Growing Onions

Cold weather may still occur.

For culinary purposes we are interested in the modified leaves of the plants. They are concentrically wrapped to form a cylindrical ‘bulb’, which we blanch, in the case of leeks, while the familiar bulbs or bulbils of onion, shallot and garlic are made up of swollen leaf bases which contain the plant’s stored food. Like all members of the Alliaceae the onion tribe contain pungent and aromatic materials which give them their familiar taste and smell. The British climate is well-suited to growing these vegetables but the drying and ripening of onion bulbs is sometimes hampered by wet conditions in late summer and early autumn.

The genus Allium contains several species which have been grown as food crops for centuries. The Egyptians were known to eat onions and garlic while there is reference to the Israelites eating leeks during their time in Egypt.

Members of the onion tribe are widely used either as separate vegetables or as flavourings for other dishes. They are biennial or perennial plants which we mainly grow as annuals in our gardens. The biennials—onions and leeks—will flower if they are allowed to grow on into a second year and may even ‘bolt’ if the young plants are put out—in an attempt to produce bigger or earlier crops—at a date when prolonged.

Bulbs and Salad Onions

Allium cepa

This group, which includes dry bulb onions, salad (or spring) onions and pickling onions, can be grown anywhere in Britain but uncertain weather can make harvesting difficult. Every year Britain imports over 100,000 tons of bulb onions from all over the world. With modern cultivars and an understanding of storage requirements there is no reason, however, why you cannot have your own bulb onions in all but the early to midsummer period. Most commercial crops of bulb onions in this country are grown from seed which is sown directly outside in the early spring. Small, partially developed bulbs or sets are used on a limited scale and are particularly useful in the colder, wetter areas.

Seed may also be sown under protection in the New Year and the resultant plants transplanted in the spring. Sowing onions outdoors in the autumn—to produce an earlier maturing and ripening crop of bulbs the following summer—is an established practice. The danger here however, is that, after the winter cold, the young plants will immediately flower rather than grow on to produce a bulb. Cultivars of Japanese onions have recently become available which can safely be sown in the autumn and which then produce an early crop of good quality bulbs in July.

Closely-spaced sowings of special, quick growing cultivars should be made in the spring if you wish to grow your own pickling onions. Other cultivars have been developed for use as salad—or spring—onions. Once again these are grown very close together to restrict bulb development. If pickling and salad onion cultivars are grown at bulb onion spacings then good sized bulbs will develop but they will not keep in store.

The growth of spring sown bulb onions in this country is largely influenced by day-length. The young plants continue to produce leaves until the day length reaches about 15 hours, when the bases of those leaves begin to swell to form the familiar bulb. To produce large bulbs you must, therefore, make sure that the plants have as many leaves as possible at this ‘switch-over’ point in their growth. This will occur, depending on location and cultivar, in late

Japanese F1 hybrids such as ‘Express Yellow’ are only for autumn sowing May or early June and final bulb size will then depend on the plant’s ability to produce food and store it in the leaf bases.

Soil requirements

In traditional kitchen gardens, the onions usually had a permanent site and this onion bed was heavily manured each year in attempts to maintain healthy bulb growth. There are considerable dangers in this non-rotational practice—mainly from soil-borne pests and diseases. Onions must have a long leaf-producing season so choose your site with this in mind. Onions, in common with other Alliums, do not tolerate acid soil conditions. Those crops (bulb or salad onions) which are sown in the autumn and over-wintered must be grown on well-drained, ‘warm’ soils so that winter losses are minimized. For the same reason it is better to grow these crops after a well-manured summer vegetable, such as a brassica or potatoes, and to withhold dressings of inorganic fertilizer until growth begins again in the spring. Too much soft, lush growth in the autumn can lead to considerable crop losses if severe weather follows. Prepare the ground for spring-sown which should be fine, level and firm. Onion seedlings take a long time to emerge after sowing. During this time fine-textured soils—like silts and clays—may develop a crust, or ‘cap’. Emergence of the seedling then becomes more difficult so don’t prepare an ultra-fine tilth on soils which ‘cap’.

Seed sowing and plant raising

Open-ground sowings of Japanese cultivars should be made from the middle to the end are large enough. Gradually harden the plants off from late March onwards prior to planting outside in mid-April. Space the plants 7cm (3in) apart in rows which are 30 to 40cm (12 to 15in) apart. Outdoor sowings in the spring should be made as soon as a seedbed can be prepared. Mid-February is the ideal time, the spacings being similar to those used in the autumn. Seed can be sown more thinly in the rows, however, since the or planted (sets or plants) crops in the autumn and leave it in a roughly dug condition over winter for a crumbly tilth to develop. Rich textured soils are ideal for this purpose but even the heaviest soils can be used if you prepare them carefully.

Suitable fertilizers

All the fertilizer for spring sown/planted onions is applied in the base dressing. Work it into the soil during the preparation of the seedbed. Fertilizers with too much nitrogen will encourage soft, sappy growth and the production of bulbs which do not store well. Try to use a material with nitrogen and potassium in the ratio of 1:1 and apply it at the rate of 50 to 70g per m2 (2 to 3oz per sq yd).

