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How to grow organic Onions
by Frann Leach
Do you know your onions?
Family: Liliaceae (Group 6)
Onions are an essential ingredient in many recipes, although it is quite likely that the onions in your local supermarket are in fact shallots, which may taste the same, but grow in groups, instead of singly like the true onion. You can grow both organically. This page only gives cultivation data for onions proper, though. Shallots are dealt with on a separate page.
Onions may be divided into 3 groups: those grown from sets, those grown from seed sown in early Spring, and Autumn-sown varieties. Onions grown for storage must be of spring-sown or spring-planted types.
Onions require good, free draining soil with a high organic content. If you are growing from seed, you need to prepare an onion bed, basically similar to a seedbed in that the soil must be very fine in texture, but with much higher organic content. The soil should be dug over in early winter, incorporating plenty of manure or compost. Leave the ground rough so that the soil may be broken up by the frost. In spring, add lime if necessary and rake the soil flat, firming well. Leave undisturbed for at least 10 days to become 'stale': onion flies are attracted by freshly disturbed soil, but will not lay eggs unless there are onions for their larvae to feed on.
The earlier these are sown, the larger the final crop. Seed should be sown in January indoors at about 50-60ª F (10-16ª C). Either sow in seedtrays and prick out or in modules. Harden off and plant out in March or April.
Ultimate size is determined by the amount of leaf produced by 21 June (Midsummer's day), from which point no more leaf grows, but the bulbs start to swell. Transplants should be kept well weeded so that they receive maximum light and available nutrients.
Sets are immature bulbs. The advantages of growing from sets instead of seed are that they are easier to grow, less prone to disease, more tolerant of poor soil conditions and often escape onion fly attacks. They also mature earlier. The disadvantages are that they are more expensive, there is a smaller choice of cultivars, and they have a greater tendency to bolt. However, the risk of bolting is reduced by selecting smaller sets(!), and by using sets that have been heat-treated.
|North||2nd week of August|
|Midlands/East||3rd week of August|
|South||4th week of August|
Autumn-sown varieties are mainly Japanese cultivars, for harvesting from June to August for immediate use only. Sow seeds 2.5cm (1") apart in 30cm (12") rows. Timing is critical:
In spring, thin seedlings to 5cm (2") apart and give a top dressing of seaweed meal or a foliar feed.*
Specially selected for their hardiness, they need a well-drained soil to succeed. Plant any time from September to November, 10-15cm (4-6") apart in 30-45cm (12-16") rows.*
* Harvest Autumn-sown and Autumn-planted onions from June to August. Cannot be stored.
Pickling onions (spring-sown)
Special cultivars are available, but it is possible to produce onions of a suitable size for pickling by sowing any of the normal onions (except giant exhibition types), by growing them close together. The competition will keep them small. Either sow 1cm (¼") apart in 30cm (12") rows or broadcast in 24xm (9") bands and leave unthinned.
|Spring seeds:||Ailsa Craig, Bedfordshire Champion, Hygro, Red Baron, Owa|
|Spring sets:||Stuttgarter Giant, Sturon, Balstora, Red Baron, Centurion, Hyduro, Showmaster|
|Autumn seeds:||Express Yellow, Buffalo, Imai Early Yellow, Senshyu Semi-Glove Yellow|
|Autumn sets:||Unwins First Early, Sutton's Early Crop, Shakespeare, Radar|
|Picklers:||Paris Silver Skin, Barletto|
Sets and transplants should be planted in 22cm (9") rows, each plant 10cm (4") apart, or 15cm each way (6"x6") if equidistant spacing is preferred. Sets can be planted from February to April, though planting of heat treated sets should be delayed until the end of March. Push sets into the ground so that the tip protrudes very slightly and provide protection against birds, e.g. A netting 'cloche'.
Watering is generally unnecessary except in very dry weather once plants are established, and should be avoided after mid July, or maturity and keeping quality may be reduced.
Bulbs can be pulled for use fresh at any stage. They are ready to harvest for storage once the leaves start to die back and the tops bend over naturally. Lift the bulbs and leave them on the surface in the sun for a few days, turning them over occasionally to dry. In bad weather, they can be dried on mats stretched across a frame indoors, in single layers. After 7-21 days, inspect the bulbs, removing all soft bulbs, those with thickened necks, spots or damage for immediate use.
Autumn-sown seed and autumn planted sets do not produce onions suitable for storage
Store in strings, net bags or on wire trays.
|Spring-sown seed||15cm x 15cm (6"x6")||Indoors Jan-Feb||Mar-Apr||July-Sept|
|Spring sets - standard||-||Feb-Apr||August|
|Spring sets - heat-treated||-||end Mar-Apr||August|
|Direct sown (spring)||4cm x 30cm (1½"x12")||March direct||-||August|
|Autumn sown seed:||2.5cm x 30cm (1"x12")||direct:||Thin to 5cm (2") in spring||June-Aug|
|North of UK||2nd week Aug|
|Midlands/East||3rd week Aug|
|South||4th week Aug|
|Autumn sets||10-15cm x 30-40cm (4-6"x12-16")||-||Sept-Nov||June-July|
|Picklers||Broadcast||March direct||Do not thin||August|
Pests, Diseases and Disorders
The most serious pest is onion fly, particularly on dry soils. The fly is attracted to freshly disturbed soil to lay its eggs. It can be deterred by sowing in a stale seed bed: i.e. a seedbed prepared 10 days in advance of sowing. This is particularly important for August sowings. It is much less likely to attack sets than seed-grown plants. Crops can also be grown under fine nets.
Stem and bulb eelworm can also be serious on infected soils. The best way to deal with this pest is rotation, growing brassicas or lettuces for two or three years. Keep the ground weed-free, as some common weeds harbour these pests.
Downy mildew, neck rot and white rot are the most common diseases. Of these, white rot is the most serious, worst in hot dry summers. Symptoms are yellow, wilting foliage and fluffy white mould on the base of the bulbs, in which round black fruiting bodies appear. Affected plants should be lifted and burnt. No alliums should be grown on infected land for at least 8 years.