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Wednesday, 24 March 2021 18:40

A Guide to Onion Production

Written by Prime Seedco
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Onion requires cool conditions for good vegetative growth with a temperature range of between 12° C and 24 °C. Winter months provide these conditions as the long cool winter months gives better vegetative growth before bulbing, resulting in better and higher yields. As the summer months approach and high daily temperatures of 25° C to 27° C accelerates the bulbing process. 

 

If a cold snap occurs with temperatures of around 13°C while bulbing this can trigger bolting. The bulbs need a month of dry weather towards the end of its growing period for maturity. Onions are sensitive to length of daylight. Certain cultivars like “Texas Grano” an open pollinated variety have short daylight requirements that is why they are termed short day cultivars. 

 

Cultivars that can form bulbs in less than a 12-hour day length must be grown. Late or indeterminate day cultivars require longer day length, therefore should not be planted above the 28 Degree latitude, as the closer to the equator the shorter the day length. Choice of cultivar is determined by whether long term storage is required or not. Some Seed Co hybrids such as Elad, Dina, Ada and Saturn will store for up to 5 – 6 months whereas open pollinated varieties do not have good storage properties. As onions remain in the ground for a long time weed control is very important because the soil should not be worked deeper than 50mm to 75mm then, only during the early growth. Leaf disease control is also very important.

 

Variety sowing windows

NB: Sowing windows differ with varietal maturity and day-length requirements. E.g. growers in Zimbabwe will be advised to sow some Seed Co varieties from the beginning of the year as follows: Regent F1 (weeks 4-11 and 16-22), Elad F1 (weeks 5-22), Ada F1 (weeks 5-17) and Saturn F1 (weeks 9-18). Consult Seed Co Agronomists.

 

Site & Soil Selection

Onions have a relatively shallow root system to a depth of about 200mm to 300mm. Onions can be grown on a wide range of soils. Heavy clay soils however are difficult to work with and can cause serious problems at harvest. Soils prone to capping i.e. T factor should be avoided.

 

Onions respond to well composted kraal manure at a rate of up to 20 tons per hectare which has been incorporated into the soil. Soil PH of 5.5 – 6.5 is optimum as at a low PH; certain diseases like white rot become more prevalent. Soil analysis is important and needs to be done for fertilizer application and to understand the cation balance e.g. Ca: Mg: K:Na ratio should be 65%:23%:10%:2%.

 

Soils should be deep ripped to achieve good drainage, with a good fine to medium seedbed tilth with no large clods.

 

Fertilization

Soil analysis will help to decide how much fertilizer to apply. Onions generally do well with 1,000kg to 1,300kg of compound “C” (6:17:15). Calcium nitrate can be used as a top dressing at the rate of 300kg per hectare starting at 3 weeks after transplanting or emergence. Another top dressing at 5 weeks, 7 weeks and 9 weeks up until seventh or eighth leaf stage. 

 

Micronutrient sprays of Nutrifoil every 2 weeks at the rate on the label should be done until the bulb starts to form. Avoid late or excessive Nitrogen top dressings as this will cause “Bull necks”, it will also cause the plant to go back to vegetative mode if Nitrogen levels are high at bulb formation.

Production Methods

Onions can be produced in the following way: -

Production of seedlings in an open field seedbed for transplanting.

Doing seedlings in trays with up to 3 or 4 seeds per plug.

Seeding direct in the field with a precision planter.

Onion sets for transplanting.

Field Seedbeds

A common method used. Sowing is done from early February to mid-April for transplanting from May to June. Harvesting is then normally done from September to mid-October. Seed is sown in rows 150mm – 200mm apart and seed sown to a depth of 10mm to 12mm. About 7 grams to 10 grams seed is used per square meter. Allow 3kg to 5kg of seed to produce 600,000 seedlings to be planted per hectare. Transplant seedlings when pencil thick about 6 – 8 weeks after sowing.

 

Seed Trays

Sowing time is similar to open seedbeds. Seedlings normally only get to 3.0mm to 3.5mm in diameter. Once the plug is pulled, separate the seedlings and plant individual plants. Some growers plant the plug without separating the seedlings but at slightly wider spacing. This is not normally suggested as these clusters do not develop sufficiently and some bulbs are malformed with flattened sides.

 

Transplanting seedlings from field seedbeds or plugs.

Seedlings are planted in shallow furrows which have been made on top of the bed. The number of rows will depend on variety and the growers decided final population. Roughly 5 to 6 rows on a 1,2metre bed top is the normal. Seedlings are placed at a slant on the sides of the furrow and

the roots are covered with soils to a depth of 1cm to 2.5cm. After transplanting, the seedlings will lie flat but will recover in a few days after the settling in irrigations and will start growing upright.

 

Direct Seeding

Direct seeded crops are normally 6 weeks earlier to maturity and avoids the labor-intensive practice of transplanting. A big disadvantage is that the entire field must be kept weed free, seedlings are still very small, weak and vulnerable. Weed control pre and post emergence is very important. Irrigation is a problem as it needs to apply small amounts often and evenly over the entire crop until germination is complete. Seed requirement is between 6kg to 8kg per hectare.

 

Sets

Sets are more robust after they have been transplanted and therefore easier to manage. Seed is sown in an open field nursery on beds that are 1.2metre to 1.6metre on top with 6 or 8 rows per bed 20cm apart this figuration depends on if a planter is being used or being done by hand.

