Centurion F1
Onion (Bulb) in the UK

Right, for onions, you've got a couple of options. You can start them from seed indoors in February or March, that's what I often do. Sow them thinly, about 1 cm deep, in trays or modules, and keep them somewhere warm, like on a windowsill or in a heated propagator. Once they've sprouted, give them plenty of light. Alternatively, you can plant onion sets directly outdoors from March to April. Sets are little immature onions, and they're proper easy to get going, just push them into the soil so the tip is showing. Whichever way you go, make sure the soil's not too cold.

If you've started seeds indoors, you'll need to prick them out into individual modules or small pots when they're big enough to handle, usually after a few weeks. Before planting out in late April or May, once the risk of a hard frost has passed, harden them off for a week or two. That's just gradually getting them used to the outdoors, you know. Plant them out about 10 cm apart in rows, leaving 20-30 cm between rows. You can get plug plants too, which saves a job, just pop them straight in.