1. PRODUCTION OF ONIONS
Introduction
Onion (Allium cepa) is the main vegetable crop in the genus Allium, with other alliums like
Garlic, Leeks, Shallots and Chives. Some species are grown for food, while others are used
for ornamental and medicinal purposes. Onions are hardy, cool season crop which produce
prominent bulbs, and characterized by a unique onion-like flavor or pungency.
Onions are of economic important as it is used as a food in salads or cooked, where it
conditioning or flavouring food, can also be used in soups and sauces, and small bulbs are
pickled brine or vinegar.
The crop is also claimed to have many medicinal uses like treatments of atherosclerosis,
coronary heart disease, diabetes, cancer and asthma. It can also be used to relive insect bites
and sore throat. Generally; onions have many uses, they may be eaten raw, boiled, baked,
creamed, steamed, fried and pickled. Onions can also be dried or dehydrated and shredded
into flakes.
History, Classification and Origin
The cultivated onions (Allium cepa), originates and is thought to have been domesticated in
the mountainous regions of central and southern Asia, including Afghanistan, north Iran,
Pakistan and southern china. It entered Europe by way of Italy, through North Africa, The
crop introduced to United States by early Spanish explorers, and records of the first
cultivation of the onions date back to around 1625. Onions are a major contributor to U.S.
diet.
Classification
Onions belong into a group (Allium genus) consisting of more than 500 species. The botanical
classification is as follows;
Class: Angiospermae
Sub class: Monocotyledoneae
Family: Alliaceae
Genus: Allium
Spicies: cepa
The botanical name of onions is Allium cepa.
Production and Industry
Onions are grown and marketed as two main types; Dry bulbs onions and Green bunching
onions. Dry bulb onions are mature bulbs that are usually harvested after the tops have died,
i.e about 90% of leaves have fallen down. Green bunching onions are pulled in the immature
stage, when the tops are still green but before the bulbs are enlarged.
Worldwide, the major nations producing both of the types are China, India, Russia, Spain,
Greece, and Turkey. U.S. production accounts for approximately 7% of the total world crop.
For Tanzania, onions are cultivated almost everywhere, but mostly in Singida, Morogoro,
Iringa, Mbeya, Tanga, Dodoma and Mara.
2. Plant Growth and Development
Onion is a biennial vegetable crop belongs into sub class of monocot. The crop usually
develops a bulb which consists of a vegetative stem axis and bases of concentric and
vegetative leaves. In the first season the crop produce a bulb and after being exposed to cool
temperature, develops a seed stalk on the second season.
The onion seed consists of seed coat, endosperm and embryo, during germination the
cotyledon elongates and pulls the embryo through the seed coat. Onion has shallow roots,
most occurring within 15-20cm on the soil depth.
All roots are adventitious after the seedling stage. Seedlings however, initially produce a
primary root. Onion roots are short-lived and are continuously been produced as old roots dies
(about 3-4 new roots every week), as bulbs mature, new roots are formed at a slower rate than
the old die.
The leaves are arises from the underground stem and have long sheathing bases. Leaves are
usually shallow and cylindrical in shape. The stem remains plate-like structure during the
vegetative stage, but will elongate and develop a flower stalk when the reproductive stage is
initiated.
During the leaf formation, the inner leaf swell, forming the bulb, while the outer leaf bases
remain thin, as the bulb matures, the outermost leaf bases become dry and papery, forming a
protective layer around the bulb.
Bulbing in the onions is determined by the interaction of photoperiod and favorable
temperature for bulb induction. Bulbing occurs when there is a change in leaf morphology
when sufficient exposure to a critical day length is exceeded. Buibing is associated with
acceleration of growth of leaf sheaths at the expense of leaf blades which during earlier
growth stage they grow at a more rapid rate. Bulb is formed at a strong photoperiodic
stimulus, even a one-leaf plant can bulb. Generally; onions bulb quicker at warmer than at
cooler temperatures, Increase temperatures in the range of 10-27o
c will hasten bulbing,
provided day length is favorable. Cold temperature below - 4o
c may inhibit bulbing and result
in mature plants with very thick and elongated necks.
Bolting: This refers to the production of seed stalks for reproductive growth (emergence of
reproductive shoots) It most commonly occurs under vernalization stimulus. A period of
exposure to 5-10o
c for 1-2 months in onion is adequate for vernalization of many cultivars.
Agro ecological climate
Temperature
Onion is a cool season crop. It has some frost tolerance but best adapted to temperatures
between 13-24o
c. The crop needs mildly cold climate for vegetative growth; and warm
temperatures during bulb formation (20-27o
c). Growth is less vigorous when night
temperature is above 20o
c and it is drastically slowed down at temperature above 30o
c.
