Many people grow onions from sets - mini, immature onion bulbs - to get a head start. The advantage of growing onions from seeds, instead of from sets, is that it is far cheaper if you are going for a big harvest. So if you eat a lot of onions then seeds are worth a try but you need to get them in ASAP now.
Make sure you use fresh seeds as the germination rate reduces the older seeds get. They will still germinate but if you are using a packet from last year you may need to sow a few more to get the crop you are hoping for. Sow the seeds on a windowsill or in the greenhouse, from February to April.
They will germinate without extra heat, but providing a little heat underneath the seed trays or pots will speed up the germination process. So, add a lid or enclose in a plastic bag and put it on a heat mat or somewhere warm like on top of the fridge. Germination should take around 7 to 10 days.
Onion seedlings sometimes have trouble shedding the seed husk and end up doubled up like an ostrich with their head in the sand. If you want to help them move along then you can snip the thinner bit, pull it out complete with seed husk and discard it. The thicker side of the loop can then get on with growing. This is fiddly and they will sort themselves out eventually so you can decide if you have the time and energy to do this or want to just let them get on with it.
Whether you are growing in the ground or containers make sure that that the young onions get plenty of light. If you are not growing in a greenhouse, then put the seedlings outside on warm sunny days to get maximum light benefit and to help harden them off. Use a large Bell cloche, poly lantern cloche or poly tunnel to help protect from wind and temperatures below 10˚c. Once you are happy that night time temperatures are well above 8˚C then the onions can stay out without protection.
Transplant them outside in May or June when they produce a third leaf and are about 3” (8cm) high. Dig some rich fertiliser into the ground where you are going to plant them. Make sure you put it directly under where the onions will be as their roots are concentrated directly down from the bulb.
Plant them vertically and handle them gently. The bulb should be ½” (1cm) below the surface. Depending on the onions final sizes, plant them between 2-10” (4-25cm) apart, with 9" (22cm) between rows.
If you want to grow onions in containers then transplant them at the same stage as for outdoors. The container will need to be at least 10" (25cm) deep and each onion will need about 8cm (3 inches) of space to grow. So, the wider the container the better. Make sure that the compost you use to fill the container is not too high in nitrogen. If it is you will get a lovely leafy display above ground and very little below ground.
Looking after your Plants
The important thing while they are growing is to keep the weeds down. Onion seedlings don't compete well with weeds and it will affect the size of your onions. So weed regularly.
You can also keep trimming them back to around 5" so that they don't flop over. Once again they will be OK if you leave them to their own devices, so if you're not growing them for the Village Show you may want to miss this step.
Keep them well watered especially when it is dry. When the leaves start to turn yellow at the ends, bend the tops over to help with the ripening. Possibly even clear a little of the soil at the top of the bulb too.
Harvest them from July to October. Lift the onions as you need them from July to October. There is a danger that they can rot in the ground when it starts to get very wet so harvest and store them before the end of October. After you lift them let them lie in the sun for a couple of days.
Only store the onions that are perfect - use any that aren't straight away. The best way to store them is in a jute Veg Sack. This allows air to circulate and keeps them cool and dark. They can keep in a well aired room for up to six months.
When peeling chopped onions, light a couple of candles. This should stop your eyes watering, as the vapours from the onions will be absorbed in the candle flames.