RHS principal horticultural advisor Esther McMillan explains how to grow this bulb
A plait of home grown onions hanging in your kitchen is a satisfying sight. They are such a versatile culinary staple, it’s good to have a store of your own! Here’s how you can produce these tasty bulbs on your plot.
Select your site
Choose an open site for your onions. They like fertile, free-draining soil, so if yours is heavy, dig in some well-rotted organic matter before planting. If you have acidic soil (a pH below 6.5), you can apply a lime dressing to raise it. It’s also wise to rotate your onion patch each year to avoid some of the common pest and disease troubles.
For summer cropping get on and sow or plant as early in spring as conditions allow. The longer days of late spring trigger bulb formation, so a good plume of leafy growth at the point is important.
Succeed with sets
The easiest way to grow onions is to plant sets, immature onions for gardeners to grow on. They’re sown in early to mid-spring for harvest later that year. If in the past, your onions have ‘bolted’, which is when the flower stem emerges leading to a poor crop, then try heat-treated sets. Red onions are especially prone to bolting. Heat treatment kills the flower embryo so they’ll keep growing for longer and often gives a harvest of huge onions, even though you generally start them off a little later than ‘normal’ sets.
Plant sets directly into the ground by pressing them into a prepared bed with just their tips showing. For average sized onions, space at 10cm apart, but if you’re aiming for larger, run to 15cm. Rows can be 25-30cm apart. If the weather is cold and wet, you can get them going in modules, then they’ll start to develop a root system before planting out in better conditions later on.
You may find the need to cover sets with fleece until their roots give anchorage if you have playsome cats or interfering birds.
Starting with seeds
If you want to grow from scratch, try sowing onion seed. You’ll find a far wider selection of cultivars if you choose this route and it’s a much cheaper method if you can start early. You can sow outdoors in late winter, but if there’s a cold spell, this may trigger the plants to subsequently bolt. In a prepared seedbed, sow thinly in a drill 2cm deep. Wait until they reach the size of small salad onions and thin to 5-10cm. You can then wash and eat your thinnings! Why not add them to salads?
Sow 1cm deep into modules in your greenhouse or coldframe in midwinter to early spring. Sow two seeds per module and thin to leave the strongest see
