  • Frequently weed between the onions by shallow hoeing, onions do not trap much incoming light due to their sparse leaf forms so weeds can take full advantage of the availalle light.
  • If your crop has been sown from seed then you will need to thin the onions when they reach about 5 cm in height. Thin them so that they are spaced about 10cm apart.

  • Onions are ready to harvest a week after their tops have started to fall over and are yellowed.
  • Use a fork to lift the onions out of the ground. Take care not to damage the skins as this invites decay organisms in to attack the onion flesh.
  • Onions should be harvested on a sunny day, cleaned of any soil still attached to them and then placed on top of the soil where they will dry out with the help of the sun and wind. Leave the onions out for a few days (until the tops dry out). 
  • Remove the tops with a sharp knife about 2cm above the onion top so that decay organisms do not have direct access to the onion bulb. 
  • Discard any onions that show signs of decay or damage as these can affect healthy onions if they are stored .

  • Leeks and onions are occasionally attacked by the Onion Fly, which is particularly troublesome in the larval stage, when the maggots tunnel into the plant tissue.  
  • The Onion Fly, which looks like a small grey house fly, lays its eggs in the spring and early summer, near the base of the leek or onion, on the leaves and in the soil nearby. The maggots are white and about half an inch (1.25 cm) long when fully grown.
  • Lift and destroy infested plants. When lifting the infested plants, make sure there are no maggots left lying on the ground.
  • Because the flies are attracted to the smell of fresh manure, make sure that leeks and onions are not planted on freshly manured soil.
  • There are chemicals to treat the soil where onion fly maggots are a big problem. These are readily available at most garden centres and nurseries.

  • If you want to store the onions over winter then you can cure them by hanging them in a well aired place (such as from the roof of a summer house veranda). 
  • Mesh bags or strings can be used to group and hang the onions and they should hang for about 3-4 weeks.
  • Spring onions (scallions) can be harvested when they are about 30-40cm in height and will store for up to a week in the fridge. 
  • Onions are used in a huge range of culinary dishes, both raw and cooked.
  • To prepare scallion onions simply remove the outer set of leaves and wash.


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