Have you ever wondered how onion[allium cepa] seeds are produced? two approaches are used. My discussion will consist of production, harvesting, threshing, conditioning, 
There are two ways of producing onion seeds, namely;
- Bulb to seed
- Seed to seed
Onions can be open pollinated or hybrid seed. Hybrid seeds have greater uniformity, increased yield and disease resistance and also give greater seeding.
BULB TO BULB METHOD
Bulb to seed method, is where onions seeds are produced from previously harvested bulbs. Bulbs are harvested and stored and replanted the following season. Seeds are produced in the second season.
This method is said to expensive than seed to seed method, but it allows the grower to easily discard the off types, diseased or undesirable bulbs.
Bulb to bulb method is done for the preservation of onion seed stock.
SEED TO SEED METHOD
In this case, seed is used as the initial planting material and has the following advantages;
- It results in a higher seed yield than bulb to seed method, because of the closer spacing achieved when seeds are sowed meaning that more bulbs will be produce and latter flower to produce seeds.
- Less time is invested in bulb establishment.
- Overall cost is less than bulb too seed method.
The above reasons, make this method the most preferred for seed production. The major disadvantage of this method is the production of off-types thus rouging is required.
IMPORTANT CULTURAL PRACTICES IN ONION SEED PRODUCTION
Some of the very important cultural practices have a role in ensuring the seeds produced are viable It’s to pay close attention to pest and disease management when using seed to seed method. .
1. Tillage
Leave the field fallow for at least 4 years and ensure it’s free of perennial weeds and soil borne diseases.
The soil should be ploughed to a depth of 10-15 cm, worked until a firm seedbed is achieved ensure it’s friable, fertile and well supplied with humus.
2. Planting
Plant seeds at a depth of 1-3 cm in rows of a width of 50-70 cm at a rate of 4.5-6.7 kg per hectare. Higher seed rates and close spacing yield more plants, thus more seed production.
In some varieties, more than one stalk is produced. Where soil fertility allows, adequate and reliable irrigation or rain is present, the inter row spacing can be reduced to 30 cm.
3. Fertilizer application
Onions respond well to fertilizer application because rooting structure is shallow and limited to a depth of 5 cm.
It is important to add organic matter to improve soil moisture retention ability and soil structure. An equivalent of 56 kg/Ha of Nitrogen should be applied as farmyard manure.
Application of nitrogen and potassium is not recommended at flowering because flowers become unattractive to bees; that are so important in pollination.
When the onions experience a copper deficiency, they become thin, poorly colored and have a poor bulb storage life. This can be improved by application of 22 kg /ha of Cuso4.
Manganese deficiency common in onions; it is seen as leaf chlorosis. This is often observed in alkaline soils thus adding an acid based fertilizer e.g. ammonium sulfate will solve the problem.
Maintain the soil PH at 6.0-6.5 which is slightly acidic to prevent the incidences of manganese deficiency through regular soil testing. Ameliorate the soil PH when it becomes unfavorable.
4. Irrigation
You need a permanent source of water to keep soil moisture above 65% field capacity because the crop is shallow rooted. When the bulbs initiate flowering; stop irrigation.
5. Weeding
Onions are poor competitors against weeds as they are shallow rooted. They have a poor canopy structure thus can be easily shaded by weeds.
Weeds also interfere with harvesting and become contaminants of crop seed. Effective weed control is achieved through proper cultivation, crop rotation or using selective herbicides.
6. Pest and disease control
Onions are subjected to many insect pests and fungal diseases.
a) Diseases
Umbel blight is caused by Botrytis alli that infects the onion seed stalk consequently reducing seed yield and quality.
Onion downy mildew is caused by Peronospara destructor that is common during cool season’s causes chlorotic lesions on leaves and root stalk.
Pink root disease caused by Pyrenocheata tertris is a soil borne pathogen that infects onions at any stage of development and causes the root to turn pink then brown then black then die.
Fusarium basal rot- caused by fusarium oxysporum showing yellowing, die back from the top of the leaves.
b) Pests
Onion thrips feed on leaf surfaces causing them to turn white or silvery. They also feed on flowers, thus affecting seed formation. Scales are also a major problem of onions.
HARVESTING AND THRESHING
Harvesting commences, when onion heads contain some capsules that have turned black and are opening; the black color indicates that the seeds have ripened.
Although all onion seeds heads do not mature simultaneously, most farmers do a once all harvesting when seed heads are about 30% mature.
The harvesting is done by hand, cutting the plant 10-15 cm below the umbel head. The cut umbel head is put in sacks and stored for 1-2 days.
After the 2 days, the seeds will be dry enough for threshing as the capsules become brittle and readily break releasing small onion seed.
Seeds are dried at a temperature of 320 C until they achieve a moisture content of 10% then at a temperature of 430 C until a storage moisture content of 6% is achieved.
CONDITIONING THE SEEDS
After threshing, the onion seeds are conditioned to eliminate weed seed and chaff; onion seed that is light is unviable thus it’s also eliminated.
Initial cleaning is done by an air screen cleaner and gravity table. The cleaner and gravity table allows the offloading of on account of weight differences, heavy seeds sink on the other hand light seeds float and are unviable.
Wash and treat them for not more than 3 minutes; they dried and stored as long as seed moisture content is less than 10%.
STORAGE of onion seeds
Onions seed deteriorate so fast when stored under hot, humid conditions. They become unviable in less than a year; proper storage will result to higher viability.
It has been established that when onions seed is stored at a moisture content of 6%; the seed will have a viability of 3 years and when stored at a moisture content of 3%, the viability will be more than 3 years.
returns per acre of onion
In most cases seed yield range will be 550-800 kg per hectare for open pollinated lines and 300-1200 kg per hectare for hybrid lines.
When one is aiming to produce bulbs, one hectare can produce about 12-20 tons of onion bulbs.
Most farmers prefer growing red creole variety because can grow over a wide range of altitude and is suitable for open field planting and planting in the greenhouse.
Other varieties available in Kenya are
- Red tropicana produces red bulbs.
- Red tropicana F1 hybrid produces large red thick flat onions with a firm pungent flesh. it is highly productive and therefore demands high levels oif managment. it keeps well in a dry aerated store.
- Bombay red this variety is for dry warmer areas. It is small to medium sized, globe shaped, purplish red and pungent.
- Yellow granex F1 hybrid is an early maturing and high yielding variety, that is thick, flat with thin yellow scales. The flesh is medium firm, crisp and mild in flavor.
- Texas early grano is an early maturing variety that takes 100-120 days. However, it has a short shelf life. Its a heavy yielder for high altitudes and ideal for fish salads.
- Green bunching is a spring onion that does not produce bulbs. It has dark green attractive leaves. Its an early and highly productive onion grown for stems rather than bulbs and tolerant to sun scotch.
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