Onion (allium cepa)

Onions Why plant
Easy to grow and store.
Essential cooking ingredient in many recipes.
Grows easily from seed.

Interesting uses
Got cold feet? Rub with an onion to get the circulation going again.

To get rid of the smell of paint from a newly decorated room simply place a cut onion in its centre.

The source for the plant we know today is somewhat unclear. There are five possible wild plants it could have evolved from, all of which grow in the central Asian region.

Certainly eaten in the Bronze Age, definite evidence of cultivation appears in ancient Egypt about 3000BC with onions being fed to the slaves building the pyramids. Roman gladiators were rubbed down with onions to firm up their muscles, and it was the Romans who named and introduced the onion to Europe… the Latin name was ‘unio’ for large pearl, changed to ‘oignon’ by the French.

The status of the onion rose substantially after French Onion Soup was made popular by Stanislaus I, the former King of Poland. In the Middle Ages onions were such an important food that people would pay their rent with onions and even give them as gifts. Doctors prescribed onions to facilitate bowel movements and to relieve headaches, coughs, snakebite and hair loss.

Health benefits
Onions may be a useful herb for the prevention of cardiovascular disease, especially since they diminish the risk of blood clots. They also protect against stomach and other cancers, as well as certain infections. Onion can improve lung function especially in asthmatics.

Although onions may be yellow, red, or white, seventy percent of onions grown are yellow. Yellows are good all-purpose onions and the best winter keepers. They also have tougher skins, are more disease-resistant and less susceptible to insects. Reds are the sweetest but are generally the worst keepers. Whites are grown for salad or spring onions, harvested as "green onions" before their bulbs form.

Onions prefer a sunny position with a rich but light soil, but they’ll do well in most soils. Don’t plant in freshly manuredSpring Onion soil since they need little nitrogen.
Can be planted as sets (small, immature onions) in spring or late summer. These increase in size and each forms one full-sized bulb when ready to harvest. Generally growing onions from sets is easier and more reliable than from seed. In cooler, damper areas the sets should give a better yield of larger bulbs than if grown from seed, but there are less varieties available and the cost is higher than if grown from seed. Plant 10cm apart with 20cm between rows, pointed end upwards so the tip just below soil level.

My preferred method is to plant Japanese onion seeds (senshyu semi globe yellow) in autumn in drills 1cm deep with rows 40cm apart. Leave until spring, when the seedlings should be thinned to 5cm apart. If leaving late in the season try planting in a greenhouse in trays & transplanting 5cm apart when hardened off.

Spring (salad) onions should be sown thinly each fortnight from March to July in short rows 1cm deep and 10cm between rows.

There’s very little else to do… just keep them weed free!

When the leaves start to turn yellow in mid summer bend them over to encourage early ripening. In late summer carefully lift the bulbs and allow to dry out.

Spring onions are just pulled up whenever needed from May to September.

Common problems
  • Bolting is a very common problem and happens when the onion suddenly puts up a central stalk that develops a seed head. Usually caused by weather conditions… a cold spring followed by a hot summer seems to make it worse. Loose soil can also cause bolting, the plant roots being disturbed the plant thinks it’s starving and reacts by trying to spread its seed. Cut the stalk off an inch above the bulb and use these onions first since they don’t store well.

  • Grey mould on onions in store and general rotting is usually caused by the onions being insufficiently dried out prior to storing or damp storage conditions. Check regularly and discard rotting onions before the problem spreads to the rest.

  • Mould or rust may occur during prolonged wet periods. The bulbs need to be thrown away or burnt. During rainy periods give protection with a cloche, but allow ventilation.

  • Onion white rot causes the foliage to go yellow and wilt… check for fluffy white growths to confirm it’s onion white rot. Plants should be thrown away (not composted) and don’t grow garlic or onions in the same area for at least 8 years.

  • Downy Mildew gives the leaves slightly lighter patches in the early stages that turn to brown as the disease gets worse. Parts affected will eventually fold over and die. When the stalks are affected they weaken and fall over. Use crop rotation and proper drainage to avoid.

  • Onion Fly lays its eggs by the base of the onion which then hatch into maggots who eat away at the base of the onion and its roots. Usually only strikes at direct sown onion onions, being attracted by the scent of them when thinning. Early signs are yellowing and drooping leaves. The only cure is to prevent access using fleece.


  1. This is a very thorough run down of the onion. Thank you! I tried growing onion from seed this year, since, due to migraines, I can only eat mild, sweet onions and they don't tend to supply sets for these. I did manage to get them to "set" size, but that really isn't big enough to do anything with. I'm wondering if there is a way to store these for planting next year.

    Until I figure out how to grow onions in my garden there are always leeks, shallots, and spring onions!

  2. Hi Sylvana... maybe you didn't start them early enough. How about trying now?

    The sets you have should be OK to plant for a proper crop next summer.


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