Onions are well worth growing and pretty easy as they do not suffer from many pests and diseases. However, they are prone to bolting in dry weather which can be a problem in a typical summer in France.
It is a good idea to buy onion ‘sets’ rather than growing them from seed; sets are immature onions, part grown and then dried and stored in a dormant state. They have usually been given ‘hot water treatment’ which can help to kill off any diseases and prevent bolting. (bolting is the term used to describe early production of the flower which will make the bulb unsuitable for storing and reduce the yield considerably). Sets not only give you a head start and therefore a longer growing season but are easier and more reliable than growing from seed.
This very cold wet start to the year means that for most of us the soil is still far too wet and cold for sowing or planting but you can get your onion sets off to a good start by planting them into trays of modular cells or seed trays. This will endure that a good root system has developed before you plant them out into the soil and that the sets are viable. It can also help to prevent birds or rodents from disturbing the newly planted sets, without roots they are easy for them to steal!
They don’t need any warmth or protection – a cold frame, unheated greenhouse or polytunnel or just a sheltered spot in the garden. After about 3 weeks the shoots will be about 5-10cm long and ready for planting out in rows at about 15cm apart. You can start garlic and shallots off in the same way.
Before planting make sure that you have added plenty of well rotted organic matter to the soil to retail water and therefore helping to give the onions an even supply over the summer. If necessary you can irrigate using a seep hose laid alongside the row, keeping the soil moisture topped up by watering in the evenings when necessary. This will deliver water directly to the soil. Onions need a regular water supply to swell to full size and can split and rot if a period of drought is followed by heavy rain or watering so managing the irrigation is important.
Onions are ready to harvest when the tops have dried out and tumbled over, lift the onions on a dry day and allow to dry out on well aired racks in the sunshine for several days before storing. Onions should be stored in a light, airy and frost free place – keeping them in the dark will induce growth. You can plait them into strings or hang them in onion sacks.