Seasonal variations of natural products in European herbs

The present review gives an overview about the status of research on seasonal variation of natural products in herbs growing in or grown in Europe. Due to pronounced differences in weather patterns, papers covering plants from the Mediterranean, the temperate, and the cold climate zones are reviewed separately. Apart from trying to give an overview of the existing newer literature after the year 2000, we try to identify some repeatedly found seasonal trends and discuss some possible explanations for these trends. Moreover, some suggestions, which encompass both research bias and desirable quality standards concerning experimental designs for future studies, are given. The covered investigations are mainly focused on aerial parts and leaves. Some publications are also dealing with flowers and roots. The composition of essential oils of aromatic plants are particularly well investigated. Phenolics are the most often studied compound class, including different types of phenolic acids, flavonoids, and tannins. Additionally, some papers assess the seasonal variation of alkaloids and lipophilic compounds.