Onions are one of the most widely grown and consumed vegetable crops in South Africa. Their versatility, long storage life and strong market demand make them an attractive crop for both emerging and commercial farmers. Successful onion production begins with proper planning, including selecting suitable soils, understanding climate requirements and establishing healthy seedlings. In this first part of the series, we look at the fundamentals of onion production from site selection through to transplanting.
The art of cultivating onions
If dried correctly, onions can be stored after harvest until needed or marketed at a time when prices favour the grower. But each production area in South Africa has only a limited time on the market before the next area comes into production. Onions (Allium cepa L.) have been used as food for centuries. 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. They 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 120cm. Onions grow best in soils with a pH of 5,5–6,5. 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.
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Temperature
The optimum temperature for onion growth is 18–22°C. Higher temperatures (25–27°C) speed up bulbing and bolting (flowering) is triggered by low temperatures (8–13°C). Day length Onions are sensitive to photo periods.
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. They are grown south of Welkom.
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.
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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 15cm apart and sow seed at a depth of about 10–15mm.
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 1m2 (about 7g of seed per 1m2). 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, 7–14 days after sowing. If left too long, the plants will become leggy and get sunburn.
Seedlings 8–9mm in diameter (the thickness of a pencil) and 12–20cm high are ready for transplanting. Short-day onions will be ready to transplant within 6–8 weeks. Do not trim the leaves before transplanting. Make furrows 2–4cm 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 deep, as this tends to produce elongated bulbs.
Transplanting
After transplanting in the field, keep the soil moist for the first five days to allow the plants to overcome the shock of transplanting and the root systems to settle properly. Prepare beds 1–1,2m wide in the field, with 0,5–0,7m paths between beds.
Remove soil from the path to build the up the beds to 8–10cm high. Plant onions 7–10cm apart in rows that are 20–25cm apart (50–60 plants per 1m²).
Establishing a healthy onion crop requires careful attention to soil conditions, climate, seedbed preparation and transplanting practices. These early production stages lay the foundation for good bulb development and high yields. In Part 2, we examine crop nutrition, irrigation, harvesting, storage and the management of pests and diseases that can affect onion production.
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Source: Agricultural Research Council – Production Guideline for Winter Vegetable Production
