Common ragweed (Ambrosia artemisiifolia) is the most widespread of this genus in North America. Each ragweed plant is able to produce up to a billion grains of pollen over a season, and the plant is anemophilous (wind-pollinated). It is highly allergenic, as the greatest pollen allergen of all pollens, and the prime cause of hayfever. The plant blooms in the northern hemisphere from about mid August until cooler weather arrives. It usually produces pollen more copiously in wet seasons. Two species, Ambrosia artemisiifolia and A. psilostachya are considered among the most noxious to those prone to hay fever. The ragweed was accidentally imported to Europe during World War I, it had adapted to the different environment successfully and has greatly spread since the 1950s. Hungary is currently the most heavily affected country in Europe (and possibly the entire world), especially since the early 1990s, when abandonment of communist-style collective agriculture left vast fields uncultivated, which were promptly invaded by ragweed.
