Onions are a cool season vegetable which can be grown from seed or from small bulbs called sets.

Growing from sets is a more reliable way of ensuring a good crop, particularly under cool or damp conditions which are not absolutely ideal for onion growing.

When to plant Onions

Onions are cool-season vegetables that can be grown from seed or from small bulbs called sets. Growing from sets is a more reliable way of ensuring a good crop, particularly under cool or damp conditions which are not absolutely ideal for onion growing.

How to plant Onions

Onion sets are small, immature onions which will grow to full-size by the time of harvest.

Dig over the soil and turn in manure two or three months before planting. Do this in advance as the high nitrogen of fresh manure is not required by these plants. Improve drainage in heavier soils by adding grit and organic matter, and build up your raised beds.

The soil pH should be neutral to mildly alkaline, so use lime if it is acid. Plant out the sets in shallow drills, covering each bulb so that the neck is just protruding from the soil surface. Leave a spacing of around 15 cm between plants. If you have a raised bed, plant out two rows on each bed, 20 cm in from the side of the bed. Weed diligently and be sparing in your water regime.

Growing from seed, sow into modules in January or February for a summer harvest, or in the early autumn for an early harvest the next year. Use a special seed compost, and reckon on around five seeds in each module.

You won’t have to fiddle around separating out the plants as the bulbs will push themselves apart as they expand and grow. Don’t forget to label each tray with sowing date and variety.

Harvesting Onions

At the time of maturity, for each ring of an onion, there is a leaf top – and the larger the ring, the larger the leaf will be. The time to harvest can be gauged by the yellowing of the leaves, which will then begin to die back.

Lift out the complete plant, bulb and leaf, and lay these in a dry and warm place to dry out. Once dry, store in a dark and cool location by hanging in traditional plaits or nets or alternatively pack in layers in a box.

The length of storage time varies according to the individual variety, although a period between 3 and 6 months is usual.

Through the year

Onions can be prone to disease. Keep an eye out for neck rot, a fluffy grey growth around the neck of the onion – remove onions affected and try to avoid occurrences by rotating the location of your onions year on year; onion thrips, a mottling on the foliage of your plants is the sign that these little yellow or black insects have arrived, the leaf damage resulting in a smaller crop – there’s no need to remove the plants; onion white rot, growth around the base and roots of the bulb – destroy any affected plants; onion fly, a larval fly which eats the bulb roots and burrows into the bulbs – remove affected plants as quickly as possible as the flies will move into the soil to pupate.

An attack of onion fly is likely if you grow from sets; you can also try interplanting carrots with onions as a preventative measure.

Handy tips

If you want to harvest some of your crops as green onions in the middle of the growing season, you can plant your sets close together so that, when you pull the green onions at the early bulb stage, there will be space for the remaining onions.

Ready to plant your own vegetables now you know how to grow onions? Take a look at our vegetable seed collection.