Methods of seed production in onion

         
There are two methods used in onion seed production which are:

·         Seed  to seed method

·         Bulb to seed method

In the seed to seed method growing plants are left  in the field to flower  and  produce seeds without  going through a bulb stage. When this  method is used planting should be done  at the correct weather condition for inducing flower production. This method, however, is not commonly used because of the following disadvantages:

·         Proper bulb selection cannot be done

Segregation may occur thus resulting in seeds, which are not true to type.

In the bulb to the seed production method, bulbs are raised first and seeds are produced from the planted bulbs. This method is commonly used in Tanzania and has the following advantage: It is possible to select the bulbs to maintain the quality of the seed stock and to discard off-types for example splits and misshaped bulbs.

The first step in the bulb to seed method is to raise the mother bulbs. The procedure for the production of the mother bulbs is the same as producing a normal bulb crop but much care is taken to minimize diseases.

Bulb selection for seed production

When the bulbs have been produced the selection of proper bulbs( mother bulbs) for seed production is done  as follows:

·         Select only  ‘whole’ bulbs and discard all splits

·         Discard all off-types and use only those bulbs which are true to type

·         Select bulbs whose diameter is between 4-6cm.

·         Discard all misshaped bulbs

When the process of selection is completed the selected bulbs are stored until the time of planting.

Time of bulb planting                                             

The best time of planting bulbs is when the weather is cool enough in order to induce more flowering. Also, the timing of planting should be in such a way that seeds can be harvested during dry conditions.

Bulb treatment

Before the bulbs are planted out in the field, they are treated with a fungicidal solution. The chemical treatment helps to eliminate most of the fungal diseases that may have been infecting the bulbs. The bulbs are soaked in the fungicide solution for 10-20 minutes and then planted immediately.

Planting and spacing

The common spacing, which is recommended for bulb planting is 75 cm between rows and 30cm within rows. The bulbs are planted 10-15 cm deep and covered by light soil. The field is irrigated immediately after planting.

Isolation

Since onions are highly cross-pollinated by insects( particularly bees) it is important to maintain varietal purity by separating different cultivars intended for seed production. An isolation distance of about 1000 m between different onion cultivars is recommended. A wider isolation distance of 1,500 m is maintained in case of parental( foundation) seed production.

Irrigation

Adequate soil moisture is very important from bulb planting to harvesting time. Water is particularly very critical from flowering to the physiological seed maturity stage. Lack of proper soil moisture during this stage may lead to low yields and shriveled low-quality seeds. The frequency of irrigation depends on the weather condition with an increased frequency of irrigation depends on the weather condition with an increased frequency during dry spells.

Weeding

The seed field should remain  weed-free throughout the growing period. The weeds are removed by hand hoe as soon as they appear. Onions are very poor competitors during the early stages of growth and it is very important that weeding be done as early as possible.

Manure and fertilizers

Farmyard manure at a rate of 20-30 tons per hectare should be incorporated into the soil during land preparation. Nitrogenous fertilizers    such as urea and sulfate of Ammonia is applied as a top dressing. The rate of application depends on the fertility level of the soil.

Field inspection

Field inspection is done to remove all of the types    and diseased plants. Also during a field inspection, all abnormal plants are discarded from the field.

Onion seed harvesting

Time of harvesting, seed crop should be harvested when about 10 percent of the fruits are open and showing black seeds. Harvesting too early may cause weight and germination loss. Harvesting too late causes yield loss due to shattering.

Method of harvesting

Hand harvesting using secateurs or sickle is common in Tanzania

Number of harvests

Normally 2-3 harvests are necessary