Plant phenology



Ragweed (family Asteraceae, genus Ambrosia) is an invasive species of annual herbaceous flowering plant, originally native to Northern America, which has started to spread in Europe from the 19th century. This weed can often be found on roadsides, riverbanks, abandoned lands and fields, but it doesn’t grow on cultivated fields. Ragweed seeds are able to survive for many decades in the soil and the plant grows again under favorable conditions (Fig. 1).

Ragweed plant

Fig.1. Atmospheric ragweed pollen concentration (grains/m3) now (left) and predicted for the future (2041-2060 - right) – Lake I, Jones N, Agnew M, Goodess C, Giorgi F, Hamaoui-Laguel L, Semenov M, Solomin F, Storkey J, Vautard R, Epstein M. Climate change and future pollen allergy in Europe. Environmental Health Perspectives. 2016


Climate change (particularly rising temperatures and carbon dioxide levels), urbanization and pollution led to the increase of ragweed biomass, length of pollen season and atmospheric pollen counts, while current forecasts show an upward trend in the future (Fig. 1). These factors play and will continue to play a major role in the increasing burden generated by ragweed sensitization for people living in affected areas.

The most prevalent species is A. artemisiifolia (common or short ragweed), which is also clinically the most relevant for its high potential to cause allergic sensitization (Fig. 2). Ragweed flowers between June and October. A single ragweed plant is estimated to produce a billion pollen grains during a season. Ragweed pollen can be transported over great distances, while trade and traffic globalization have been identified as the most important factors for ragweed distribution. Ragweed pollen has been identified at great distances, of more than 600 km offshore and over 3 km up in the air.

Fig. 2. Ragweed – the plant and the pollen (source:
Fig. 2. Ragweed – the plant and the pollen (source: