Onion

Scientific Name: 

Allium cepa

Order / Family: 
Aspargales: Alliaceae
Local Names: 
Kitunguu, (Swahili), Gitunguru (Kikuyu)
Other pests: Botrytis leaf blight, Sedges

Geographical Distribution of Onion in Africa

 

General Information and Agronomic Aspects

Onion is a biennial vegetable grown in temperate zones as an annual. In the tropics the varieties that do well are in effect annuals as they can produce seed within the first year of growing. Nutrient-wise 100 g of onion provides about 30 g calcium, 0.5 mg of iron, vitamin B, 0.2 mg of riboflavin, 0.3 mg nicotinamide, and 10 mg ascorbic acid (vitamin C).

In general, onions are used for salads (bunching onion or sliced full-grown bulbs), pickling (e.g. silver skin onions), cooking (such as in soups) and frying (for example, with meat). Onions are particularly suited to smallholder farming in most countries. It also plays an important role in traditional medicine (e.g. as a diuretic). In the tropics onions can be grown year round where irrigation is possible. 

Nutritive Value per 100 g of edible Portion

Raw or Cooked Onion Food
Energy
(Calories / %Daily Value*)
Carbohydrates
(g / %DV)
Fat
(g / %DV)
Protein
(g / %DV)
Calcium
(g / %DV)
Phosphorus
(mg / %DV)
Iron
(mg / %DV)
Potassium
(mg / %DV)
Vitamin A
(I.U)
Vitamin C
(I.U)
Vitamin B 6
(I.U)
Vitamin B 12
(I.U)
Thiamine
(mg / %DV)
Riboflavin
(mg / %DV)
Ash
(g / %DV)
Onion raw 40.0 / 2% 9.3 / 3% 0.1 / 0% 1.1 / 2% 23.0 / 2% 29.0 / 3% 0.2 / 1% 146 / 4% 2.0 IU / 0% 7.4 / 12% 0.1 / 6% 0.0 / 0% 0.0 / 3% 0.0 / 2% 0.4
Yellow Onions sauteed 132.0 / 7% 7.9 / 3% 10.8 / 17% 0.9 / 2% 20.0 / 2% 33.0 / 3% 0.3 / 1% 133.0 / 4% - 1.8 / 3% 0.2 / 10% - 0.0 / 3% 0.0 / 2% 0.4
Young Onions, Tops only 25.0 / 1% 5.6 / 2% 0.1 / 0% 1.8 / 4% 61.0 / 6% 33.0 / 3% 1.9 / 11% 260 / 7% 4000 IU / 80% 45.6 / 76% 0.1 / 3% 0.0 / 0% 0.1 / 5% 0.1 / 8% 0.7

*Percent Daily Values (DV) are based on a 2000 calorie diet. Your daily values may be higher or lower, depending on your calorie needs. 

 

Climate conditions, soil and water management

In temperate zones onion is cool-season biennial, and is tolerant to frost. They produce bulbs with growing day lengths. Optimum temperatures for plant development are between 13 and 24°C, although the range for seedling growth is narrow, between 20 and 25°C. High temperatures favour bulbing and curing. In the tropics only short day or day neutral onion varieties will form bulbs. These thrive in warm to hot climates of 15-30°C. If the temperature greatly exceeds that required for bulbing, maturity is hastened and bulbs do not grow to maximum size, consequently lowering the yields. 

Onions can be grown on any fertile, well-drained, non-crusting soil. The optimum pH range is 6.0 to 6.8, although alkaline soils are also suitable. Onions do not grow well in soils below pH 6.0. On light sandy soils irrigation is necessary. Irrigation could be either overhead or on drip. Onions at the bulbing stage need a substantial amount of water, but excessive moisture must be avoided during the growing season. Avoid application of fresh manure to the crop, as this will cause the plants to develop thick necks and too much leaf at the expense of bulb formation.

Propagation and planting

Prior to planting, soils should be ploughed and disked sufficiently to eliminate debris and soil clods. In most commercial areas, beds 0.9 to 1.0 m wide are common, and 2 to 6 rows are seeded or planted on the bed. If two rows, they may be two-line (twin) rows with plants staggered to achieve proper spacing and high population density. 

Proper seed selection is recommended to minimise problems of splits and doubles. Over-fertilisation, uneven watering, and temperature fluctuations also influence bulb formation. Onion is propagated by seed (most common in the tropics) or sets (immature bulbs ripened during the previous season - in temperate zones). 

