Abstract
The onset, duration and intensity of the period when pollen is present in the air varies from year to year. Amongst other things, there is effect upon the quality of life of allergy sufferers.
The production and emission of pollens are governed by interacting environmental factors. Any change in these factors may affect the phenology and intensity of the season.
Readiness to flower in a plant, and the amount of pollen produced, is the result of conditions during an often long period foregoing flowering. When a plant is ready to flower, temporary ambient circumstances e.g., irradiation and humidity, determine the timing of the actual pollen release.
In order to understand variatio between years and to be able to safely predict futuresituations, not least due to the ongoing climate change, it is necessary to know the determinants of all related processe and differences between and within species, here reviewed.