 

Seeding is normally done in June in the hotter parts of the country and July – August in the cooler areas as a rule of thumb one hectare of seedbeds should supply enough seedlings for 10 hectares of crop. It is very important that not too much Nitrogen is given to the growing

seedlings. Approximately 30kg – 50kgs of Nitrogen needs to be applied per hectare. After germination the bulbs will start to form within 4 – 6 weeks. Bulbs will continue to grow until November when their size gets to 20mm – 25mm in diameter, then will begin to lodge. Lifting will be done in late November to Mid-December. After the sets have been lifted allow the necks to start drying off for 2 – 3 days then the sets can now be dried in bulk bins, on wire racks or cages. Make sure that the necks of the sets are completely dry before storage.

 

It is important that the storage facilities are adequate to hold all the sets as they need to be kept for between 2 – 3 months before planting out. Before storage, sets can be graded into roughly 3 sizes, 15mm – 20mm – 25mm so immature and over size bulbs can then be discarded so they do not take up storage space. If sets are stored in bulk do not go higher than 1metre – 1.5 meter in the drying room. There are two methods of storage, either cold or warm. Cold storage needs temperature ranges from 1° C to 4° C with a R.H of 65% - 75%. Warm storage has a temperature of 26 °C – 27° C with a R.H. of 60% - 75%. Sets can be stacked on a slatted or

perforated floor so that an air flow can then pass through. There are problems with cold and warm storage. High temperatures can result in slow emergence after planting out the sets due to the lack of the breakdown of the inhibitor (diallyl-disulfide) which is formed in the sets during the latter stages of growth.

 

After planting out frequent light irrigation of 1mm to 2mm can be applied twice a day to help cool the soil down. Warm storage is best for sets larger than 25mm, as large sets are prone to bolting with cold storage conditions. Cold storage can lead to unwanted flowering if sets are kept for the whole period of storage. The inhibitor is broken down by cold storage. Before

storage contact your Seed Co Agronomist to recommend a fungicide to prevent disease and decay during storage and how best to apply it. Remember to dry the bulbs again if dipped in a chemical solution.

 

Make sure at transplanting time that the sets must be completely covered with soil as any part left exposed will probably get sun scorched resulting in possible pathogen attack. After transplanting out the sets the smaller size sets just start enlarging in size and begin to become normal onions. Large sets over 35mm in diameter often split. The idea of using sets is to get a crop to mature earlier than the normal plantings thereby catching higher prices on the market.

 

However, this method does result in a rather uneven crop. The percentage of bolters, thick neck bulbs and split bulbs is often quite high. Spacing of the sets in the field is the same as for planted out seedlings or direct seeded crops. Planting is done by hand or a specially designed planting machine. Seed count is normally 270,000 seeds per kilo.

 

Spacing

Plant density can be from 600,000 to 800,000 plants per hectare. This gives yields of 80 000kg to 120 000kg per hectare of fresh market bulbs.

Plant on beds of 1.2m – 1.6m beds center to center. Five to eight rows per bed 20cm apart and seedlings or seeds 5cm to 10cm apart. Planting depth is critical as this has a noticeable effect on the bulb shape. The onion stem plate forms at the point that the seed germinates. The entire bulb forms above this point thus the bulb may form below or above the soil surface depending on the placement of the seed.

Irrigation

After sowing the seed direct in the field or in the field nursery seedbed the soil surface should not be allowed to dry out. This entails short irrigation cycles two to three times a day depending on weather conditions. After transplanting seedlings, they should be watered daily until the plants stand up. Onion roots can penetrate up to 800mm but most roots feed in the top 200mm – 300mm of soil. 

 

Care of irrigation management in the first 9 – 10 weeks is important, before bulb formation and the next important period is just before harvest. No stress should be allowed during bulb formation; however, no irrigation should be applied for the final 3 weeks before maturity to allow the bulbs to cure properly. A good onion crop will require 600mm – 700mm of irrigation.

Harvesting

Some growers harvest a small portion of the crop once the bulbs are a good size and sold as green onions tied in bunches. Mostly growers start harvesting when the tops of over 50% of the crop have collapsed and fallen over, even though the leaves still show a green color. A good way to lift is to loosen the soil with a blade pulled below the root zone. Leave the onions for 3 – 4 days to dry then pull them out by hand. Even if the crop is not going to be stored, the bulbs should be dry before marketing. Pull up to about 10 rows at a time and place in a single windrow in the field with the leaves being put over the bulbs to stop sun burn.

 

These are left for a further 2 – 3 days to dry and then the roots and leaves are cut off. If rain should fall before the onions have been removed from the field the windrows should be turned over and opened to dry before re doing the windrow. Once off the field and in the shed onions can be graded to size before pocketing for marketing. If onions are to be stored the keeping quality is related to the number of cells per volume of the bulb, the amount of growth inhibitor (diallyl disulphide) is formed in the leaves then translocate to the bulb at maturity. Make sure the onions have lodged properly for this process to take place to enhance the keeping quality.

 

Effective drying is when the temperature is around 27° C and low humidity of 75% R.H. with good ventilation. This temperature also reduces sprouting in the stored onion. Where long term storage is required it is important that high relative humidity of 75% - 80% R.H. are avoided as this is the enemy of stored onions. High R.H. promotes root growth and the development of storage pathogens that could result in big losses. Low relative humidity of less than 65% R.H. leads to excessive moisture loss resulting in shriveled bulbs and big losses in weight.

 

For long term storage good management of the shed is vital. Before filling the shed it should have been cleaned thoroughly to prevent fungal and bacterial infections. All damaged onions should be removed at grading before coming into the building. Good ventilation, temperature control and humidity control of all the bins should be checked regularly, and any onions with Basal, Neck

and Soft Rots which are common storage problems must be removed. Keep the floor clean of debris and sweepings.

A well cured onion will store and remain in a good state for a long time.

 

- Prime SEEDCO

Read 408 times Last modified on Wednesday, 24 March 2021 18:54

 

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