- Onion seed is usually produced in the temperate and subtropical countries and in regions where high temperature prevails throughout the year especially during seed ripening.
- Only the early bolting type of onion requires relatively little low temperature exposure to produce seed.
- Onion is a biennial crop for the purpose of seed production.
- In one season, bulbs are produced from seed and in the second season bulbs are replanted to produce seed.
- Onion seeds are poor in keeping quality and loose viability within a year.
- Therefore, it is essential to produce seeds freshly and use the same for bulb production.
- It is highly cross-pollinated crop which is facilitated by protandrous nature of flowers.
- Cross pollination is carried out by honeybees.
- There are two methods of seed production.
- Most commonly used method of seed production is bulb-to-seed method.
- Another method is seed-to-seed method.
- In the first year, the crop is raised in the same manner as raised for table purpose (first year)
Bulb-to-seed method
- The bulbs produced in the previous season are lifted, selected, stored and replanted to produce seed in the second year.
- Mostly the bulb to seed method is used for seed production because of following advantage over the seed method.
- It permits selection of true to type and healthy bulbs for seed production.
- Seed yields are comparatively high.
- The seed to seed method, however, can be practised for varieties having a poor keeping quality.
First season
All the package of practices are same for raising bulb crop as followed for table crop.Planting of bulbs and seed production (second year)
Selection of Bulbs
- For seed production, bulbs harvested during warm weather should be selected and stored carefully till middle of October.
- Temperature ranging from 4.5-14oC with an optimum of 12oC is the best for storage of mother bulbs which are to be planted for seed production.
- The roots of the bulbs should be left intact after harvest.
- Bulbs selected for replanting should be free from disease infection.
- Double and long necked bulbs are discarded and only true to type are selected.
Time of planting bulbs
- The best time of planting of bulbs is the second fortnight of October
Preparation of land
- Prepare the field to good tilth by one deep ploughing, followed by three to four harrowing and land leveling.
Seed rate
- The seed yield is affected by the size of bulbs.
- The bigger sized bulbs give high seed yield.
- However, a very large size bulb of 2.5-3.0 cm diameter is used for planting.
- Approximately 15 quintals of bulbs per hectare are required.
- If the bulb diameter is 3-4 cm, it will need three times more seed bulb.
Method of planting and spacing
- Selected bulbs are planted 8-10 cm deep in the soil at a distance of 30-45 × 30-45 cm, the size of beds depend upon the source of irrigation.
- In certain species, the upper half portion should be removed, leaving the disc like stem and roots intact, the removal of upper tops hasten sprouting.
- To avoid rotting due to fungal infection of bulbs in the field, carbendezim @ 0.2 per cent is used for dipping the bulbs before planting.
- The treated bulbs are planted along one side of the ridge.
Irrigation
- Fortnightly irrigation is given during winters and weekly irrigation is given during the hot weather.
- Discontinuing irrigation when the seeds reach the milk stage gave high yields, good quality seeds with economy of labour and water.
- The doses and methods of application are same as for the first year.
Interculture
- Same as in first year
- Higher seed yield is obtained if planting is done in the first fortnight of October.
- Mulching practices record higher yield.
- Herbicide treatment (pendimethalin and fluchlorlin at 2.5 l/acre) and fungicide sprays (0.3% mancozeb) at 10 days intervals to control purple blotch also increase seed yield and quality.
- In this method, the first season crop is left to over winter in the field, so as to produce seed in the following season.
- Bulb-seed method of seed production not only takes two years for seed production but it is more expensive as large quantities of bulbs are to be stored for planting in the second year and losses in storage may also be considerable.
- In this method more seed yields are expected and the time to ensure 100 per cent bolting is also eliminated.
- Seed yield from this method is much higher but seed quality may be poor as rouging process of bulbs is not practised.
Roguing of seed crop
First year
- It is desirable to begin roguing in the field before bulbs are harvested.
- Any plant having different foliage colour, or plant type, or maturing bulbs should be rogued out from the field.
- After harvesting, the bulb should be carefully rogued for colour and off types such as thick-necks, doubles, bottlenecks, as well as many other types, which don’t confirm to varietal type.
Second year
- Plant only true to type bulbs and remove plants not confirming to varietal characters before flowering.
Flowering
- Inflorescence in the form of umbels with small flowers, a single spathe which split into segments, young flowers emerges having a superior ovary above the six stamens and petals like perianth.
- Flowers are borne in simple umbels at the apex of flowering stem.
- Flowers may number from 50 to 2000 depending on cultivars.
- It is protandrous.
- The anthers of inner whorls dehisce first and usually it occurs between 9.00 am and 5.00 pm.
- Style becomes receptive when it elongates to a length of 5 mm.
- It is a cross-pollinated crop.
- It is pollinated chiefly by honeybees, which visit the nectarines.
- Though cross-pollinated, selfing also takes place as insect visit many flower in a single umbels of the same plant before leaving for another plant.
Isolation requirement
- Onion is basically cross-pollinated crop, but some self-pollination does occur.
- It is chiefly pollinated by honeybees.
- For pure seed production, the seed field must be isolated from fields of other varieties of onion, and fields of same variety not confirming to varietal purity requirements for certification, at least by 1000 meters for foundation seed production and 500 for certified seed production.
Harvesting and threshing
- Seed is ready for harvest when first formed seed in the heads get blackened.
- Two to three pickings are necessary to harvest the heads at just the right stage.
- Seed heads are cut, or snapped off, keeping a small portion of the stalk attached.
- Seed heads, after harvest, should be thoroughly dried.
- Air circulation is important while drying seed heads.
- Hence, the tray or canvases should be filled only to a depth of 15 cm.
- Heads can be threshed when seeds separate easily from them.
- Much of the seed falls from the capsule during drying.
- The seeds can be cleaned by immersing them in clean water for 5-10 minutes and then drying in the sun or artificially.
- Before storage, the seeds must be dried for 6-8 per cent moisture.
Seed yield
- The average seed yield varies from 8.5-10 q /ha.
Factor influencing seed yield
Seed yield is influenced by many factors, among them important are:
- Cultivars
- Bulb weight
- Soil and climate
- Spacing
- Fertilizer application
- Seed stalks per bulb
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