Vegetable production: The art of cultivating onions

Question: I would like to start growing onions. Do you have any advice?

If dried correctly, onions can be stored after harvest until needed or marketed at a time when prices favour the grower.

Onions (Allium cepa L.) have been used as food for centuries. They are an important vegetable crop and are grown worldwide. Mature and immature onions are used to flavour food, as well as in salads and pickles.

They are sometimes used as a repellent for insects attacking other vegetables. Onions are one of the few vegetable crops that can be kept for a longer period and can withstand rough handling and long-distance transport.

SOIL AND CLIMATE REQUIREMENTS

Although the crop adapts well to a variety of soil types, the best results are obtained on a loamy soil that is fairly deep and well-drained to a depth of about 120 cm.
Onions grow best in soils with a pH of between 5.5 and 6.5.
Rotate onions with crops that are not the same family. Legumes, e.g. beans and peas, are recommended, because these crops also help increase soil fertility.

Temperature

The optimum temperature for onion growth is 18°C to 22°C.
Higher temperatures (25°C to 27°C) speed up bulbing and bolting (flowering) is triggered by low temperatures (8°C to 13°C).

Day length

Onions are sensitive to photo period.
Bulbing is initiated once the period of light exceeds a certain minimum requirement.
Short-day onions have a fairly short day length requirement.
Intermediate day cultivars require longer day lengths for bulbing.

CULTIVARS

If you are planting onions, choose cultivars adapted to the specific production area.
In South Africa, two groups of cultivars (cvs) are available: short-day cvs for northern areas and intermediate-day cvs for southern areas.

SOWING TIMES

Sow onion seeds directly into the soil, or grow seedlings to transplant later.
The best way for beginner farmers is to transplant seedlings; sowing onions directly requires some experience.

SEEDBED

It is important that the soil in the seedbed be fine in texture.
Sow seeds in rows to ensure enough ventilation and make weed control easier.
Make furrows 15 cm apart and sow seed at a depth of 10mm to 15mm.
Do not sow too thickly, as this results in spindly plants which transplant poorly.
Sowing densities should be between 1 500 and 2 500 seeds per 1 m² (about 7 g of seed per 1 m²).
If the days are very hot, a thin layer of grass should be sprinkled on the soil as mulch. Remove the grass when the plants start to emerge, 7 to14 days after sowing.
If left too long, the plants will become leggy and get sunburn.
Seedlings 8 mm to 9 mm in diameter (the thickness of a pencil) and 12 cm to 20 cm high are ready for transplanting.
Short-day onions will be ready to transplant 6 to 8 weeks after sowing.
Do not trim the leaves before transplanting.
Make furrows 2 cm to 4 cm deep and lay the white part of the seedling in the furrow.
Use a rake or spade to cover the roots and compact the soil around them with the back of the rake, or by hand.
Take care not to plant the seedlings too deeply, as this tends to produce elongated bulbs.

TRANSPLANTING

After transplanting in the field, keep the soil moist for the first 5 days to allow the plants to overcome the shock of transplanting and the root systems to settle properly.
Prepare beds 1 m to 1.2 m wide in the field, with 0.5 m 0.7 m paths between beds.
Remove soil from the path to build the up the beds to 8 cm to 10 cm high.
Plant onions 7 cm to 10 cm apart in rows that are 20 cm to 25 cm apart (50 to 60 plants per 1 m²).

FERTILISING

During soil preparation, work in 100 g of 2:3:2 (22) or 2:3:4 (30) per 1 m².
The crop is a heavy feeder, and needs nitrogen and potassium.
But remember that too much nitrogen (N) late in the season can cause too-vigorous leaf growth, delayed bulb development and thick-necked plants.
Phosphorus (P) and potassium (K) are required throughout the growing season.
Use 10 g LAN per 1 m², as well as 10 g potassium chloride (KCl) per 1 m², 3 weeks after transplanting and again at 6 weeks after planting, especially if the soil is sandy.
Apply them 5 cm to 15 cm from the plants.
Carefully work in the top dressing with a fork or a ghrop, but be careful not to damage the roots.
Water immediately after applying the top dressing; this will allow the fertiliser to start working immediately.

