A few things you may find of interest while you are watching the cold and snowy weather conditions outside that we are facing at the moment in most parts of the country.
In general most winter sown species will survive down to about -15°C without snow cover. However, wind-chill can of course affect this figure and could already bring damage to plants, even when -15°C is not reached.
With snow cover, most species can take a further -5 to -10°C; however, long term snow cover brings its own problems with various dis eases taking hold. If the snow cover is blown away it can cause damage to non covered part of the fields, while the covered areas are protected against the low temperature.
For WOSR specifically, there are no valid winter hardiness scores from UK or continental trials. Last year too many trials failed completely due to the very difficult establishment conditions through very late sowing and very wet sowing conditions. In previous years the winters were simply too mild and did not affect the plants. DSV breeders say that hybrids are more likely to survive cold conditions, but it is impossible to prove, or demonstrate on a yearly base, because the winter hardiness cannot be tested every year. The reason why hybrids have shown (in DSV private trials) a higher survival rate after very cold and frosty winters is that hybrids have a deeper rooting system and more vigorous plant development in spring, and for seem to survive the harsh conditions better.
Under snow the main disease to affect OSR is Typhula root rot, we have an information sheet available on request.
The winter hardiness in the plants is mainly determined by the concentration of different makro elements, like sugar, in the cells of the plants. We do know that every time the weather warms up after a cold snap, the concentration of these makro elements in the cells goes down and the winter hardiness of the plants decreases. So the highest winter hardiness of plants is at the beginning of the winter or the first cold period. When we get more of these cold - warm circles the plants decrease their potential winter hardiness. This means for our current situation, that when we will see a few more ‘warming ups’ (above freezing) followed by freezing periods in the next few weeks until the spring temperature is achieved, then winter damage can be expected.
We do know that a plant at 6 – 8 leaves is the most winter hardy, before and after this growth stage can give more problems.
When spring finally comes, growers should inspect their fields. If there is green cover, then a hybrid at 6 – 8 leaves, with an evenly spread population of 10 plants/sm will deliver a good yield, as long as spring conditions are not too difficult, and the field is relatively weed free. A conventional needs at least double this population.
If the field is bare, then as long as the growing point is not damaged (the top 1 – 2 cm of the root) then the plant will still perform OK. If there is damage, then either the plant is dead, or the plant will shoot several tillers. If this happens, then yield and management will be compromised and the decision to keep the crop or start again will have to be made.