Winter in the UK has been very mild and very, very wet this year. According to the BBC and the Met Office:
Long-standing weather records have been smashed by a stormy, yet warm December, the Met Office’s early figures suggest.
Scotland, Wales and the north-west of England all had the wettest December in more than a century.
A UK mean temperature of 8C (46F) broke records too and would have felt more like a day in April or May.
This has caused a bit of consternation in the gardening world, with everyone remarking on how early the daffodils are coming up and wondering what will happen when the weather reverts to the miserable mean.
For growers of fruit, I think there are two possible issues:
- If the mild winter continues, will plants get enough chilling hours before Spring?
- Will plants make an early start on growing/flowering and be set back by a sudden cold snap?
To me, (1) is more interesting since sudden cold snaps are nothing new to British gardeners.
So what counts as a chilling hour, and how many do plants need? Many temperate fruiting plants expect a certain number of cold hours before spring in order to properly complete the cycle of dormancy then waking up when temperatures start to rise again. If they don’t get enough hours of cold temperatures then they wake up cranky and don’t do as well as they should do. The temperature they’re expecting? The boundary is a bit fuzzy, but is well below 10 celsius (Wikipedia suggests 7C).
The number of hours needed varies by species. Below I’ve sorted some common UK species from highest to lowest requirement:[table “2” not found /]
These values come from the US supplier Raintree Nursery. The numbers are inexact and vary a lot by variety, so other sites may give sightly different values. But all sites should agree on a general ranking, with the ribes and apples perhaps needing more cold, and mediterranean fruit being less demanding.
The BBC actually hunted down a farmer growing blackcurrants for Ribena and mentioned this issue:
But it’s not good news for all crops. Herefordshire blackcurrant farmer Jo Hilditch, who sells to Ribena, says her fruit is missing out on vital time it needs to chill.
The crop has only had 192 hours of chill time – but one variety needs 1,400 hours and another 2,200.
So… if the mild winter continues and your blackcurrant yields are low in the coming year, chilling hours could well be why. If this is a sign of the future under climate change, could this be the end of UK produced Ribena? Or will ice melt shut down the gulf stream and give us Siberian winters to go with the hotter summers?