Ragweed (Ambrosia psilostachya) pollen, coloured scanning electron micrograph (SEM). This pollen is an allergen. 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. One of the main contributors to fall allergies is the ragweed plant. 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. Magnification: x340 when shortest axis printed at 25 millimetres.