1. INTRODUCTION
Bulbs of the onion family have been used as a food source for millennia and traces of onion remains dating back to 5000 BC have been found in archaeological sites in western Asia and eastern Mediterranean. They were grown in large quantities by the ancient Egyptians and are thought to have been a favourite of the Jews in their desert wanderings. Onions are grown throughout the world and the UN Food and Agriculture Organisation recorded the world production in 2010 as over 74 million metric tons, with the top producing countries being China, India, United States, Egypt and Iran.
Onions are widely grown throughout Southern and central Africa on a small scale and large commercial ventures and are second only in importance to tomatoes. The major producing areas in South Africa are the Western Cape, Northern Cape, Free State, North West, Mpumulanga and Limpopo. Onions have fairly limited nutritional value but are highly prized as a condiment owing to their strong flavour.
Climate and Soils
Onions require a cool climate during the initial growing season and high temperatures once the bulbs begin to form. Low temperatures during the bulb forming period will not only retard growth but could also trigger bolting. Warm, dry conditions will promote bulb growth and reduce the risk of disease. Most onion cultivars are sensitive to day length and bulbing is initiated when the minimum daylight requirement of the particular cultivar is exceeded.
Condiment: a substance suchas salt, mustard, or pickle that is used to add flavour to food.
Hybrid: bred as a hybrid from different species or varieties.
Onions can be grown on a wide range of soils varying in texture
from coarse grained sands to clays. From a management point of view the lighter soils are preferred. Ideally, soil pH should be in the range 5.5 ‐ 6.5. Hybrid cultivars have a low tolerance to low pH soils. Acid soils should be corrected by liming preferably using ground dolomite to obtain a pH reaction of 5.0. The applied lime should be incorporated at least 4 weeks before planting. Rotational cropping with legumes such as cowpeas, sunhemp or velvet beans is recommended and also accepted as a means of improving soil fertility. Soils should be well drained to a depth of around 120cm. Rooting is fairly shallow and although roots penetrate to a depth of 600mm, a soil depth of 450mm is adequate for growth.
2. VARIETIES
All onion cultivars require a certain number of daylight hours before they will begin to bulb and if these requirements are not met the plant will simply continue to grow vegetatively without forming a bulb. Temperature also plays an important role in initiating and influencing the rate of bulbing. For any given day‐length high temperatures ensure early initiation and rapid bulb formation. Short day‐ length cultivars will start bulbing earlier; produce a crop quicker when grown at low altitudes and at the same latitude, but in a cooler locality.
Onion varieties are grouped according to this sensitivity. Short‐day and intermediate‐day cultivars are successfully planted in South Africa. Long day cultivars require approximately 16 daylight hours and are therefore not suitable for planting in Southern Africa. The day length of an area is determined by its degrees of latitude and time (season) of year. In Northern areas, between 22 South and 28 South and short day cultivars (requiring less than 12 hours daylight) are grown as a Winter crop.
Intermediate cultivars can only be grown at latitudes below 29 South during the Summer months when the day length is greater than 12 hours allowing the initiation of bulb formation. These areas are restricted to parts of the Karoo and Western Cape. Detailed information regarding suitable onion cultivars for any specific area should be obtained from ARC Vegetable and Ornamental Plant Institute at Roodeplaat in Pretoria.
The main onion varieties are tabled below:
Short day cultivars (Northern areas)
Bon Accord
A large light brown elliptical‐shaped cultivar with a medium growth period of 170 ‐ 200 days and are best planted in March.
Hojem
A mid‐season large cultivar with a round shape and darkish scales. Hojem has a fairly long growing period of 220 ‐ 230 days and are mainly used for the home and fresh market gardens.
Texas Grano
An early mid‐season cultivar that yields heavily with extra‐large top‐shaped bulbs and mature from 220 ‐ 230 days. Texas Grano are widely grown.
Granex
An F1 hybrid onion which can be planted for early harvesting in the season or planted late and will still give a good crop. Medium growth period of 170 ‐ 200 days with large top‐ shaped bulbs.
De Wildt
An early short‐day cultivar producing a large flat bulb.
Long day cultivars (Southern areas)
Australian Brown
An excellent late long‐day cultivar with a distinctive reddish‐brown skin and extra‐large round bulbs with a very strong flavour. 250 ‐ 290 Days to maturity. Should only be grown in the Cape and Southern Free State. Australian Brown stores well.
Caledon Globe
A late long‐day cultivar with an extra‐large round globe shape with pale yellow flesh. 240 ‐ 250 days to maturity. They are very popular in the Western Cape and grown widely in the Cape Province and Southern Free State.
