© by aha! Swiss Allergy Centre - Info on allergies - Ambrosia – Characteristics

Ambrosia is a weed with two outstanding properties: the dispersal potential of the plant is enormous and its pollen is highly allergenic. The plant from the Compositae (Asteraceae) family originates from North America and in the last 20 years has spread considerably in Europe, notably in the Balkans, in the Po valley and the French Rhone valley. In Switzerland it can currently be found mainly in housing development areas, less commonly on farmland. The weed was successfully halted in good time thanks to the official obligation to report its presence and to combat the plant.

Modest and inconspicuous

Depending on the location, common ragweed can grow up to 5 cm to 1.5 metres in height, has a reddish hairy stem, pinnate leaves, green flower spikes with yellow anthers. It is often confused with “common mugwort”. It can thrive on all soils and spreads very rapidly. Ambrosia germinates from early April and can even germinate two weeks earlier after mild winters. Flowering lasts from mid-/end of July to October. The seeds are produced from mid-August until the end of October. Ambrosia seeds remain germinable in the ground for up to 40 years. One study showed that common ragweed is a pioneer plant with high dispersal potential, like the widespread “white goosefoot”.

Millions of pollen grains

Pollen is produced from mid-July until the first frosts in autumn. The main pollen season of Ambrosia is the months of August and September. A full-grown plant will release millions of pollen grains. Depending on wind and weather, these can be blown 200 kilometres or further. Part of the quantity of pollen measured in western Switzerland and Ticino is hence imported from the areas of neighbouring countries “contaminated” with common ragweed. Just 11 Ambrosia pollen grains/m³ are enough to trigger allergic reactions of the eyes and respiratory tracts. By comparison, 50 grass pollen grains/m³ are required to produce the same symptoms. This is why experts regard Ambrosia pollen as particularly aggressive. It has not yet been conclusively determined how many common ragweed plants can be tolerated so that no allergies occur in sensitised people.

Distribution methods

Ambrosia seeds get into housing development areas as a contaminant in grain mixes, for example from birdseed and pet food. They stick to harvested crops, agricultural machinery and vehicles, are found in excavated material on building sites or are carried along transportation routes. Thus the seeds are primarily distributed in the country by means of human activities. Domestic gardeners, homeowners, professional gardeners, farmers, workers in road and rail construction as well as public highways maintenance staff are called upon equally to combat Ambrosia.

Occurrence in Switzerland

The Agroscope Changins-Wädenswil research body (ACW) has been observing for a number of years that common ragweed is starting to colonise more and more regions. In the Geneva canton and in western Vaud, Ambrosia has managed to establish itself in a few fields in agricultural use, but also in gravel pits and topsoil stockpile areas; in Ticino it is actually found along transport routes. 

Facts and figures

In Switzerland around 1.2 million people, roughly 20% of the population, are affected. The flower of a single blade of grass contains around 4 million flower pollen grains. Conifers are among the flower pollens that do not trigger any allergy. Visible as a yellow dust-fall in springtime, at most they cause irritation of the conjunctiva.

Editors: aha! Swiss Allergy Centre in co-operation with the Scientific Advisory Board. For prevalence figures, see source references.

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