Onions are easy to grow from baby onions; otherwise known as sets. It is also possible to grow them from seed which is very cost-effective if you use a lot of onions. However, sets are a lot easier and quicker. If you still want to grow from seed, check out our Grow at home: Onions from seed blog. If not, read on.
Onions grow best in open ground. However, they do grow well in containers. Just choose a deep planter to allow room for the developing onions. Potato Planters work very well if you only have a small space. A Raised Bed System that comes with a cover to protect them would also work if you have more room.
Wherever you plant them, onions need a sunny, sheltered site with fertile, well-drained soil. For best results test your soil. Inexpensive kits are available from your garden centre to make sure the pH is above 6.5. You may need to improve the soil before planting. A bucket of well-rotted manure or garden compost to every square metre (yard) and some general-purpose fertiliser will do the trick.
You can buy your onion sets from your garden Centre. There are many different varieties to choose from. So, select something that you would like. Maybe something out of the ordinary like giant onions that you can show off, red onions for a bit of colour or shallots for your winter’s stews.
When to plant your Onions
You can plant onions in spring or autumn. Depending on their final size, plant the onion sets 5-25cm (2-10in) apart in rows 25-30cm (10-12in) apart from mid-March to mid-April for spring planting.
Autumn onions should be planted in mid-September to mid-October. They will pretty much look after themselves over the winter. You need to take care as they have a long growing season and won’t be ready for harvesting until next summer. As a result, they will still be in the ground when you start planting other crops in spring.
There are two ways to plant: either directly into your ground or planter or into Rootrainers.
Which you use depends on the number of birds you have in your area. Birds can be a problem lifting the new sets. They aren’t after the sets themselves but the earthworms that congregate in the microbe-rich area around the roots. Starting your sets in Rootrainers means by the time that you plant them out the roots will be strong enough to keep your plants where you planted them!
If you choose to plant directly into the ground or planter then either cover with a Fleece Tunnel or stretch some birdscare across your bed until the roots are established. This will give the plants time to establish and be too firm for birds to pull out.
However, you choose to plant do it by gently pushing the sets into soft, well-worked soil so that the pointed tip is just showing, and firm the soil around them.
Weeding and Watering Onions
It is important to keep the weeds down as this can affect the size of your onions. Water when dry and give an occasional feed with a general liquid fertiliser. Stop watering and feeding once the onions have swollen in mid-summer
When the leaves start to turn yellow at the ends, you can bend the tops over to help with the ripening. Some gardeners swear by this but not everyone agrees with it anymore so you may want to try it and see how you get on.
Remove any flower spikes as soon as you see them.
Harvest & Storage
Onions can be harvested when the foliage starts to turn yellow and topple over. For spring-planted sets this will be in late summer to early autumn. And for winter planted sets this will be early to mid-summer.
Lift the bulbs as you need them, ideally before the foliage completely dies down. Importantly, don’t let them rot in the ground so harvest and store them before the end of October. After you lift them let them lie on a rack in the sun outdoors or a well-ventilated greenhouse for one to two weeks to ripen fully. They are ready for storage once the foliage is dry and papery,
Only store the onions that are perfect. Store them either in natural jute sack hung up or in old tights knotting after each onion. They can keep in a well-aired room for up to six months.
Pest & Diseases
Fungal diseases are the main problem for onions. White Onion rot, Leek Rust and Onion Downy mildew are the main culprits.
There is little you can do about any of these once they have taken hold, so prevention is the answer. Use the correct spacings to make sure there is plenty of light and air around each plant as humidity will encourage the spread of fungus. Weed regularly and avoid overhead watering if possible. Remove infected leaves and dispose of them away from the garden. Fungus can be transported in contaminated soil, for example on muddy tools or boots. So, take particular care not to pass it on to the next garden or allotment when you visit.
When peeling chopped onions, either use a ceramic knife – the extra sharpness means less crushing and so less vapour. Or light a couple of candles. The candle flames should absorb most of the vapours from the onions and stop your eyes from watering.