Onion nursery
© A.A.Seif, icipe

 

In the tropics the seed is usually sown in a nursery under a mulch cover. In the nursery prepare raised beds maximum 1 m wide and incorporate plenty of well-decomposed compost as well as additional rock phosphate. Make rows about 15 cm apart, sow the seeds and cover lightly with soil and mulch. Irrigate liberally for the first 10 days. Seed rate is 2-3 kg per ha. After the seed emerges, the mulch is removed. About 6-8 weeks after sowing, when the seedling has a base as thick as a pencil and is approximately 15 cm tall, the seedlings are transplanted to the field. 

The ultimate yield of onion is determined by the number of leaves that are formed prior to bulbing
 

 

 

 

 

Common varieties grown in Kenya (short day or day neutral varieties)

  • 'Red Creole'. This is a popular standard variety in high demand because of its good keeping quality. It produces mainly single onions from transplants, red, flat-round and with a pungent taste.
  • 'Red Tropicana': Red bulbing type
  • 'Red Tropicana F1 Hybrid'. Produces large, red, thick flat onions with firm pungent flesh. It is highly productive and therefore demands high levels of management. It keeps well in dry aerated store.
  • 'Bombay Red'. It is a variety for dry and warmer conditions. It is small to medium sized, globe shaped, purplish red and pungent.
  • 'Yellow Granex FI Hybrid'. This is an early maturing high yielding attractive, thick flat onion with thin yellow scales. The flesh is medium firm, crisp and mild in flavor. The shape and size is uniform leading to higher market prices, and the storage quality is good.
  • 'Texas early Grano'. This is a fresh market, early maturing variety (100 - 120 days) with a rather short shelf life. It is yellowish, mild and not very pungent. The bulbs are high top shaped with dry yellow scales. It is a heavy yielder for high altitude regions. Ideal for fish salads.
  • 'White Creole'. This is a white variety normally used for dehydration.
  • 'Green bunching': Non-bulbing spring onion. It has attractive tasty dark green leaves. It is an early and highly productive onion grown for stems rather bulbs. It is tolerant to sun scotch and it is recommended for salads and fresh market.

When buying seed and not recognizing the variety name as one of the above, ask if it has been grown in Africa before. If not, better stick to a known variety in order not to lose the whole production. 

Planting systems
  • Nursery seeding and transplanting is the most common and practical option in the tropics. Transplants normally have 3 to 5 well-formed leaves at transplant time. Roots are pruned during planting, in order not to be bent upwards when transferred to the field. This facilitates early establishment of the plant.
  • Any germinated bulb of above mentioned varieties would produce 3-6 good size bulbs in about 3 months when planted with the rains. Choose only healthy bulbs for propagation.
  • Sets are used in some areas in the temperate zones to ensure large bulb size and uniform maturity. Sets are small dry bulbs, approximately 12 mm in diameter, which have been produced the previous season by seeding thickly or growing under conditions that favour rapid bulbing.
  • Direct seedling is possible and gives excellent results where herbicides can be used and the season is sufficiently long to provide early pre-bulbing growth. In the tropics this method is impractical due to enormous weeding costs in an organic system.
Husbandry

Do not plant onions after the field has been planted with other Allium plants (e.g. garlic). Mulching onions with composted leaves and straw is highly recommended to maintain soil organic content, prevent soil-borne diseases, and suppress weeds. Planting onions in raised beds improves drainage and prevents damping-off diseases. 

Weeding and harvesting are mostly done by hand, although chemical weed control is possible but not organic. Crop rotation is important to avoid the build-up of pests and diseases such as nematodes, Sclerotium and Fusarium.

Nutrient management 
Onions respond very well to well decomposed organic manure. Organic manure at 25 to 40t/ha is recommended to obtain high bulb yield. 

Harvesting

Harvesting takes place 90-150 days after sowing. Onions are ready for harvest when the leaves collapse. Alternatively the leaves can be bent over and left to dry for 10-12 days. The crop is pulled out by hand and kept for some days in the field with the bulbs covered by the leaves (= windrowing). The leaves are then cut off and the mature bulbs are bagged or packed in crates if they are to be stored. 

Freshly harvested onions are dormant and will not sprout for a variable period of time (this depends on the variety). Storage will extend the dormant period. Sprouting will increase in storage temperatures above 4.4°C. It will decrease again as temperatures exceed 25°C. 

Fresh Quality Specifications for the Market in Kenya

The following specifications constitute raw material purchasing requirements
 
© S. Kahumbu, Kenya
 

 

 

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Last Updated on:
Tuesday, March 7, 2017 - 17:24
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