The Giant ragweed (Ambrosia trifida) along with its smaller relative, the Common ragweed (Ambrosia artemisiifolia), are the primary cause of most pollen allergies. Notice the pollen on the leaves awaiting a wind gust to carry to an unsuspecting hayfever sufferer. The plants seem to thrive despite the dry Oklahoma summers. The giant ragweed plants can grow to over two meters in height.