How to grow onions
Onions are an easy crop for minimum effort. Onions are easiest to grow from sets which are tiny onion bulbs which have been treated to produce good bulbs. The treatment also reduces the possibility of the onion producing a flower and bolting which would spoil the crop. When growing onions the best time to plant out is mid to late March or April, depending on the spring weather when the soil is a bit drier and warmer. You can buy onions suitable for planting out in the autumn and over wintering, if that suits your plot. As with all vegetables the soil needs have organic compost or manure incorporated into it. To grow onions, the ground doesn't need to be really finely raked as it does for carrots and salad crops, just reasonable, and not compacted.
I like growing onions, the taste of home grown is good and they keep well so you can pick and use all through the winter. If it is a bad summer and the veg crops are struggling you can always depend on onions to produce a good crop.
New video How to grow onions and garlic
How to grow and plant onions
Onion sets are the easiest, although you can raise from seed if you have time and inclination but it is a faff to raise onions from seed. Onions can be grown in containers although given onions are planted about 10-12 cms apart and 30 cms between rows, they would need to be planted in a large container; for containers garlic is more suitable. Onions are a bulb, so when planting the hairy bulb end goes downwards . Make a small indentation in the soil, you can easily use a finger or a dibber, and place the bulb down so only the tip of it is showing above ground. It is important to appreciate that onions sit just under the soil with a tiny top bit pointing out of the earth. Make a small indentation and ease in gently as the growing source is the hairy bulb end and if you damage it when planting you may slow the growth. Also, if you push the bulb in, onions have a tendency to push the bulb back out again when it roots. In a raised bed /good soil plant rows each about 10cms apart. Growing onions is easy and onions have very few enemies in terms of pests but they do have one terror; the pigeon. Over to the plot it waddles and pecks out the little tab of Onion set above the ground and yanks out the bulb. They are a menace. If you have a lot of pigeons in your area you will need to deter them by covering the onions with a cloche /netting, otherwise keep some spares bulbs to replace the pigeon pecks.
Companion planting suggests that Alliums, (Garlic, Onions and Leeks,) because of their smell, can deter carrot fly and slugs which suggests it is a good idea to plant onions near the carrots and salad crops. If you are also interested in the appearance of your veg plant planting red and white/brown onions in rows looks attractive. Some veg plots, as seen in the show gardens such as at RHS Harlow Carr are really inspirational. A more practical reason for planting in tidy straight rows is that you know everything outside the row is a weed which makes the weeds are easier to find and get rid off before they take root. (see tip)
A row or two of sweet peas to bring in the pollinators adds colour and looks lovely. If your garden shed is next to the veg plot, the side of the shed is an ideal place for a bit of trellis and a few sweet peas. If you have no border use a trough and fill with rich compost as sweet peas are greedy. More about growing sweet peas
Which Onions are the best onions to grow
In 2013 the RHS undertook a plant trial of onions. 21 different onion types were trialled all planted the same and at the end assessed for bolting, size, yield and then 25 of each were stored in a uniform way to test for storage quality. After that, 12 were given the AGM Following these trial buying onions with the RHS award is an easy way to pick a good variety and those which were given the award were: Red onions Marshals Red Fen, Red Baron, and Garnet. Heat treated yellow and brown skinned onions Rumba: not heat treated Sturon, Centurion, Hercules, Autumn Gold Improved, Griffon, Setton, Vulcan, and Turo. Curiously, nearly all of the AGM were not heat-treated and the trial suggests that it does not in practise make any difference. The trial gives good guidance as to reliable variety to buy and grow.
In September the tops of the plants start to turn golden brown and look like dry growth rather than green growth. Harvest Onions on a warm day or preferably a warm spell, lift them and sit on the warm dry earth for a few days to dry out throughly and then store in cool place in string bags .Often the weather isn't good and you cannot dry onions outside. Since it is very important onions are dried well before storage if the weather is poor another possibility is in the greenhouse where the slats make an ideal spot to dry them out for a week or so before bringing in to store somewhere light and cool. You can dry onions, garlic and shallots in this way as shown below. It is very important that onions are fully dried out before hung up for storage overwinter, any moisture will cause the bulb to go mouldy. Onions need to be stored in a cool light environment, an unheated conservatory, garage or porch is suitable. The overwintered variety does not store well.