Onion (Under development)

Introduction

Onion is a biennial vegetable grown in temperate zones as an annual. Onion (Allium cepa) is classified within the family Amaryllidaceae and the genus Allium. The genus is one of about fifty-seven genera of flowering plants under the family with more than 500 species. Other significant species within the genus Allium include: Garlic (A. sativum), Green onion or scallion (Allium fistulosum), Leek (A. porrum), and Chives (A. schoenoprasum).
Allium cepa is believed to have originated in Central Asia (between Turkmenistan and Afghanistan). This region is notable as it is home to some of its wild relatives to this day. Its cultivation has traversed the globe, adapting to varied climates and soils. Flourishing as a cool-season crop, it predominantly thrives in temperate regions.
Onions boast diverse and widespread utilization across various domains: In general, onions are used for salads (bunching onion or sliced full-grown bulbs for use as topping for sandwiches, burgers, and hotdogs etc ), pickling (e.g. silver skin onions), cooking (add flavor, aroma, and texture to a wide array of dishes, including soups, stews, salads, curries etc.). Onions have been historically recognized for their potential health benefits. They contain bioactive compounds like quercetin, which possess antioxidant and anti-inflammatory properties. These properties may contribute to their use in traditional medicine (e.g. as a diuretic, alleviating coughs, treating colds and infections, and making poultices for wounds and insect bites). The pungent smell of onions has been used to deter insects and pests in gardens and households. 
(Messiaen & Rouamba., 2004,PROSEA, 2016,Plantvillage, (n.d)
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. 

Species account

A. cepa is a biennial plant grown in temperate zones as an annual (harvested after one growing season). The plants can grow to a height of 50 cm with adventitious and fibrous roots. Stems: flattened disc with tubular leaves that form a pseudo stem where their sheaths overlap. Leaves: 3-8 leaves per plant that are either erect or oblique. Flowers: pink or white flowers cluster on its stalks. Bulb: made up of layers, each of which corresponds to a leaf, generally oval, but their shape can vary globose, ovoid or elongate and its size varies depending on the cultivar. The outer leaf base dries and becomes thin and variously colored, forming the protective coat, while the inner leaf bases thicken when the bulb develops.
(Messiaen & Rouamba., 2004, PROSEA, 2016, Plantvillage, (n.d)
Variation in onion
The extensive diversity present in A. cepa has resulted in many suggestions for subgroup classifications, leading to a certain level of confusion in its taxonomy. Currently, the commonly followed informal classification divides the species into two cultivar groups:

1.    Allium cepa var. cepa. (Allium Cepa (Cepa Group). This is the common what is commonly referred to simply as onions. This features large, typically solitary bulbs. These plants reproduce through seeds or by using seed-grown bulbils (sets). The vast majority of A. cepa belong to this group and include yellow onions, red onions, white onions and green onions. White onions have a white flesh with a sharper and more pungent flavor than yellow onions. Green onions are onions harvested while their shoots are still young and sold with the shoots attached. They have a mild flavor hence often used as a garnish. Scallions are green onions. Spring onions are also immature onions. They have a white, round bulb with long stems, and are also sold in bunches.

2.    Allium cepa var. aggregatum( Allium cepa (Aggregatum Group) are the Shallots. In this group, bulbs are smaller and cluster together, forming several to many aggregated bulbs. The group includes shallots, which produce a cluster of mainly small pointed bulbs (similar to garlic) and potato onions. Reproduction occurs vegetatively through lateral bulbs (daughter bulbs). Shallots have a rather mild garlic flavor. Their skin is pale brown to copper. Leaves are tubular and long.
 
3. Chives (Allium schoenoprasum)
Chives should not be confused with green onions. They are smaller, thinner, and more delicate than green onions, Compared to green onions, chives have a much milder onion flavor. The bulbs are slender and grow in dense clusters from the roots. The stems are long and thin, hollow and tubular with a soft texture.
 

Nutritive Value per 100 g of edible Portion

*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 in 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.