Onion is the biennial crop and takes two full seasons to produce seeds. In the first year bulbs are formed and in the second year stalks develop and seed is produced. It is a long-day plant. The day length influences bulb onion, but has little effect on induction of seeding. It appears to be day-neutral for seed production. It requires cool conditions during early development of the bulb crop and again prior to and during early growth of seed stalk. Varieties bolt readily between 10 to15 degree C. In the early stages of growth, a good supply of moisture is required and temperatures should be fairly cool. During bulbing, harvesting and curing of seed, fairly high temperatures and low humidity is desirable. Seed production is widely adapted to temperate and sub-tropical regions.
There are two methods of seed production
1. Seed to seed method. In this method, the first season bulb crop is
left to over-winter in the field so as to produce seed in the following
season.
2.Bulb to seed method. The bulbs produced inthe 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
the following advantages over the seed to seed method.
a) It permits selections of "true-to-type" and healthy bulbs
for seed production.
b)Seed yields are comparatively very high. The seed to seed method, however,
can be practised for varieties having a poor keeping quality.
Production and storage of bulbs (first year)
1.Sowing time (nursery). Middle of October to middle of November in the
plains and April to June in the hills. 1/20 hectare nursery is sufficient
for raising seedlings for one hectare.
2.Seed rate. Eight to ten kilogrammes per hectare.
3.Fertilisation. Add 20 tonnes of well-rotted farmyard manure at the time
of land preparation and 250 kg superphosphate (single) and 45 kg potassium
sulphate at the time of planting. 250 to 375 kg. of ammonium sulphate
or CAN may be applied as top-dressing in two to three doses during the
growing period.
4.Transplanting. Eight to ten weeks old seedlings are planted in small
beds in well-prepared fields. January is the best period.
5.Spacing. Spacing depends upon variety and bulb size and varies from
10 to 15 cm.
6.Irrigation. Fortnightly irrigation during winter weekly irrigation during
hot weather. Irrigate sparingly during maturity.
7.Interculture. Keep field free from weeds. Frequent interculture is essential
for good bulb development. For controlling weeds, post-emergence application
of tenoran at 2 kg per hectare in 800 litres of water, two to three weeks
after transplanting, is recommended. Oxadiazon one kg active ingredient
per hectare has also given effective control of weeds.
8. Insect and disease control
| Insect | Control measures |
| Onion thrips | Dust with 5% BHC dust at 25 to 30 kg per hectare, or spray malathion 50 EC at 600 to 700 ml per hectareor thiodan 35 EC at 600 to 700 ml per hectare. Three to four applications may be required. |
| Onion maggot | Spray sevimol |
| Diseases | |
| Damping-off | Use treated seed. In cases of seedlings mnortality, drench nursery with 0.3 per cent captan, or thiram,or dithane Z-78 at weekly intervals. |
| Purple blotch | Spray with copper fungicides such as blitox 50 at 0.2 per cent. |
9.Harvesting and curing of bulbs. Well-matured bulbs should beharvested.
Maturity is indicated bythe tops drooping just above the bulb,while the
leaves are still green. After harvesting, the bulbs should be topped leavingan
half inch neck. Before storage, a thorough selection and curing of bulbs
should be done. The length of time required forcuring depends largely
on weather conditions and may take three to four weeks.
Storage. The essentials of successful storage are:
a)The bulbs should be well-matured, dried and cured before storage.
b)Storage should be well-ventilated.
c)Storage should be done inshallow trays with perforated bototoms.
d) Storage temperatures should range 0 to 4.5 degree C until three to
four weeks prior to planting, when the temperature should be increased
to around 10 degree C.
Planting of bulbs and seed production (second year)
1. Time of planting bulbs. The best time for planting Bulbs is the second
fortnight of October.
2.Preparation of land. Prepare the field to good tilth. One deep ploughing,
followed by three to four harrowings and land levelling are enough.
3. Seed rate. The seed yield is affected bythe size ofthe bulb. The bigger
the bulb size, the higher is the seed yield. However, verylarge sizedbulbs,
if used, will needa very high seed rate. If bulb size of2.5 to3 cm diameter,
is used for planting, approximately 15 quintals of bulbs per hectare are
required if the bulb diameter is 3 to 4 cm.
4. Fertilisation. Same as described for first year.
5. Method planting and spacing. Selected bulbs are planted 8 to 10 cm
deep in the soil ata distance of 45 x30 cm. The size of beds depends upon
the source of irrigation. The sprouted bulbs are planted as such. In unsprouted
bulbs, the upper half portion should be removed, leaving the disc-like
stem and roots intact. The removal of the upper tops hastens sprouting.
6.Interculture. Insect and disease control. Same as described for first
year.
After harvesting, the bulbs should be carefullyrogued forcolour and such off-types as thick-necks, doubles, bottlenecks, as well as any other types which do not conform to varietal type.
Second year, plant only selected true-to type bulbs and remove plants not conforming to varietal characters before flowering.
Heads can be threshed when seed separates easily from them. Much of the seed falls from capsules during drying. The remainig seed is removed by flailing. The seeds can then be cleaned by immersing them in clean water for five to ten minutes and then drying in the sun or artifically. Before storage, the seed must be dried to six to eight per cent moisture.