White Welsh
A green ‘bunching’ onion that can be raised from seed and propagated by division.
Pyramid
Early maturing for February ‐ March sowing. Bulbs are flat‐round and store fairly well. This variety can be used to produce onion sets.
Note: Spring onions are not a separate cultivar, but are the seedling stage of bulb cultivars. Thepreferred varieties for use in this role are White Welsh and White Pearl.
3. PLANTING
Whatever type of production is used, such as, seedling, direct sowing, and sets or planting big onions, the soil must be well prepared beforehand. The seedbed must be level and have a good fine tilth to ensure maximum contact with the seeds and even germination. If the bed is not level, seedlings growing in the higher parts develop very slowly are weak and very susceptible to pest attack. Those in the lower parts are susceptible to waterlogging and often remain yellow and stunted.
The most common planting method is to sow seed into a specially prepared seedbed, followed by transplanting the seedlings into a field. Alternatively seed can be sown directly in the field or sets (small mature onions) may be used. Finally, though less common, large onion bulbs can be planted.
SEEDLINGS
This is the most common method of production. Seedbeds are prepared as mentioned above and high fertilizer levels are often necessary. Exact amounts will vary according to soil type and region and a proper soil analysis should be conducted.
As a rough guide 90 ‐ 120 kg phosphorous (P) per ha will be required. On sandy soils 100 ‐ 140 kg/ha nitrogen (N) and 150 ‐ 250 kg/ha potassium (K) should be applied. On heavier soils with a higher clay content 60 ‐ 90 kg/ha N and 75 – 150 kg/ha K will be adequate. High nitrogen levels should be avoided in seedbeds producing tender seedlings that are inclined to bolt. Trace elements should be supplemented as required, either by incorporation into the soil or by means of foliar sprays. Seeds should be treated before sowing with a dressing of thiram to control seed‐borne diseases such as botrytis of damping‐off.
Dormant: having normalphysical functions suspended or slowed down for a period of time; in or as if in a deep sleep.
The seed is either broadcast or sown in rows 150 mm apart and covered to a depth of approximately 10 mm to 15 mm. The latter system is much easier since furrows are merely opened to the required planting depth and subsequently covered. This method also provides for ventilation between rows which facilitates disease control. Broadcasting requires the seed to be raked in and covered with soil brought from areas adjacent to the beds. Weed control in the drill‐sown seedbed is also much easier.
The seed germinates in 7 ‐ 10 days. For calculation purposes, 28 g of seed contains 7 000 seeds therefore 1kg will contain between 250 000 and 270 000 seeds. A seedbed of 300 m2 sown at the rate of 10 g/m2 will produce sufficient plants for 1 hectare. Seed is usually sown in late February, March or April, depending on the cultivar chosen and the locality.
Seedbeds are watered twice daily until seedlings appear thereafter, watering is gradually reduced until the seedlings are ready for transplanting 6 ‐ 8 weeks after sowing. The plants should then be about 5 – 7 mm in diameter and pencil thick.
The seedlings are lifted and rough‐graded. Small plants should not be used because uneven plant size land results in varying maturity rates and consequent management difficulties at drying‐off and harvesting. In addition, plants which show premature signs of bulbing should be rejected as these invariably fail to continue growth and after planting and will dry off to form minute bulbs. Such plants can remain dormant in the soil for 3 ‐ 4 months after transplanting.
There is a correlation between seedling size and ultimate bulb size as the larger the seedling, the larger the bulb. Leaves and roots are now seldom trimmed when planting out and is no longer considered necessary unless the plants are too long whereby foliage trimming may be desirable.
For rapid planting, shallow furrows are opened 230 – 300 mm apart. The seedlings are placed 50 ‐ 75mm apart on one side of the furrow with the crown about 25 mm deep. Soil is drawn back over the base of the plant which can be done by hand or mechanically. This system considerably reduces the need to weed and thin out in the field. The final plant population should be in the region of 550 000 to 650 000 plants per hectare. Transplanting into wet soils should be avoided due to the compaction caused by the feet of the planting team which will lead to poor growth.
Table 1: Recommended sowing, transplanting and harvesting dates of onion cultivars in differentregions of South Africa.
DIRECT SOWING
With this technique seed is sown directly where the crop is required to grow therefore soil preparation and good seedbed conditions are important. Advanced maturity is one of the main advantages of this system. Experiments in South Africa show that a direct‐sown crop of de Wildt will yield a mature crop 4 ‐ 6 weeks before the transplanted crop as the seedlings are not subjected to the shock of transplanting. The incidence of premature bulbing is lower and combined with a good stand produces much higher yields. Another advantage is that incidence of bolting is reduced.
