Coloured scanning electron micrograph (SEM) of ragweed (Ambrosia psilostachya) pollen, showing the tips of the grains. Ragweed is a main cause of weed allergies. Ragweed pollen is notorious for causing allergic reactions in humans, specifically allergic rhinitis. Up to half of all cases of pollen-related allergic rhinitis in North America are caused by ragweeds. A single plant can produce one billion pollen grains per season. Ragweed grows abundantly throughout the South, North, and Midwest, and its lightweight pollen grains can travel up to 400 miles in the wind.