Onions are a popular vegetable to grow at home as they are used continuously in the kitchen. Bulb onions store well and it is possible to have them from your own garden from mid-summer to the following spring. They are relatively easy to grow, not needing a great deal of effort, and a good crop of onions is a very satisfying achievement.
Site and soil:
Onions have a relatively short growing season, at least if sown in spring, and they need good soil in a sunny position to grow quickly and make good size before bulbing up begins. The soil must be fertile but not too rich. Very rich soil with a lot of organic material tends to make the plants very leafy, large, and with a greater number of ‘thick-necks', which do not store well. If the soil fertility is a bit low, apply some general fertiliser at about 70 grams per square metre. The soil should be light and well-drained without being dry. Ideally, onions can follow a crop such as potatoes or courgettes for which lots of organic material is applied. The rotted organic material will boost onion growth and retain good reserves of moisture without promoting too-vigorous growth.
Onions can be grown from seeds or from sets, which are small bulbs. The standard variety for many years from seed is ‘Bedfordshire Champion', a round brown-skinned onion, which stores well. ‘Buffalo' is a relatively new variety which can be sown in spring or in August and can be used to provide early onions. ‘Napoleon' is new with flattened bulbs that stores well, spring sown. ‘Carlos' is recommended for resistance to bolting and very good storage, spring sown. The main variety from sets is ‘Sturon', a good reliable variety that stores well. In general, onions grown form seeds store better than those grown from sets, but it is easier to grow from sets, which are spring-planted although autumn sets are sometimes available. Japanese bulb onions can be sown in autumn to provide onions in mid-summer and the variety generally used is ‘Senshyu Yellow'.
Onions can be sown in August or early September, using a suitable variety, or in early spring. They are sown early to make some size before bulbing starts. Bulbing is regulated by day length and begins in late May and June. If the plants have not made much growth by then, they will start to make bulbs before they should, resulting in smaller bulbs, which actually might be preferred. For large bulbs, it is essential to sow early. This can be achieved by sowing in a greenhouse or under other cover as early as December or January. Seeds sown in cell trays can be grown on as four or five seedlings per tray, or even one to get greater size, for planting out in April.
When sowing outdoors, the ground needs to be well cultivated in advance and a fine tilth made at the surface. It may help to rake in some silver sand for very early-sown crops outdoors. Sow about one centimetre deep and watch early-sown crops for slug attacks.
Plant out onion plants sown indoors in April. These can be grown in cell trays or small pots and transplant very easily after being hardened off. It is also possible to transplant onions sown outdoors, lifting them carefully and re-planting immediately, watering them in. This can be a way of extending a planting if there is patchy germination or losses from an early spring sowing.
Onions sets are planted out in April usually although they can be planted out earlier if required. These are spaced 10 to 15 centimetres apart, with rows 30 to 40 centimertres apart. Often a double row of sets about 20 centimetres apart is planted. If more than one double row is grown, the rows can be laid out at 50 to 60 centimetre centres. Seed-grown onions can be thinned to about the same spacing or a group of seedlings from a cell tray planted about 20 centimetres apart.
After setting sets, transplanting, or thinning seedlings, the main aftercare required is weed control. With seed-sown plants, weed control will often need to start before the onions seeds are up to control weeds seeds that germinate more quickly. The best method is to hoe the weed seedlings when they are tiny. In this way, they are easily controlled and never offer any competition to the onions. If they get large, it can be very difficult to remove weeds without pulling up onion roots as well, and onions suffer badly from weed competition as they offer vey little leaf cover of their own. Consider watering if there is a dry spell, which often happens in May. Onions react to drought by bulbing and make very little growth subsequently. Overwintered onions often show signs of lack of nutrients and can benefit from a liquid feed in early spring as growth begins.
Onions are generally harvested in September from a spring sowing or April-sown sets. But if the seeds were August sown, they will be ready in mid-summer. Summer onions can be used straight out of the ground while the remainder of the crop ripens. The sign of readiness is when the tops of most of the plants fall over. The bulbs can then be loosened in the ground with a fork and this triggers bulb ripening. This will happen any way but often the weather can turn wet and the bulbs may get neck rot, a storage disease. In a week or so, when the leaves have withered and yellowed, the bulbs can be lifted and the soil shaken off. They can be placed on a paved area or a wire rack to dry, and they can be placed in an airy greenhouse or tunnel to ripen fully. This helps to improve storage quality. Store in net bags or in trays, or make an onion hank by twisting the onion tops around a strong wire.
Pests and diseases
Onions are relatively free of pests. Occasionally greenflies affect the young growth on plants in very sheltered gardens and may need to be controlled. White rot disease is a very damaging soil borne disease that builds up and can ruin an entire crop by causing the base of the bulbs to rot and the plants to wilt. Use a different part of the garden and avoid contamination. Neck rot is a storage disease that may be seed-borne or picked up late in the season. Some seed is sold with fungicide treatment, but all old onions should be destroyed to prevent spread to new season crops.