While it is possible to grow onions from seed, most people will choose to grow onions from sets, which are small, immature onion bulbs. Onions are a great crop to grow in a polytunnel and they make a good companion crop to a number of other vegetables since their strong smell can confuse and deter a range of different pests. They are of course a versatile addition to a kitchen garden and can be used in a wide range of different recipes.
Onion sets come in a range of different varieties. There are red onions and white onions, of which some are best sown in the spring and others best sown in the autumn for overwintering. If you are looking for a perennial onion alternative, there are several plants to choose from that will produce bulbing onions for a number of years. Should you wish to go for the cheaper but more tricky option and grow onions from seed, these should be sown indoors under cover around 8-10 weeks before the last frost date in your area. Containers used for food packaging are ideal for sowing onion seed in, as long as you make sure that you have cut plenty of holes for drainage. It is important to choose a type of onion that is appropriate for where you live.
Harden off the onions grown from seed or plant out onion sets when the soil begins to warm in spring. You can then also plant sets for overwintering in September or October. Choose a sunny and reasonably sheltered spot for your onions (a mixed polytunnel bed is ideal). Water in and then keep plants moist during drought conditions. The main thing is to weed well between your onions as onions, both those grown from seed and from sets, will suffer if there is too much competition. Usually, you should give a spacing of around 10cm between each of your growing onions in their final growing position. A mulch is a good idea to keep down weeds and to feed onions as they grow. Sometimes birds will lift the sets. If you have this problem you can lessen it by removing the loose skin from the tops of the sets. If onions flower, remove the flowers as soon as possible. The flowers can be eaten.
While established gardeners used to recommend bending over the foliage when the time to harvest approaches, or gently lifting bulbs to break the roots, it is generally now recommended that you wait for the foliage to turn yellow and flop over, wait a week or two and then lift. Onions for storage must be thoroughly dried for two to three weeks (a wire shelf erected between the crop bars of your polytunnel is ideal for this). Eat any blemished onions first and leave the firm and disease free onions for storage. You can braid the onions or string them in netting (or old tights) for storage.
Planting – February to April
Expect to lift – July to September