Altitude
Onions grows best at elevation between 800- 1500m a.s.l.
Light
The crop is also sensitive to day-length; some of the cultivar can not be grown to develop
bulbs in day-length less than 14 hours.
Rainfall
Onion requires uniform moisture throughout the growing period. Over the growing season,
the crop needs about 400-800mm of moisture.
3. Soil
Good onion soil should be friable, fertile, well drained. Good soil range from sands to heavy
clay loams. The crop is sensitive to water logging, high water table and also water stress. Soils
with high holding capacity are best. The soil must be sufficiently structured. Favorable pH for
onion is about 6.5-8 in mineral soils and 5.8 in peat (organic soil). Even low pH can be
tolerated in organic soils. Onions are very sensitive to soil salinity.
Planting and crop establishment
Propagation
Propagation of onions can be direct sowing with use of transplants or set.
Direct sowing
This can be least costly practice for bulb and especially for green onion production. Direct
sowing must aim at a spacing of 40- 80 plants per m2
.
Transplanting
This is the most reliable method of propagation worldwide. This method is especially suitable
in areas where growth periods are too short to permit production from direct seeding.
Seedlings are produced in nursery and transplanted after 8-12 weeks (can be ready for
transplanting after 56 days, in tropics after 7-9 weeks (45- 60days) or when stem diameter is
3-4mm. Most favorable condition for transplant growth is 17o
c day temperature and 10o
c
night temperature. Onion is most often handled as bare root transplants.
Propagation using sets
Sets are small bulbs, usually 2-3g fresh weight, produced by growing a crop from seed sown
at very high density (1000-2000) plants per meter. These small bulbs are usually less than
25mm diameter (mostly 15-20mm).
Spacing and Plant population
Bulbs size and shape are affected by the population density, particularly the spacing within
the rows. Spacing between plants to plant is 8-10cm, while the spacing between rows is 30cm.
Cultural Practices
Land Preparation
Onions can be planted on raised beds, sunken beds for flood irrigation, or ridges. Soil must
have been cultivated deep enough (30cm). FYM is usually incorporated into the soil and well
mixed, about 20t/ha. The field should be prepared about a month prior to transplanting.
Weeding
Onions like other vegetables, can not withstand weed competition. Weeds before bulb
formation will reduce yields, although weeds may shade bulbs and keep onion from drying
quickly. Chemical weed control may be used extensively in commercial onion production.
Mechanical weeding should be very shallow to avoid root damage, because of the shallow
rooted crop like onions.
Irrigation
The root system of onion is shallow; therefore onions must receive a fairly constant supply of
water for good yields. For a good deep soil with moisture retention capacity will produce a
good crop without irrigation. For mineral soils, irrigation is required, together with in drier
growing areas.
4. Mulching
Mulching can be very important in onions especially when grown on raised beds. Various
materials can be effective as mulches; wood shavings, sawdust, Rice husk, Grass e.tc.
Mulches conserve moisture and suppress weeds.
Fertilization
Onions needs N, P, K, in addition, some micro-nutrients, for proper growth. Yield increases
with increasing application of N in the range of o-150kg N per ha, High N application should
be avoided, which may results on vegetative growth in thick necks, Multiple centers and even
bulb splitting. Onions have very low efficiency of utilizing N fertilizer; this is because of
shallow and sparse root system. Because of onion’s shallow root systems, low root densities
and lack of root hairs it needs high concentration of P and K in soil solution to diffuse into
root surface at a rate sufficient to satisfy the potential demand.
In Tanzania recommended application of N fertilizer is about 3 and 6 weeks after
transplanting as top-dressing. About 3 bags of SA or CAN three weeks and 4 bags per ha, 6
weeks after transplanting. The fertilizer should be applied in straight line along rows not to
touch the plant. Apply when the soil is moist, each plant needs about one teaspoonful
approximately 5g.
Cultivars
There is a very large of cultivars and landraces developed over centuries to diverse climates
and food preferences all over the world. The reason of great diversity is due to bulbs and
inflorescences developments are considered to have been closely adapted to temperature and
photoperiods that prevail where they are grown.
The distinguishing features of onion varieties include:- Foliage characteristics ( Foliage
colour, leaf length, leaf erectness), Bulbs characteristics (Shapes, colour, uniformity of bulb
shape and size), Inflorescence characteristics (Presence and absence of bulbils, presence and
absence of inflorescence)
Most commonly onion cultivars are classified into short day (11-12hrs), Intermediate day (13-
14hrs), and long day (>16hrs) cultivars.