IRRIGATING

Onions require approximately 400 mm to 600 mm of water during the growing season.
Onion roots are concentrated in the upper 30 mm of the soil and for this reason the soil must be kept moist.
Do not water onions for the three weeks before harvesting.

HARVESTING

In home gardens, onions can be harvested when 100% of the leaves have lodged.
For commercial plantings, onions are usually harvested once 50% of the leaves have lodged. Do not leave onion plants in the soil for too long.
Onions are lifted by loosening the soil with a fork or a harvester.
Collect onions, make bundles and tie the leaves together.
Hang from the ceiling of a storeroom to dry.
Plants can be dried on the field by placing them in wind-rows in such a way that the leaves protect the bulbs against sunburn.
If it rains, turn the plants over.
Once the neck of the bulb has dried completely, the leaves can be cut and the bulbs stored.
For fresh marketing of short-day onions, the onions are partially dried and then marketed immediately in order to realise high prices.

STORAGE

After drying and cleaning, onions must be stored in a dry place.
Onions for storage should be mature, thoroughly dried and not damaged.
The storeroom should be well-ventilated and have a low temperature and a dry atmosphere.
Turn the bulbs often, so that all sides of the entire bulb are exposed to air and light at regular intervals.
Store in layers no thicker than 10 cm.
Rotten bulbs must be removed immediately.

YIELD

Under favourable conditions, yields of 30 tons/ha to 40 tons/ha can be achieved.
Top farmers can harvest up to 60 tons/ha.
Factors such as planting density, cultivar, planting date and growing period do, however, influence yield.

CROP MANAGEMENT

To ensure optimal growth, never let the soil dry out.
Onions have a shallow root system and should be watered regularly.
The plants are sensitive to waterlogging and so it often happens that the seedlings on the lower portions of the plot remain stunted and turn yellow without making any progress.
Onion plants, particularly seedlings, do not offer much resistance to weeds so weeds must be controlled to avoid competition for nutrients and water, particularly when the onion plants are small.
Weed carefully to avoid damaging the roots.
Stop working between the rows as soon as the foliage becomes dense, as the leaves may be damaged.

PESTS AND DISEASES

Thrips are very small insect pests, which feed on the leaves by sucking plant sap.
Attacked onion plants become silvery and flecked.
Farmers should practise crop rotation to combat this.
Nematodes attack the roots, but can be controlled.

The most important diseases are

Downy mildew causes grey to purplish mounds on the leaves. Leaves turn pale green, then yellow and die off. Downy mildew is favoured by low temperatures and high humidity.
Pink root appears on the roots of seedlings and older plants. The root turns pink, shrivels and dies. More tolerant cultivars are available.
Purple leaf spot or Alternaria blotch cause large brown lesions to appear on the leaves and eventually kill the leaves.
With white bulb rot is a white fluffy fungal growth that occurs on the bottom of the bulb. The bulb becomes rotten.
Basal rot causes the leaves die from the tips and if you cut through the bulb you will see brown rot.

Storage diseases

With black mould the bulb shows blackening just below the skin and later becomes rotten.

General control measures for storage diseases

Do not store damaged onions and those showing disease symptoms.
Inspect bulbs in storage and remove infected ones and clean containers with a bleach solution.

Also read:
Vegetable production: Prepare to plant onions
Vegetable production: Beat pests with crop rotation and companion planting
Drying methods for vegetables and fruit – getting more from your produce
Your library to fresh produce production

This article was written by dr. Ian Du Plooy, Sunette Laurie and Andre van den Berg from the South Africa Agricultural Research Council and first appeared in Farming SA.