Pickling and successional crops of summer salad onions are not for storing so use an equally balanced fertilizer to encourage crisp, rapid growth. The final operation before sowing is to prepare the seedbed of August. This will ensure that the plants are the right size to overwinter because if they are too large they are more likely to ‘bolt’ and to be damaged by frost. When growth begins again in the spring the plants should be 5 to 7cm (2 to 3in) apart in the row but the initial sowing will have to be closer than this to allow for germination losses and winter die-back. The drills should be 1.0 to 1.5cm (1/2 to 1-1/4in) deep with the rows 35 to 40cm (12 to 15in) apart. Overwintered bulb onions will be ready for harvesting 6 to 8 weeks before spring sown crops. On no account sow Japanese cultivars in the spring.

A sowing of bulb onions can be made under slightly heated glass (10°C/50°F) in January. Sow the seed in seedpans or trays and prick out into seedtrays or into individual containers—allowing 25cm2 (10 sq in) per plant—as soon as the seedlings danger of winter loss does not apply. Thinnings from autumn or spring sown bulb onion crops can be eaten in salads.

Onion sets are bulbs which have been partially grown in the previous season, then lifted and dried, and finally kept in temperature controlled stores so that the small bulb continues to grow vegetatively when it is replanted rather than running to seed. Growing onions from sets allows less suitable soils to be used and, since some of the growing has been done already, they are good for districts with shorter growing periods. Sets are planted 5 to 7cm (2 to 3in) apart in drills, which are themselves 30 to 40cm (12 to 15in) apart, with only the necks of the small bulbs protruding above the soil. Plant them firmly; even then some will be disturbed by birds, earthworms or frost. Replant them immediately. March or April is the best time for planting onion sets.

If you want your own salad onions at Easter then sow the seed in late August in much the same way as for overwintered bulb onions. This time, however, we require young, immature plants with no bulb so space the rows very closely. If the seed is sown less than half an inch apart no thinning will be needed in the spring. Salad onions in the summer are obtained from successional sowings made outside from February or March onwards. The overwintered crop will probably carry on until late May. The earliest spring sowings be ready in June.

Silverskin onion cultivars for pickling should be sown outside in April and grown rapidly to produce small, crisp and succulent bulbs. Seed may either be broadcast or drilled in rows which are 15 to 20cm (6 to 8in) apart. Sow them very closely to make sure that the individual bulbs remain small.

Crop management

There must be no check to the growth of onion plants. Weeds are a major form of competition especially when the onions are small so hoe them out with a short-handled onion hoe. Be careful not to damage the developing bulbs or to pull soil up around them as this will slow down ripening. It is not usual to water bulb onions but it may be necessary in dry seasons until ripening begins. Salad onions may need watering to keep them growing rapidly.

Typical distortion (above) caused by eelworms feeding on young onion plant In August (June for autumn sown crops; July for picklers) the leaves of bulb onions will turn yellow and topple over. This marks the start of the ripening process which will be hastened if the leaves are folded over neatly. The leaves of thick-necked (’bull-necked’) onions remain standing and the bulbs from these plants must be separated from the rest since they do not store well.

Harvesting, ripening and storage

The condition of the bulbs at harvest time and the efficiency of the ripening process will largely determine how effectively they will be stored. When the leaves have become brittle choose a warm, sunny day on which carefully to lift the bulbs. Lay them out in the sun to continue drying and turn them frequently to prevent damp patches developing. Warm conditions are needed to ripen the outer scales of the bulb, seal the neck and produce a rich skin colour and finish.

In wet autumns it means that onions will have to be ripened under protection. It may take 4 weeks to ripen the bulbs fully. Unless you do this carefully they may rot during storage or ‘sprout’ early in the spring. Use ‘bull-necked’ onions immediately and do not attempt to store damaged or soft bulbs. For storage they can be made into ropes or, alternatively, can be kept in wire-bottomed, wooden trays with corner posts to allow air to circulate. Keep bulbs in a cool, dry, frost-free place and inspect them regularly. All being well they can be kept until the following Spring.

Pull salad onions as needed. Remember that large-bulbed plants are usually very hot! Pickling onions will be ready in July. Pull them up when the tops begin to shrivel and pickle them as soon as possible.

Although seed can be saved from non-F1 hybrid cultivars of onion it is often difficult to dry and ripen in Britain.

Pests and diseases

Onion crops need to be rotated to prevent a build-up of stem eelworm in the onion bed. These microscopic pests can remain in the soil for many years and will not be content only to attack the onion family. Infested plants take on a ‘bloated’, twisted appearance and produce bulbs which quickly go rotten in store. Make sure that you follow a carefully programmed crop rotation and don’t buy untested seed.

Onion fly is the other major pest of onions. Like cabbage rootfly on brassicas, its eggs are laid near to the onion plants—particularly those growing on light land—and the resultant larvae tunnel into the young plants and, eventually, into mature bulbs.

White rot is a serious disease of onion and related crops since it, too, is soil-borne and remains viable for several years. The leaves go yellow, the roots rot and the base of the plant becomes covered with a white fungal growth. Crop rotation is the only satisfactory control.

