… the start of hazel and alder flowering was up to 26 days earlier between 1961 and 2017; the start of flowering of birch and grasses was also shifted forward by an average of one to one and a half weeks between 1991 and 2017. For grasses and herbs in particular, a lengthening of the pollen season is … the start of hazel and alder flowering was up to 26 days earlier between 1961 and 2017; the start of flowering of birch and grasses was also shifted forward by an …