The ragweed is a plant belonging to the genus Ambrosia, a flowering plant from North America that is now invasive in many areas of the world. Ragweed plants occur in several different types and grow in a variety of regions. However, all of them produce the fine powdery pollen which causes hay fever. There are 22 allergens known to occur in ragweed, of which six cause major allergies. The most prominent species of ragweed causing allergy is A. artemisifolia.
Photo credit: Asja Radja [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons