While most spring allergies are caused by pollen from budding grass, weeds, trees and plants, fall seasonal allergies are most often caused by ragweed, a plant which usually blooms in mid-August. Ragweed releases pollen from August to November, with the highest pollen counts in September, according to the American College of Allergy, Asthma and Immunology (ACAAI). The plant grows virtually everywhere, and if that's not enough to trigger your fall allergies, there are plenty other fall growers releasing pollen. Click here for a three-step guide to beat the effects of fall allergies.