A further advantage is that the labour intensive process of transplanting is eliminated.
However, the system does have numerous disadvantages. Whereas the seedbed occupies a relatively small area, the direct sown crop requires as much attention to the soil preparation but over a much larger area. The irrigation system must be capable of applying small amounts of water evenly over the entire area on a daily basis.
As young onion seedlings are particularly sensitive to adverse conditions it is necessary to compensate for poor stands with high seeding rates in the order of 5 – 8 kg/ha. Later, areas which
have too heavy a population will have to be thinned. Although the thinned seedlings can be used as transplants in the conventional manner, a large labour force will be required for the process.
Fresh seed, a fine tilth and weed‐free conditions are essential if the stand is to be even. The cost of production will be increased since chemical weed control is probably indispensable and will be in the ground longer than the transplanted crop. Direct sown onions grow rapidly.
In Gauteng, onions may be directly sown in mid‐January to mid‐February for harvest in July or August. In the Western Cape late onion varieties are generally not sown directly as this would entail sowing during the cold, wet Winter months of July and August when cultivation, weed control and germination will all prove problematical. In the warmer drier regions of the Cape direct seeding of late onions is feasible but earlier harvests should not be expected.
SETS
This is a specialised form of production and primarily practiced in the Northern areas and aimed at the Winter market. The seed is sown in the seedbeds at a rate of 10 g‐13.5 g/m2 during early Spring (August‐September) in the conventional manner. This is out of season so the bulbs will form immediately as a result of the already adequate day lengths, and together with the high seedbed population will result in very small bulbs approximately 3 months after sowing (mid‐December). They are dried off in situ, lifted and stored in a dry place either in pockets or on wire netting racks. Sets which are smaller than 20 mm can be troublesome to plant. Storage of sets should be either at moderate temperatures of between 10°C ‐ 24°C which will minimise the percentage of bolters which may develop.
Ideally sets should be approximately 25 mm in diameter and weigh out at 120 ‐ 135 bulblets per kg. When planted out in the land sets require 12 times the area of seedbed used. Sets are again planted out in the field during January and early February in a similar fashion to seedlings, in furrows 10mm deep and at a close spacing with rows 150 mm apart.
About 12 ‐ 15 pockets (12.5 kg) of saleable onions are obtained from a pocket of sets.
Field Transplanting ‐ Large Bulbs:
Spacing 50 ‐ 75mm in the row and 230 – 300 mm between rows (seeding rate in direct‐down crop 5 – 7.5 kg/ha).
Variations on this are obviously possible, e.g. double rows 150 mm apart with 450 mm in the row. This is recommended for conventional crops, including sets, but when planting large bulbs a spacing of 300 mm x 150 mm is preferable.
4. FERTILIZATION
The fertilizer requirements of a particular land should be assessed by soil analysis and is recommended that farmers submit a soil sample from the area where onions are to be grown, to their nearest soil analysis laboratory.
As a rough guide, it is estimated that 1 ton of onions will remove the following nutrients from the soil; 1.2 kg N, 0.35kg P and 1.2 kg K. in other words, a crop of 60 tons per ha will require 72 kg N, 21 kg P and 72 kg K per ha.
Onions respond well to organic manure which may be applied at a rate of 50 tons/ha. Severe problems will occur from heavy applications of manure or nitrogen especially if coupled with a low
plant population. Ripening will be retarded and large thick‐necked bulbs will be encouraged. Manure should ideally be applied to the preceding crop and carried over.
Early onions in the Northern areas of South Africa are generally fertilized with 80 – 100 kg/ha N, 80‐ 100 kg/ha P, and 120 – 200 kg/ha K, depending on soil fertility. Nitrogen is normally applied early in the season when the weather is warm and seedlings more responsive.
It is advisable to apply 50% of the nitrogen requirement at planting and the remainder over 1 or 2 top dressings 4 ‐ 6 weeks from transplanting. Seedbeds should receive 1 000 kg/ha of 2:3:2 compound fertilizer and when direct seeding 25% of the nitrogen should be applied pre‐planting and the balance in two top dressings at 4 and 8 weeks after sowing.
Early onions in the Western Cape are transplanted in cool weather at the beginning of Winter, so the nitrogen is generally applied at a later stage. The basic requirements are 120 – 140 kg/ha N, 120 kg/ha P, and 250 kg/ha K, which is dependent upon inherent soil fertility. The phosphorous and 70% of the potassium are applied pre‐planting together with 0 – 20 kg/ha of nitrogen. The rest of the N is applied as a 40 kg/ha top dressing at 2, 4 and 6 weeks after transplanting with the remaining 75 kg/ha K applied with the last N application.
