On the Highveld, the past winter was considered to have been a mild one. There was a very cold front which affected the temperatures for about a week. Thereafter the weather warmed up and then another quick cold front passed through. This front caused a fairly widespread frost but was considered innocuous compared to the cold snap experienced earlier on. But this cold snap actually did do a heap of damage, and farmers started wondering why their crops weren’t performing up to scratch, despite the warm spring temperatures experienced during the day.
The problem was not caused by the intensity of the cold but rather by the warm conditions experienced before the cold. Plants are programmed to perform well when conditions are favourable and man has contributed to this by selecting the varieties with the highest performance, which is reflected in a higher yield.
With this higher performance the plant becomes “soft” and more vulnerable when there is a quick change in the weather. Their cells had become more tender and the frost, even though it was relatively light, caused a lot more damage than if the weather had remained on the cool side before the cold event. Manifestations were physical cold burn in some cases, but in most the damage was barely visible, although there was some distortion in the leaves. often looks like small holes, and when the problem is observed it can be attributed to a number of possible causes from early insect damage to deficiencies.
Plants need a long, warm period to become soft, and their growth rate will be affected for quite a long time after a climatic setback. So, what can be done to prevent this? To reduce or stop the damage, we have to harden the plants off. This entails reducing water to cause a little stress or keeping the nitrogen levels down to slow down growth. Both would minimise damage, but the downside is that yield potential will also be reduced. It’s a Catch-22 situation where we find we can’t decide which is worse.
The best approach is work on averages. If this occurs on an annual basis, then we can consider lowering the nitrogen a little at this time. it occurs occasionally, we just have to learn to live with it, just as we learn to live with hailstorms. But on average we score. We must learn to work more carefully at this time of the year and also take care that we don’t over-stimulate with too much nitrogen. The important thing is to recognise what’s happened, so that we’re in a position to make these kind of decisions. – Bill Kerr
Contact Bill Kerr on (016) 366 0616 or e-mail [email protected]. |fw
