The planting date has to do with the problem of bolting – that is, going to seed. This results in a poor harvest as the plant diverts its energy into developing the seeds, and the leaves suffer. It occurs in many crops apart from onions.
Bolting is influenced by plant mass and cold units in a process called vernalisation. The rule of thumb is: the larger the plant going into winter, the less cold is required to initiate vernalisation.
Plants have evolved to bolt after vernalisation, as the severe cold followed by the warming weather is a clear signal to the plant that it is approaching the season most suitable for producing viable seed and thriving.
Unfortunately for the grower, bolting is genetically programmed into the plant. The best you can do is to slowly change the parameters through selective breeding to suit the climate and planting date.
Some varieties bolt after much less vernalisation than others. One of the best-tasting, most attractive and highest-yielding short-day onion varieties I’ve ever evaluated had to be scrapped in this country due to early and profuse bolting.
Planting later to avoid bolting is only a partial solution. This is because bulb initiation is determined by daylight length and, to a lesser extent, heat. If you delay the starting point, you won’t significantly alter the end, which translates into a shorter growing period and therefore less chance of reaching a better size.
In other words, the planting date is a compromise – the earlier you plant, the greater the potential to produce large onions. However, if the bulb is vernalised, a smaller, poor-quality product results. The ideal is therefore to plant as early as possible with little or no bolting.
One difficulty of getting this right is that the cold units vary from year to year. You will therefore need to keep accurate records of planting dates and bolting percentages. After a few years, you’ll be in a position to identify the most likely safe date. You’ll know you have it right when you have just a few bolters; if you have none, it means you could have planted earlier and obtained a higher yield.
Another difficulty is that each variety has a different optimal planting date, so keep a separate record for each variety.
Growing conditions are also important. As mentioned, bolting depends on plant size (mass) as well as cold units. Good soil and well-fertilised plants have a greater mass and are thus more likely to bolt.
A rep from a Texan company once told me that they planted their onions without nitrogen so that the onions would develop to a point and then stop growing. If the onions were allowed to grow normally, they would all vernalise. If they were planted closer to winter, the young seedlings would be damaged by the severe cold.
The answer therefore was to plant when germination and early development were favourable, ‘hold’ the plants until winter had passed and then stimulate growth by a generous application of nitrogen.
Onion farmers in the Cape do not have the same problem. They grow onions that mature in midsummer, so there is plenty of time from winter until then to regulate growth in order to obtain the size and yield required. These farmers plant much later than those up north, and with May/ June planting, the plants are unlikely to become big enough to vernalise before the cold is over.
Onion growing will keep you on your toes! Yet when all the hazards are avoided, it becomes routine and easy.