Approximately fifteen species in the genus Ambrosia are commonly referred to as ragweed. The plants feature rough stems, small yellow-green flowers, and lobed leaves. They often grow in disturbed soil that cannot support other types of vegetation and flourish in conditions of low humidity and high heat. In North America, ragweed is most abundant in the midwest and central United States, but few areas are free from it altogether. However, Portland, Oregon and Seattle, Washington, which claim that they do not have ragweed seasons, could be considered safe havens for hay fever sufferers.