GE-I-3: Contamination with pollen of ragweed

The image shows a close-up of a blooming ragweed plant.Click to enlarge
Climate change favours the spreading and establishment of highly allergenic species such as ragweed.
Source: Photograph: © Elenathewise /

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


GE-I-3: Contamination with pollen of ragweed

The spreading and establishment of ragweed is presumably favoured by climate change. However, so far the results of pollen measurements do not indicate any significant trends.

The line graph shows the mean ragweed pollen total since 2006, differentiated by the central, western, southern and northern regions and for Germany as a whole. The curves run almost parallel. The values were particularly high in 2014. Significant trends could not be determined.
GE-I-3: Contamination with pollen of ragweed

The line graph shows the mean ragweed pollen total since 2006, differentiated by the central, western, southern and northern regions and for Germany as a whole. The curves run almost parallel. The values were particularly high in 2014. Significant trends could not be determined.

Source: PID (Pollen Traps)

Allergenic plants gaining ground

In Germany some 15% of adults suffer from hay fever at least once in their life (lifetime prevalence) and 9% from Asthma bronchiale. Allergenic pollen are primary triggers for hay fever, and several clinical studies have indicated a connection between the occurrence of airborne pollen and the occurrence of hay fever. Notably, the occurrence of pollen is heavily dependent on the weather or the climate. An overall milder climate with longer growth periods favours pollen being airborne for longer; it therefore favours higher pollen concentrations. It is also conceivable that the allergenicity of pollen allergens may increase as a function of temperature rise. In a warming climate, thermophilic plants may immigrate which were hitherto not endemic to Germany, including plants with high allergenic potential.

Common ragweed which emanates from North America (Ambrosia artemisiifolia) had for a long time been a comparatively rare and inconsistent weed. Its occurrence did not start to increase until the beginning of the 1990s. Nowadays common ragweed occurs in all Länder, and especially in southern and eastern Germany, it has started to form extensive, well established localised patches containing thousands of plants. This plant thrives in gardens, on uncultivated or fallow ground, in arable fields and cut-flower fields, in set-aside areas, on building sites as well as road and path verges. Causes for the spreading of this species are, for instance, the importation of bird food or seed material for deer and wildlife food plots, where these seeds are contaminated with ragweed seed. Other causes are the transportation of soil from affected areas in connection with building projects as well as the adhesion of seeds to agricultural machinery or mowers used on road verges. An EU Directive came into force in 2011 with the objective of restricting the contamination of feedstuffs with ambrosia seed.

The fact that ragweed is able to spread and establish itself in Germany is, however, also attributed largely to climate change because the annual plant can only achieve the seed maturity required for dispersal when the climate is warm or moderate with mild autumn weather. Unfortunately, it was not possible to find any systematic scientific evidence for this relationship. Similar concerns regarding variables such as the spreading and establishment exist with respect to other highly allergenic thermophilic plants such as lichwort or asthma weed (Parietaria officinalis, P. judaica) or the olive tree (Olea europaea).

Ragweed pollen is considered highly allergenic. In sensitised people even minor pollen concentrations, i.e. some ten percent pollen per cubic metre of air can trigger hay fever and, in the case of allergy sufferers, can also trigger asthma. Besides, there are reports of skin reactions after skin contact with the flower head or other components of this type of plant. Another contributing factor to the spreading of ragweed is the fact that it is one of the late-flowering plant species, which extends the period in which its pollen is airborne to the end of October. For sensitised allergy sufferers, this entails additional exposure to pollen owing to the extended period when it is airborne while undergoing the splaying process, thus prolonging the time when affected individuals suffer from related complaints.

For the time being mean ragweed pollen concentrations in Germany are still on the low side although concentrations vary from region to region. Moreover, long-distance transportation of goods from more strongly affected neighbouring countries can lead to heavy pollen burdens in some areas. Any trend statements on the development of pollen totals measured in Germany are still subject to uncertainties as they are based on relatively short time series. With respect to exposure situations in the four major regions of north, west, centre and south of Germany there is no distinct pattern discernible. The nationwide high contamination with ragweed pollen in Germany in 2014 was caused by long-term wind dispersal of pollen from the Hungarian basin during the plant’s flowering period. Ragweed is particularly wide-spread in Hungary and surrounding countries, especially Slovakia, Romania, Serbia, Bosnia-Hercegovina and Croatia; wind dispersal can thus emanate from those countries on ‘long-haul’ flights.

Total pollen count measurements do not permit any conclusions as to the risk of the population actually coming into contact with this type of pollen or in respect of developing sensitisation or allergic reactions. Nevertheless, for precautionary reasons every effort should be taken, subject to the rules of proportionality, to curb the further spread of this plant in Germany.



GE-R-3: Information on pollen



Examining measures to curb the spreading of ragweed throughout Germany’s Länder (DAS, ch. 3.2.1) 

To keep Germany as far as possible free from the occurrence of this species (Action Programme Ragweed by the Julius Kühn-Institute – ongoing since 2007).

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