How to Grow Onions from Seeds


  • Sowing Instructions (Indoors)
  • How to Transplant
  • Sowing Instructions (Outside)
  • Crop Maintenance
  • How to Harvest
  • Storage
  • Pests & Diseases

Sowing Instructions (Indoors)

  • Sowing time: From early spring
  • Sowing Location: Indoors in seed trays
  • Sow in 3 inch pots or in seed trays, one seed per tray cell
  • Sow using a general potting compost and perlite mix at 3:1
  • Water soil well before sowing, and keep pots or trays moist at all times
  • Sow indoors in January & keep under glass in a bright and warm position

How to Transplant

  • Move outdoors in mid April
  • Harden plants off one week before planting out
  • Soak well before transplanting & do so on a dull day
  • Prepare soil prior to transplanting
  • Space transplants 10 to 15cm apart
  • Take care not to distrub roots when lifting
  • Place in hole & firm in well
  • Assess regularly for water

Sowing Instructions (Outdoors)

  • Sowing time: For main crop onions best to Sow in mid to late spring
  • For main crop onions best to Sow in mid to late spring
  • Can be sown in autumn & over wintered under a garden cloche but this not advisable in cold regions such as the midlands
  • Sowing location: A sunny site with good drainage is ideal
  • Sow very thinnly, 1 cm deep
  • Space rows 15cm apart

Crop Maintenance

  • Water if the soil is dry and cover with soil
  • When large enough to handle thin the crop in two stages
  • Dispose of all thinnings to avoid attracting the onion fly
  • Spacing is important. Close spacing for smaller onions and wider spacing or larger onions
  • Dig & manure your soil the previous autumn if possible
  • Make sure to remove any large and smaller stones, weeds and weed roots while preparing soil
  • It is recommended that you perform a soil fertility test at least once
  • Lime the soil if your soil is acidic
  • Keep the soil weed free
  • Apply a general fertilizer if needed and rake the surface on a dry day
  • If your soil is a heavy clay it is recommended that you improve you soils drainage
  • Alternatively place a small amount of horticultural grit beneath seeds prior to sowing


  • Harvest when the bulbs have swollen and when the stems turns yellow and falls over
  • Using a garden fork, slightly lift the onions in their position. lift to snap the onion's roots preventing further growth
  • Leave for 14 days and carefully remove on a dry day
  • You can speed up the process by bending and tieing the stems once they start to yellow
  • On a dry day lift your onions and place them out in the sun
  • Depending on the size of your onions and weather conditions it can take up to 4 weeks for the onions to dry


  • Consume any damaged bulbs first
  • Short term storage in the fridge
  • Can be stored for up to 7 months in a cool dry place
  • You can also tie up your bulbs using their own stems or some cord

Pests & Diseases

  • Bird attack is most likely the biggest threat to onions
  • Netting should be used to protect against birds that pull up roots
  • Other pests and diseases include: Onion fly, root fly, bolting and downy mildew


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