What Causes Sinus Problems During the Fall Season? Ragweed is certainly the biggest allergy trigger in the fall. This allergen usually emerges around late August and can last all through September and October. In addition to ragweed, mold is another common fall allergy trigger. Signs of Fall Allergies Sneezing Coughing Runny nose Sinus headaches Scratchy throat Poor sleep quality How Long Does Ragweed Last? Your fall allergy symptoms are going to last as long as ...