Are you freezing in January on sidewalks that are too gray? Console yourself by thinking of the good summer fruits that you will be able to eat. The amount of cold stored during the winter by fruit trees is indeed essential to their proper development. “The current cold is beneficial for our fruit trees, explains Jean-Jacques Kelner, ecophysiologist and lecturer at SupAgro Montpellier. This should give a more effective regrowth in the spring. Good news after a historically hot month of December.
“Trees need a cold quota to wake up from the dormancy they entered in the fall to protect themselves from the cold,” continues the researcher. And for that, they have a kind of internal counter. “In fact the cold is a landmark. The plant will be able to know if it is at the beginning or at the end of winter”. A mechanism whose operation is still a bit mysterious for researchers. “We researchers have modeled this in hours of cold, but in reality we don’t really know how the plant counts,” admits Jean-Jacques Kelner.
Loss of yields
According to figures put forward by the CTIFL (Interprofessional Technical Center for Fruits and Vegetables), an apricot tree needs 400 to 600 hours of cold per winter, peaches 350 to 900, depending on the variety. More reserved the researcher, does not advance a figure. “In fact, one hour of cold at 7 degrees is not equivalent to one hour of cold at 3 degrees, for example”.
A complex alchemy without which the “bud break”, when the vegetative and floral buds of the trees develop to reveal their first leaves and young fruits, goes wrong. “On apple trees that have been placed in conditions that are a little too hot, in Brazil, for example, the bud burst is not good. There is a loss of performance. Not all buds come out,” he observed. Because to tell the truth, the buds are in fact present from winter and it is precisely this envelope that protects the fruits from the cold.
“So there is a good cold and a bad cold”
And if bud burst is initiated, conversely, by a certain amount of heat received, “the lack of cold affects the number of buds that are bud burst and dormancy breaks badly. Therefore, if the temperatures warm up too quickly, the fruit trees are also exposed to late frosts. This was the case in April 2021 in Provence, with a significant loss of yield and consumer prices which had soared. An envelope of 23 million euros had been distributed to farmers in the Bouches-du-Rhône under the agricultural calamity scheme.
“There is therefore a good and a bad cold,” smiles Jean-Jacques Kelnel. A challenge for farmers with regard to global warming and climate change. “In the Mediterranean areas, we are already at the limit of the climatic impact because of the winter temperatures which are too mild and which risk not breaking the dormancy. This is already the case in Morocco, for example. On apple or almond trees there are flowering shifts between varieties, because in fact, we need interfertile varieties for them to produce fruit. »
One of the farmers’ responses to this is to raise the altitude of the crops. Others are considering solutions to fight against frosts when the fruits have already come out. Some warm the ground with candles, fires, or spin some kind of wind turbines that blow warmer air into the fields. But these solutions are costly in terms of energy. So let’s hope that this cold lasts a little longer.