How to Grow Onions – The Gardening Register

Choose an open, sunny site with good, free draining soil. Dig over the soil in early winter and incorporate compost, firm well before planting and add a general fertiliser. Keep the ground weed-free and water when the weather is dry feeding occasionally.

Onions can be grown from Sets or from Seed.

For an August or September crop sow seed late February to early April when the ground is workable, for an earlier crop sow in mid August. Sow under glass in January, harden off in March and transplant outdoors in April

Onion sets are immature bulbs which have been specially grown for planting and tend to be easier and safer to grow than seed but they are much more expensive. You can also buy heat-treated sets which prevents bolting (running to seed).

Onion sets take 11-14 days to sprout and around 20 weeks to mature, plant mid March to mid April.

Break off any flower stems as they appear and stop watering once the onions have swollen, pull back the earth to expose the bulb surface to the sun.

Onions can be harvested once the foliage has turned yellow and topples over, leave them for two weeks then carefully lift; this is best done in dry weather.

Onions to be stored should be dried by spreading them out in trays then store in a cool, well-lit place. They should keep until late Spring.

Onion Troubles
Trouble	Symptoms	Treatment	Prevention
Onion Fly	Yellow, drooping leaves. Maggots burrow into the bases and kill young plants.	None. Lift and burn affected plants.	Rake Chlorophos into soil before showing or planting. Sets are less prone than seeds.
Bolting	Premature production of flower-heads.	Cut off flower stalks and lift as usual. Do not store.	Don’t sow or plant too early in the season. Ensure soil is firmed well before sowing or planting
Stem & Bulb Eelworm	Swollen, distorted foliage. Young plants die and older plants product soft bulbs.	None. Lift and burn affected plants.	Do not grow onions, peas, beans or strawberries on land previous infected with this disease.
Saddleback	On harvesting, bulbs are split at the base. Caused by heavy rain or watering after a dry period.	None. Use affected bulbs immediately.	Keep well watered during dry spells.
Set Division	Onions from sets produce twin bulbs.	None	Plant sets in good soil and keep well watered in dry spells.
Smut	Black spots and blotches on leaves and bulbs of young plants. Twisted and thickened leaves. More likely in leeks than onions.	None. Lift and burn affected plants.	Don’t grow leeks or onions on affected ground for eight years.
Rust	Orange spots and blotches on surface of leaves. More likely in leeks than onions.	Remove and burn affected leaves.	Don’t grow leeks or onions on affected ground for one season.
White Tip	Tips of leek leaves turn white and papery in autumn.	Spray with Dithane at first signs. Lift and burn badly affected plants.	Don’t grow leeks or onions on affected ground for one season.
Bull Neck (Thick Neck)	Abnormally thick necks means the bulbs will not store successfully.	None	Don’t use too much manure. Use a feed with more potash than nitrogen. Don’t sow seed too deeply.
Downy Mildew	Downy, grey mould covering leaves. Leave die back slowly and shrivel. Bulbs are soft and don’t store well.	Spray with Dithane at first signs. Repeat fortnightly.	Grow onions on a difference site each year and ensure soil is well drained.
Leek Moth	Tunnelled leaves. Caterpillars feed inside young leaves leaving the outer skin. Also attacks leeks.	Spray with a contact insecticide at first signs of attack. Destroy badly affected leaves.	None
White Rot (Mouldy Nose)	Foliage turns yellow and wilts. Fluffy white mould on base of bulbs and round black bodies appear in the fungus.	None. Lift and burn affected plants.	None. Don’t grow onions on affected land for eight years.
Shanking	The centre leaves turn yellow and collapse, outer leaves follow later. Evil-smelling slime within scales.	None. Lift and burn affected plants.	None. Don’t grow onions on affected land for several years.
Neck Rot	During storage a grey mould appears near the neck. Bulbs are soft and rotten.	None. Remove rotten bulbs.	Dist seeds and sets with benomyl before planting. Dry bulbs thoroughly before storing. Don’t store soft bulbs or bulbs with green necks.