The mild temperatures favored the early flowering of the trees.
According to the last report published Tuesday, February 21 by the National Aerobiological Surveillance Network, only the departments of Brittany, Vendée, Charente, Center-Val de Loire and the south of the Alps remain partly preserved, with only an average allergic risk. The rest of the country is at high risk.
The mild temperatures brought some of the vegetation out of its winter dormancy. Hazelnut trees in particular (which only need a temperature of 5°C to start flowering and release their pollen). Beyond hazelnut trees, people with allergies are especially bothered at the moment by alder or ash pollen, and in the south of France by cypress pollen.
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The main risk of “false spring”, in other words mild temperatures which can cause plants to bud early, is the subsequent occurrence of late frosts, explains Guillaume Charrier, researcher in plant eco-physiology at Clermont-Ferrand. With its teams of the National Research Institute for Agriculture, Food and the Environment (Inrae), he carried out tests which show that these “false springs” have a negative effect on fruit trees. And not only on the buds which are damaged by frost, the damage can affect up to a third of the length of the branches, subsequently modifying the shape of the tree, and creating problems with fruiting. Temperatures in the orchards therefore remain under surveillance, as late frosts are possible until the end of April.
Drought threatens
An early start to vegetation can also quickly raise the question of water resources, because when the leaves unfold and begin to transpire, the trees begin to draw water from the soil. The start of vegetation is therefore not a favorable period for groundwater recharge, recharge which generally extends from September to March. The problem is that even if rains are expected from February 22, this month of February 2023 is one of the driest on record. Already in January, the groundwater level was below normal for the season.