Whether they know it or not, most hay fever sufferers in North America have Ragweed to thank for their autumnal misery. Ragweed releases its vile pollen for six to eight weeks every year, starting in mid-August and continuing through November in some areas. In most areas, ragweed pollen levels peak in mid-September. During that period of time, a single ragweed plant is capable of producing over a billion grains of pollen. It is estimated that ragweed produces 100 million tons of pollen every year in the United States.
Most ragweed species are native to North America, where they are most prevalent in the midwest and central United States. There are few places known to be ragweed-free in the United States, although Portland, Oregon and Seattle, Washington sometimes claim to have no ragweed season. Cities with very mild ragweed seasons include Salt Lake City, Utah and Bangor, Maine. Outside of North America, ragweed can also be found in Eastern Europe and in the French Rhône valley.
Ragweeds are annuals with rough, hairy stems and mostly lobed or divided leaves. Their flowers are greenish and inconspicuous. Since ragweed is an annual, it can be eradicated simply by mowing it down before it gets a chance to release its pollen. A member of the Aster family, it grows mostly in disturbed vacant soils which cannot support other vegetation. It flourishes during hot, dry weather.
There are several species of ragweed, but most ragweed allergies are caused by two species which are present in nearly every region of the United States: Ambrosia artemisiifolia (common ragweed, which gets to be, at most, 5 feet tall) and Ambrosia trifida (also known as giant ragweed because it can grow to be 15 feet tall).
Fruits such as honeydew, cantaloupe, watermelon and banana contain proteins which are similar enough to the offending bits of ragweed pollen that they can produce reactions in allergic individuals. To avoid compounding your symptoms, its best to avoid these fruits. They can cause oral allergy syndrome, which is itching or swelling of the lips, tongue, throat or roof of the mouth. Chamomile, marsh elder, and mugwort may also cause problems for those allergic to ragweed.
To reduce your exposure to ragweed pollen, it is best to stay indoors between 5 and 10 AM, and also on dry, hot, windy days. The best time to be outside is in the late afternoon or after a heavy rain, because pollen levels are lower during these times. Keep your house and car windows closed as much as possible.
After being outdoors, it is best to shower and change your clothing. Pollen can adhere to clothing, skin, and hair. Be aware that your pet can also transport pollen into your home. Wash your hands after petting them, and resist the temptation to bury your face in their fur.
Sources:
http://allergies.about.com/cs/ragweed
http://pollen.utulsa.edu/ragweed.htm
http://www.auburn.edu/~deancar/wfnotes/ragwd.htm
http://www.niehs.nih.gov/airborne/prevent/ragweed.html
http://www.ppws.vt.edu/scott/weed_id/ambel.htm