Growing onions from onion sets is about probably the easiest way to produce a good crop of quality onions and generally you will achieve better success planting onion sets compared to growing onions from seed.
To begin with you need a sunny site with good drainage but the key is to grow them in a permanent bed in order to build up soil fertility. There is a down side to this however, as you can also encourage the build up of diseases. With this in mind, it may be advisable to rotate your onion bed with the rest of the vegetable garden once in a while.
Avoid soils that have been planted to onions within the past three years, and because onions are shallow rooted and poor competitors with other plants, try and avoid sites with a history of perennial weeds.
.As mentioned previously, it is possible to grow onions on the same bed year after year but in order to maintain successful and healthy cropping a strict health routine must be followed. If there are any onions that you suspect are harbouring any kind of disease, remove not only the plant but also a small amount of soil from where the onion was growing. Hopefully this will eliminate any unwanted bacteria in the soil. It is also worth watering the bed with a dilution of Jeys Fluid once the crop has been harvested - this again will help to kill off any unwanted bacteria or fungi. There are onion beds around from over 140 years ago that are still in production today using this kind of method!
If you can, start preparing your onion bed in the autumn by digging in plenty of well-rotted farm manure. This will give the ground a chance to settle over the winter period and allow frosts to break down the soil clods. If you soil is to acidic – below pH 5.5 – you will need to add lime to the bed according to manufactures recommendations. In general, onions prefer a pH of between 6 and 7.5 and a fine tilth to be planted into. Weather permitting, the frosts should do a good job of this.
You can plant onion sets as soon as your soil will allow you to which can be any time from late February onwards, but you can steal a march here by picking a dry day a few weeks before planting and raking the soil to a fine tilth. Onions like a firm bed so tread over the area you have just raked. Try adding a general fertiliser like growmore for extra fertility, and for an even earlier crop you can plant onion sets under protective cloches at the end of January. (You can gain an advantage by setting up cloches before planting. If cloches are placed over the ground prior to planting, the ground has a chance to warm up, reducing the chances of a check in growth). The soil may require some watering to achieve a uniform moisture before planting onion sets, but try and avoid planting them into a dry bed.
Plant onion sets 4 inches apart and in rows about 1 foot apart. Plant onion sets to a depth where only the very tips of the sets are just showing through the soil. Dig a hole in the soil with a trowel and place the onion sets the hole with their necks uppermost. Do not just push them into the soil as they may grow out of the soil as the season progresses.
Micro-nutrients are also important in onion production - in particular boron and zinc - so look at giving your onions a periodic liquid feed of seaweed based fertiliser. However if your onions are clearly growing well then this will probably be unnecessary.
You will need to keep a a particular eye on the newly sprouting onion shoots as these will often attract the attention of inquisitive birds – particularly pigeons and black birds - who will lift your juvenile crops straight out of the seed beds for nothing more than a bit of mischievous fun. If you don't have some kind of protection in place you can end up loosing almost the entire crop!
Onions are not very good at supressing weed growth, and if regular weeding is neglected they will easily be out competed for nutrients. This will result in your crop becoming stunted. If you can, leave enough space between the rows to get your hoe in for weeding. However, always hand-weed any weeds close to your onions as they are easily damaged by garden tools.
To have a year-round supply, you can make a second planting during the late summer which should be ready to harvest from June, although a second planting isn't recommended in heavy, poorly drained soils. In general, onions should be given as long a growing season as possible to reach their maximum size.
Your main crop onions should be ready for harvesting any time between August to September depending on both the weather and individual varieties. The onion bulb will be mature when the foliage turns yellow and begins to tip over, but you will need to leave them for another couple of weeks before lifting. Choose a dry day, and if the onions are fully ripe they will lift easily from the ground by hand. Any problems and you can carefully ease them out of the soil with a gardening fork. They will now need to be dried and depending on the weather or the size of your onions it will take approximately 2-4 weeks for them to properly cure. They are now ready for the kitchen. If any of your onions have developed thick 'necks' over the growing season, use these ones straight away as they will not store well and will be prone to neck rot.