The amount of sunlight that onions receive every day has to be exactly right before bulb formation takes place. Farmers have always selected onion varieties that suit their day length and season.
In South Africa, we are to some extent limited by the varieties available. For example, the Western Cape and Eastern Cape have longer daylight hours in summer than the northern areas of the country.
Day length in the southern regions is classified as ‘intermediate’ and onion varieties that flourish here will not bulb properly elsewhere.
Short-day varieties are normally planted in the north, but you should avoid the Australian brown variety that is available here, as it is unsuitable for growing at a latitude north of Bloemfontein.
When the summer is very hot and there is little cloud cover, this variety could bulb in December, but this is usually too late for farmers to receive a good price. When a variety matures too far into the rainy season in the north, disease becomes a much greater hazard.
Individual varieties start bulbing at different day lengths, so even varieties adapted to an area will mature at slightly different times.
You could therefore plant several varieties at the correct planting dates and harvest them as they mature. This will allow you to use your workers more effectively, as they do not have to harvest the entire crop at the same time.
If an onion is ready and lifting is delayed, bulbs can deteriorate in the soil and even start splitting in the rain. An onion must be lifted and cured as soon as possible when ready for harvesting.
However, later-maturing varieties can be left in the ground for a further week or more, depending on the variety. This will ensure larger bulbs, allowing you to harvest more bags per hectare. This higher yield can often make up for the lower price received later in the season.
Planting too early may result in a mature plant by winter that is more likely to produce seed stalks (bolting). Energy that should be used for bulb forming is directed to the seed stalk and flower instead.
The hollow seed stalk also extends into the bulb so that even when cut off at the bulb, a hollow area remains in the centre, resulting in very low prices, if you are able to sell these onions at all.
Some varieties are more prone to bolting than others, but the important factor is planting time.
The heavier the onion plant, the less cold is required to cause it to bolt. For example, if you plant the same variety on the same day as your neighbour and fertilise and irrigate it well, while your neighbour does not, your crop may bolt and his not, because your plants have a higher mass at the time of the first cold snap.
Warmer areas are less prone to bolting and crops can be planted earlier and will produce a higher yield.
This is a result of plants growing larger in the warmer winter months and also having a longer period to grow, because they were planted earlier than those in cold areas. They can be harvested at about the same time, however, because this is determined by day length.
In warm short-day areas that are frost free or receive little frost, you can sow in late February and March and transplant when the plants are ready. In cold short-day areas, sow in April and transplant in July.
Better yield is achieved in short-day areas with late-maturing varieties, because they have a longer growing period (the planting date remains the same for early and late types).
In the southern region, where intermediate varieties are grown, the planting date is much later because the type of onion grown here only matures much later in December or January. These varieties are suitable for storage, enabling farmers to wait until they can receive best prices for their produce.
A few onions with bolt can, however, be an important tool to help you fine-tune planting according to the conditions in your area. For example, you can determine whether you should have planted earlier to achieve larger bulbs and make adjustments accordingly.