Guide to Growing Onions

• A member of the allium family and a kitchen staple. The home-grower can choose a variety to suit their taste from the very mild to extremely strong.
• Not suitable for container growing (unless the containers are really large) or close spacing as this just reduces the yield.
• Overwintering onions, often called Japanese onions, seem to do better from sets than seed. They do not store well but fill the gap until the main crop are ready.

Sowing and Growing

• There are two methods of growing onions: from seed and from sets (small bulbs). Sets are easiest for beginners, although growing from seed is not difficult. Sets are generally more disease resistant, avoid onion fly and crop better in poorer soils. Seed is less expensive and is available in many more varieties.
• Onions grow best in good, well-drained soil. A heavy, wet soil encourages fungal rots. Add a general purpose fertilizer a week before you plant.
• Sets: scrape a small hole or a drill to plant in. Don't just push the onion sets into the soil as this can damage the root plate and kill the set. Plant the sets fat end down, so that the tip is just showing above the soil, in March–April. Plant each set 15 cm (6 inches) apart each way, or space at 10 cm (4 inches) in rows 20–30 cm (8–12 inches) apart. Closer spacing will produce smaller onions. Note that large sets are more prone to bolting, so don't discard the the small sets in favour of the large.
• Seed: sow into modules under glass late-February to early-March. You can sow direct but you will get better results from sowing into modules. The temperature should be 10–15ºC. Once germinated, don't let the temperature rise above 15ºC – or the plants may bolt later when they're planted out. Harden off in a cold frame before planting out in April, at the same spacing as for sets.
• Once planted, keep your onions weed-free and water during dry spells. Stop watering once the onions have grown to harvest-size.
• Sow Japanese (overwintering) onions between mid-September and mid-November for harvesting in June the following year.

Harvesting

• Once ready, you can pull onions at any time for immediate use.
• If you want to store them, wait until the foliage is starting to bend over (don't bend the foliage over yourself as this damages the onion neck). Gently lever the bulbs out of the soil with a fork and lay them outside to dry for a couple of weeks. Ideally, place them on some sort of rack to allow air flow all around. The odd shower won't cause a problem but if the weather is really wet, they will need protecting. Be carefull if you dry them in a greenhouse – they will start to cook if it gets really hot, and won't store well.
• Once the bulbs have dried out, string and hang them in a cool, dry place. Check the base of the bulbs regularly for rot and remove those bulbs to stop it spreading.

Pests and Problems

• Birds like to pull the sets out of the soil. Keep an eye on them or cover with netting or cloches until established.
• Onions are susceptible to rust, smut mildew and white tip, where the leaves go white from the tip down the stem. A spray with a seaweed fertilizer may help prevent spreading.
• White rot is a white fungus that grows on the base of bulb, killing the plant. There is no treatment. If this happens, try growing in large containers or raised beds with brought in compost. White rot is the club root of onions – easily spread and difficult to cope with.
• The onion fly produces small maggots which eat the base of the bulb. If this is a problem, grow from sets or cover the young plants with cloches or fleece until they are well established.
• Onions can bolt and send up a flower shoot. Cut it off as soon as you see it and use those onions first as they will not store well.

Varieties

• One of my favourites is Ailsa Craig. Red Baron is an excellent red onion.
• Paris Silverskin (mild) and Brown Pickling SY 300 are suitable for pickling. However, I prefer to grow some Ailsa Craig closely spaced to produce flavoursome small onions for pickling.

Eating

• Onions contain many essential minerals, vitamin B, antioxidants and are high in vitamin C.
• They're a staple of most cuisines and can be used in a multitude of ways, from soups through to breads.

RHS Award of Garden Merit

• Red Baron
• More information on the Award of Garden Merit