Seedbeds for early onions should receive 100 – 120 kg/ha N, 90 – 120 kg/ha P and 150 – 200 kg/ha K, depending on soil type.
Late onions, grown only in the Western Cape are transplanted in Spring and harvested mid‐Summer. They require 140‐160kg/ha N, 120kg/ha P and 250kg/ha K. The application method is the same as for early onions but with the nitrogen top dressing spread over a longer period depending on the climate. In the cooler areas nitrogen top dressings are applied over the period 3 ‐ 9 weeks after transplanting and in warmer regions over the period 1 ‐ 7 weeks after transplanting. Seedbeds are normally fertilized at the rate of 120 – 140 kg/ha N, 90 ‐120 kg/ha P and 150 – 250 kg/ha K, again depending on soil type and fertility.
Nitrogen:
Great care should be taken in the application of nitrogen as it has a great effect on the yield and quality in onions. Too little N will result in poor growth, earlier bulbing and small bulbs. Excess N will result in large bulbs with a tendency to spilt or bolt, thick necks and poor storage quality.
Sulphur:
The keeping quality and pungency of onions is closely associated with the sulphur. Sufficient sulphur is usually supplied through the use of a compound fertilizer or a single superphosphate, and extra sulphur is not normally required.
Boron:
Boron is not readily moved from one part of the plant to another so symptoms of boron deficiency usually occur first in the younger roots and leaf tissues. Characteristics of boron deficiency include a stunted, distorted, corrugated appearance in the leaves which become stiff and brittle and tend to crack, leaf colour varying from dark grey‐green to deep blue‐green with yellow mottlings and shrunken areas on younger leaves, followed by ladde rlike transverse cracks on the upper side of the basal leaves. Fertilizer borate should be applied before planting at the rate of 10 kg/ha, or 5 kg/ha solubor.
Zinc:
Deficiency is indicated by distorted, spirally‐curled leaves with alternate yellow and green stripes running along the length. It often occurs in soils with high pH. 14 ‐ 20 Kg/ha zinc sulphate or 3.5 ‐ 7 kg/ha zinc oxide should be applied. Alternatively, 270 kg/ha superphosphate containing 1% zinc.
Copper:
Deficiency causes slow growth with thin chlorotic leaves. The outer scales of the bulbs are thin and light in colour. An application of 5 – 10 kg/ha copper sulphate or 2.5 – 5 kg/ha copper oxychloride should be used.
Molybdenum:
Deficiency causes stunting and leaf die‐back. The dead portion shows white in colour and the end of the green portion becomes soft and pulpy. More common in low pH soils sodium or ammonium molybdate should be added at a rate of 250 – 500 g/ha.
5. PESTS AND DISEASES
PESTS
Thrips (Thrips tabaci)
Thrips feed on young leaves which subsequently develop a silvery, speckled appearance at maturity. Growth is retarded and yield reductions of 40% have been recorded in severe infestations. Thrips can be controlled with Malathion, Karate and Thiodan.
Root Knot Nematode
Characterised by knots on the rootlets and can be avoided by ensuring that onions are not rotated with other nematode‐susceptible crops. Fumigation should be considered in extreme cases.
Seed‐Bean Maggot
Causes wilting of seedlings and is often found in manure. No insecticide is registered for its control.
Cutworms
These are often a problem although not usually serious on old lands. They chew through the stems of young plants at ground level and penetrate the sides of maturing bulbs. Cutworms can be controlled by applying endosulfan to the base of the seedling immediately after planting.
Wheat Mite
Dark red mites which feed off the leaves, which causes a light green discolouration. Spray Demeton or Anthio.
Bulb Mite
Small, cream coloured mites which infest the bulbs and feed on the plant sap causing the leaves to turn yellow and become stunted or distorted. Onions in storage will rot prematurely. Treat as for wheat mites.
DISEASES
Damping‐off
Characterised by the weakening of the stem near the soil, and subsequent lodging. Seed should be treated with Thiram before planting.
Downy Mildew
This is a serious disease of onions showing as a purple/grey fungus or mould on the foliage and is inclined to develop during humid conditions in Winter. The infected leaves turn yellow and eventually die off. Prophylactic sprays before wet periods with Dithane, followed by weekly sprays, prevent infection.