Downy mildew will cause the leaves to go grey and topple over while they develop a purple coloration in wet weather.

Onion smut fungus produces black, streak-like pustules on the leaves which split open to release clouds of soot-like spores. Fortunately smut is not very common, and grey mould (Botrytis) only infects previously damaged tissues. It sometimes appears on overwintered crops following frost damage.

Neck rot—caused by another species of Botrytis—causes bulbs to rot in store. The fungal spores are taken into store in the necks of onions which are badly sealed. This fungus is often on the seed at sowing time so—once again—clean, reliable seed must be used. A number of other rots may occur in store, caused by bacteria taken in on the bulbs. Damaged bulbs are attacked and quickly degenerate into a soft, evil-smelling mass which acts as a centre of infection for other bulbs.

Suitable cultivars

Dry’ bulb (Autumnsown) ‘Express Yellow’: F1 hybrid, Japanese; flattish bulbs. ‘Imai Yellow’: Japanese; pale yellow and globular. ‘Reliance’: flat bulbs; keeps well. ‘Solidity’: large flat bulbs; less likely to ‘bolt’. (Spring sown) ‘Hygro’: F1 hybrid; Dutch type which keeps well. ‘Wijbo’: Golden brown; ball-shaped; heavy cropper. ‘Bedfordshire Champion’: popular; mild flavour. ‘Ailsa Craig’: well known for exhibition work.

Onion sets ‘Stuttgarter Giant’: resists ‘bolting’, keeps well. ‘Sturon’: new earlier cultivar.

Salad onions ‘White Lisbon’: best known. ‘Winter Hardy’: hardier strain of ‘White Lisbon’.

Pickling onions ‘Paris Silverskin’: ripens early. ‘The Queen’ small, very early cultivar.

Unusual onions

A number of other onions can be grown in British gardens and, while they are not of great culinary value, they do add considerable interest.

Egyptian onion – Allium cepa aggregatum

This perennial, clustering onion can be grown for its shallot-like clusters of bulbils which, in this case, are produced at the top of 20-cm (8-in) high stems. The bulbils are small and of little value but the young plants may be used as a substitute for salad onions. The plant is vegetatively propagated either from bulbils or by division. The former are planted in August or September on well-drained soil at a spacing of 30cm (12in) square. More bulbils will then be ready in the following summer/autumn. Established clumps can be divided and replanted in March.

Potato onion – Allium cepa aggregatum

Another form of the ordinary bulb onion. It produces a collection of small, shallot-like bulbs just below the soil surface.

Welsh onion – Allium fistulosum—Ever ready onion

This onion has no connection with Wales— in fact it is the major onion of importance in Japan and China—but can easily be grown in Britain. It is a perfectly hardy, multi-stemmed perennial which can be used either as a substitute for salad onions or for flavouring purposes in winter. Plants grow as clumps and reach a height of up to 30cm (12in). Propagation is by division with the pieces planted 25cm (10in) apart in each direction.

Shallots – Allium ascalonicum

Not all botanists accept that the shallot is a distinct Allium species—many regard it as another type of bulb onion (Allium cepa aggregatum). It is an ancient, hardy perennial native to the near East. The small bulbs (3 to 5cm (1 to 2in) diameter) which have a milder flavour than onions, are produced in clumps and are commonly used either as pickling onions or to give flavouring. Shallots are usually propagated vegetatively by saving the best bulbs from the previous season’s crop. The old saying—’Plant on the shortest day to lift on the longest day’— provides a useful guide to growing shallots.

Soil and fertilizer requirements

Shallots require the same soil conditions as bulb onions. If they follow a well-manured crop no fertilizer need be given unless very large bulbs are wanted. Planting and crop management It is rarely possible strictly to follow the old saying but shallots should at least be planted as early as possible in February since the bulbs go soft if they are stored much longer. Plant in the same way as onion sets with the bulbs 25cm (10in) apart in rows spaced 30cm (12in) apart.

Shallot bulbs, too, may be pushed or pulled out of the soil and will need replanting. Only plant firm, undamaged bulbs and make sure that weeds are kept down by constant hoeing. Be very careful, however, not to damage the developing bulbs during hoeing. As the bulbs begin to dry off in June or July it will help if you gently remove the soil from around the clumps of bulbs.

Harvesting, ripening and storage

Shallots are usually easier to ripen than bulb onions since they mature in the warmer, sunnier conditions of June or July. The leaves will turn yellow and wither when growth has finished. At this point, lift the shallots carefully and spread them out in the sun to dry for a few days. If the weather is wet then dry the bulbs under protection. When required for pickling the shallots will be used immediately but if they are to be stored for winter use then follow the same rules as for bulb onions. The best bulbs should be saved and planted for next year’s crop of shallots.

Pests and diseases

These are exactly the same as described for bulb onions.

Suitable cultivars

Both yellow and red-skinned forms are available: ‘Long Keeping Yellow’, ‘Dutch Yellow’, ‘Red Dutch’, ‘Hative de Niort’.

27. April 2013 by admin
Categories: Tips and Advice | Comments Off on Growing Onions


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