§ Short day cultivars include Bermuda yellow, Red Creole and white Bermuda.
§ Intermediate cultivars include Hybrid like Pronto, Candy, and Rialto.
§ Long day cultivars include Redwing, Ontario, and Copra.
Diseases and Insects
Onions like other vegetable crop face various diseases and insects. The following are number
of diseases attack onions in the field
1. Neck rot
This is a common fungal disease (Botrytis sp.) of bulbs in storage, although losses may occur
in the field.
Control measures include proper curing and storing. Growers should make sure that, the bulbs
are not bruising during harvesting and handling.
2. Pink rot
It is a soil borne fungus (Pyrenochaetaterrestis) that may infect plant at any age. The disease
always affects the root and bulbs. Roots turn pink, and then shrivel and die. This will result in
failure of the plant to take up water and nutrients. Bulb size and yield are also reduced.
5. Control measures; use of disease free planting materials, planting should be done on the soil
free from disease, crop rotation, and use of cultivars with disease resistance.
3. Downy mildew (Peronospora destructor)
This attack on the foliar part of onions, then destruct in the direct seeded crop, Its symptoms
include white to purplish mold develops on older leaves, results into leaves dry up and drop
off. The disease is favored by cool and wet weather.
Control measures include the use of fungicide sprays every 7 to 10 days.
4. White rots (Sclerotium cepivorum)
This disease is mostly favored by high humidity, first appears as yellowing and wilting of
leaves. For the case of bulbs become soft and rotten, and eventually a white mold develops on
the bulbs. The infection always occurs in the field, but the symptoms may not appear until
storage. A control measure is a long crop rotation.
Insects
Onion thrips (Thrips tabaci) and onion maggots (Hylemya antique) are the most serious insect
pests of onions.
1. Onion thrips
These are similar to other thrips that cause damage by rasping the leaf surface and feeding on
the sap. In hot and dry conditions, the damage may be severe. The injury appears as white or
scorched areas on the leaves. Application of several insecticides can be effective way of
controlling thrips.
2. Onion maggots
These are larvae of a small fly that lays eggs near the base of the plant. The maggots enter
into stem and bulbs and may feed on the roots. The damage is normally more severe under
cooler temperatures. Soil insecticides, applied in the seed furrow or as a soil drench after
transplanting, will provide good control.
Harvesting and Post harvesting Handling
Onions are ready for harvesting about 80- 170 days after transplanting depending on variety.
As it approaches maturity, the crop usually shows some obvious senescence trends, with
beginning of weathering of foliage especially the lower leaves. About 2-3 weeks prior to
harvesting irrigation should be stopped to allow the soil to dry. Onions are necessarily ready
for harvest when 50- 80% of the tops have collapsed (fallen over), Onion harvested at full top
senescence tend to have short storage life. When onions are harvested while tops are erect and
fleshy, bulb yield is reduced. Harvesting involves undercutting from beneath the bulbs then
lifting the loosened soil with bulbs.
Curing of Bulbs
Soon after harvest bulbs are cured to enhance formation of well-colored intact outer skins.
Curing improves post harvest handling characteristics of the bulbs. It also limits the entry of
rot causing organisms. This process is performed in the field when weather permits, or on the
room at ambient temperatures.
Field curing involves gathering of the bulbs in wind row for up to 7-14 days, the same as
ambient temperature curing. In areas with strong sunshine, curing must be protected from
direct insulation by covering with leaves to avoid sunscald which kills the outer soft scale
tissue.
6. After curing the foliage is removed and the bulbs packed for storage. If the bulb skin is wetted
after harvest, particularly if it is covered by decaying leaves; fungal growth (Botrytis Cinerea)
may result in dark staining of the skins. Curing is like drying of the bulbs, therefore artificial
drying of the bulbs can also be achieved at 25-30o
c, 25-35% relative humidity for 12-24hrs,
with ventilation air flow.
Storage
Onion bulbs can suitably be stored for up to 8 months under good conditions. Various post
harvest losses can occur in onions due to shrinkage, decay, sprouting and rooting. Best storage
is that which suppress these causes of loss. Onion bulbs store best at or near 0o
c and 65-70%
relative humidity. Onion can also be stored at high temperature regime of 25-35o
c. Both low
and high temperature storage regime prevent sprouting or breakage of bulb dormancy.
REFERENCE
1. John M.Swiader & George W. Ware,(2002). Producing vegetable Crops 5th
Ed.