Alternaria Blotch
This causes a lot of damage where humidity and warm temperatures favour development. It is characterised by the presence of small irregular white patches on the foliage some of which enlarge to necrotic tan lesions with purplish centres. The leaf withers from the tip downwards.
Control is difficult but similar to the downy mildew programme. Preventive sprays of Dithane at a rate of 2 kg/500litres water at 5 ‐ 10 day intervals during susceptible periods are advisable. In serious infections, Iprodione (Rovral) is recommended.
Pink Root
Roots first become yellow and shriveled, later turning a distinctive pink colour as they die off. New roots are affected as they form. Bulbs remain small, adversely affecting yields. Attention should be paid to effective rotations and onions should not be planted in the same soil more than once in 3 years.
Fusarium Basal Rot
Is indicated by foliage die‐back during bulb ripening. Examination of the bulb will show that the base is rotten and roots are dead. A watery rot that spreads slowly but may only be detected during storage and is often associated with maggot damage. Planting on well‐drained soils and effective crop rotation will help to avoid contracting this disease. High soil temperatures are favourable for the development of basal rot which often affects onion sets planted in the Summer. Close inspection of set material is recommended.
White Bulb‐rot
This is a storage disease and characterised by a black sooty‐like mould which develops between the scales of the bulb. Control in stored onions is difficult but within the field spot spraying of the infected and surrounding areas with Benlate at a rate of 500 g/500 litres water has proved successful. Soil treatment with quintozene will also achieve satisfactory results but the best policy is complete avoidance of contaminated soils.
Bolting
This defect adversely influences bulb quality. Although a normal bulb develops it has a dry hollow flower‐stalk arising from the crown and descending to the bulb thus precluding the sealing of the top of the bulb by the leaf scales. This detracts from the appearance and reduces storage potential. Bolting is basically an inherited characteristic but is also influenced by climatic conditions. The choice of a resistant cultivar is advisable but sowing too early also increases the incidence.
6. HARVESTING
The first leaves to develop eventually drop off and disappear. The stem formed by the sheaves of the remaining foliage weakens just above the bulb and the leaf blades topple over indicating maturity. Onions may be lifted when the foliage of 30 ‐ 50% of the crop has fallen over. At this stage the leaf scales have sealed over the neck of the bulb and the young innermost leaves have stopped growing and are unlikely to push through and continue active growth. The bulbs have now reached their maximum mass and are at the optimum stage for harvest quality and storability.
Yields are variable with over 80 tons per ha being recorded in some areas but 40 ‐ 60 tons is considered to be a more realistic figure.
Before lifting, the heavier soils may have to be loosened, either by applying a light irrigation or using a duck‐foot type cultivator or some other mechanical implement to break up the soil. On lighter soils, the onions are simply pulled by hand and stacked in windrows with the foliage uppermost to reduce sun damage. If the crop is lifted during a wet spell it may be necessary to open the rows to allow the bulbs to dry without rotting.
It is important to allow the foliage to dry out naturally since this contributes to improved bulb weight which follows when the reserves in the leaf blades are drawn back into the leaf bases. Once dry, the foliage and roots are removed and the bulbs are packed into 10 kg pockets for marketing.
Thick‐necked onions should be removed, tied together with the foliage attached and marketed as green onions. Onions should be handled with care to avoid bruising and skin cracking which leads to weight loss through transpiration.
DRYING AND STORAGE
Onions can be sun‐dried in the field but many may be damaged by sun‐scald if temperatures exceed 35 C. The traditional method of storing onions is to plait the stems together with a strong twine and are hung under cover to dry. Alternatively the foliage may be removed and the onions stacked in bulk bins or loose on the floor to a maximum height of 3 metres. Warm air is then blown through the onions until the outer scales deepen in colour and become crisp and dry. Air humidity should be kept at approximately 60 ‐ 70% and the temperature at approximately 33 C. Under these conditions the onions will dry within 5 ‐ 7 days. Artificial drying reduces neck rot because the neck is sealed properly and reduces any chance of fungal entry.
Cultivars bred by ARC‐Roodeplaat can be successfully stored for up to 5 months allowing farmers to take advantage of seasonal price fluctuations on the markets. In some parts of the Free State and Northern Cape they are stored in wire netting cages ensuring good ventilation from all sides.
Cold storage is a skilled operation requiring expensive and specialised equipment. The onions must be properly dried in the field and are stacked to a height of 3 metres and stored at temperatures of 0-8 C for several months. The relative humidity should be kept between 50 ‐ 75%. Small bulbs will keep longer than large bulbs and borne in mind when marketing.
