- The optimal sowing dates so far should be considered early.
- The decision to choose the sowing date should not be dictated only by the course of vegetation last year's autumn.
New dates closer to practice
In recent years, almost the entire month of August was considered to be the optimal date for sowing rapeseed in Poland. Due to agrometeorological differences, the first dates were on August 5, and the last optimal dates were on August 25. However, for several years it has been signaled that these terms have nothing to do with reality. In fact, they had been operating for so long without any update that they had lost their usefulness, because sowing, according to the book, meant in practice very early establishment of plantations. There was (and still is) debate about the correct sowing date. It was proposed to shift the initial dates of the optimal deadline by 5 days. At that time, for example in Podlasie, the optimal date was not August 5-10, but August 10-15/20. In turn, in the center of the country and the middle belt from Pomerania to the southern edges, the initial sowing dates would be August 15/20 instead of August 10/15. In the western part of the country, the optimal date would not be August 20-25, but August 25-31. These "new" dates are certainly closer to autumn practice and conditions. Some people point out that these are still early sowings. But delaying sowing for too long is not the best solution.
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It must certainly be recognized that the current optimal dates are early dates in the context of rapeseed sowing. For example, if in southern Greater Poland or Lower Silesia rapeseed sowings performed around August 15 were previously considered early, now they can be said to be very early. In these regions, the date between August 20 and 25 should be considered early sowing.
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Is it worth sowing rapeseed very early?
On the other hand, too long delays in sowing pose certain risks. We don't know what autumn will be like. Will the vegetation last for a very long time? Will autumn be relatively warm? The new proposed dates are quite safe, i.e. they constitute a golden mean. Of course, if we referred the sowing date only to last year, the new sowing date proposals would also have to be considered early. But not every autumn will be like this. Last year, plantations established e.g. in the Opole region in the first week of September (or even around September 10) often managed without any problems. However, if the favorable conditions for rapeseed development ended in October, we would have a certain problem, because the rapeseed would not have time to develop the appropriate rosette before the arrival of winter.
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Not too early, not too late
It is better to sow population varieties at dates close to the optimal ones, and it is best to follow the new proposed dates, not the previously used ones. Hybrid varieties generally have greater possibilities of compensating for deviations from the sowing date and here the possibilities are more flexible. However, as long as we have appropriate field conditions, it is worth "sticking" close to the updated dates, which are quite safe in relation to both a possible long autumn and a quick end of vegetation. However, you do not necessarily have to choose earlier dates from the so-called optimal. Generally, if, for example, there are no appropriate humidity conditions on August 25, in Lower Silesia, the Opole region or southern Greater Poland, it will not be a mistake to move the sowing to, for example, August 31/September 1. It will be similar in other regions. In the context of last year, it would be better to sow rapeseed in most cases at the end of the updated sowing dates (or even later). On the other hand, looking at the multiannual period, it turns out that despite the fact that the vegetation lasted for a long time, the development of rapeseed in the second part of autumn was much slower, because the average daily temperatures were much lower.
Degree days are crucial when choosing the sowing date
And we must remember that the number of degree days, in addition to agrotechnics, determines how rapeseed will develop. Thus, rapeseed needs about 90 degree days to emerge, provided it has adequate moisture. When the sum of temperatures is about 150 ºC, we already have the first pair of leaves. Rapeseed will always need about 150 ºC (in degree days) to develop subsequent pairs of leaves. For the BBCH 16 phase, the total sum of temperatures is approximately 450 ºC. What does it mean? That if September is cold and sowing is significantly delayed, it will be difficult to obtain an optimal rosette shape (and the desired rosette is one that has at least 8-10 leaves, and preferably 10-12).
There is no single rule as to when to sow. However, too early sowing will in many cases result in the need for strong regulation, while delayed sowing is a certain risk in relation to a possible cold autumn. Generally, compared to the optimal sowing dates used so far, a safe date will be a delay of about a week.
