Winter rape is sown throughout Poland on about one million hectares. Due to its specific requirements regarding the development phase in which it should enter winter dormancy, sowing is spread out over time, and recommendations have been developed for individual regions regarding optimal sowing dates. However, these, according to many farmers, have long been out of date, not adapted to climate change, hence delayed sowing is becoming increasingly popular.
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How quickly does rapeseed sprout?
The sowing date is important
The key to choosing the sowing date for winter rape is to allow the plants to develop a rosette of 8-12 leaves and a root shaft with a minimum diameter of 1 cm. This requires a sum of effective temperatures of around 800-850°C. Several decades ago, scientists calculated based on average daily temperatures that this requires 75-80 days, and to make it before the cooling, sowing must take place, depending on the region, between August 5 and 25.
Today we know that in recent years autumn has been increasingly long and warm, which means that the desired sum of effective temperatures can be "harvested" earlier. Since rapeseed should not be too grown before winter, this is more of a disadvantage than an advantage. We will not speed up winter, but in view of this state of affairs, farmers are increasingly delaying sowing. The question remains, however, how much can the date be postponed so that it is not too late.
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Sowing rapeseed: row spacing matters
In practice, in exceptionally favourable conditions, rapeseed sowings even from the end of the first ten days of September are successful. However, this is largely a lottery and a huge risk. A reasonable delay is considered to be about a week, in relation to the recommendations for a given region. It is worth remembering, however, that the delay makes sense mainly for hybrid varieties, and population varieties may react badly to them. Moreover, if the delay were to be longer, varieties with great initial vigour should be selected.
Disadvantages of sowing too early and too late
Excessive delay in sowing, as we have already established, can backfire. Although in recent years we have been dealing with a long and warm autumn more and more often, we do not know what this year's autumn will be like, hence the time needed to "collect" the sum of effective temperatures may vary. It may turn out that rapeseed sown too late will not manage to reach the desired phase of at least 8 leaves and 1 cm of root collar diameter or a properly developed root system before winter. What then? Such plantations may have problems with proper wintering, be more sensitive to frost, and have problems with starting vegetation in spring, which will ultimately translate into lower yields. It is estimated that a two-week delay in sowing means a loss of about 10 percent of the potential yield.
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What is the heterosis effect in hybrid varieties?
Of course, rapeseed sowing should not be excessively accelerated. Yes, there are supporters of early sowing who want to be sure that the plantation will enter winter dormancy at the optimal vegetation phase, but it is not worth rushing too much. First of all, if early sown rapeseed meets a long and warm autumn, it will certainly require strong regulation, because excessively exuberant plants do not survive winter well. In addition, increased pressure from agrophages should be expected, led by flea beetles, leafhoppers and aphids, against which insecticide dressing may not be enough.
