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 too acidic – below pH 5.5 – you will need to add lime to it according to manufactures recommendations. In general, onions prefer a pH of between 6 and 7.5.
It's possible to grow good onions on heavy soil, but you must improve the drainage first before planting. Add plenty of horticultural grit and bulky organic matter to the soil and then create a ridge of soil 4 inches high to further reduce soil moisture.
You can sow main-crop onion seeds as soon as your soil will allow which can be any time from late February, but you can steal a march here by picking a dry day a few weeks before sowing time 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 sow onion seed under glass or cloches in January.
Choose a dry day to sow onion seed when the soil is moist but not too wet, then plant the seed very thinly into drills ½ inch deep. If you are planting more than one row then each row should be at least 9 inches apart. Carefully cover the onion seed with soil and gently water in. Germination should then take approximately 21 days to occur. Once the new seedlings have began to push through the soil, they can be thinned out to between 1 and 2 inches apart. After a couple more weeks these can be further thinned out to one plant to every 4-5 inches. Remember to clear away all of your discarded thinning so as not to attract onion fly.
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 little mischievous fun. If you don't have some kind of protection in place you can end up loosing almost an entire crop!
Onions are not very good at suppressing weed growth, and if regular weeding is neglected they will easily be out competed for nutrients resulting in your crop becoming stunted. Try to leave enough space between the rows to get your hoe in for weeding, but always hand-weed any weeds close to your onions as they can be easily damaged by garden tools.
To maintain a good year-round supply of onions, you can make a second planting during the late summer which should be ready to harvest from June. However, this second planting isn't recommended if you have 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 the individual variety. 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, any problems and you can carefully ease them out of the soil using a hand fork. They will now need to be dried and depending on the weather or the size of your onions this will take approximately 2-4 weeks for them to properly cure before they are 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 more prone to neck rot.
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WHEN ARE ONIONS READY FOR HARVEST?