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Ragweed is an annual weed with a branched, leafy stem. It can grow up to 5 feet tall and has deeply divided leaves. The stems are four angled and appear hairy. The leaves are green on the outer surface but gray and hairy underneath. Ragweed can be found on roadsides, in waste areas and in fields throughout the United States and southern Canada. Although it has been introduced to the northern central and western parts of the country it is not as common in these areas.
The flower heads on ragweed are numerous. The male heads appear to be nodding and are born in long flower spikes at the top of the branches. They produce clouds of pollen. This is the part of the Ragweed plant that has been the bain of allergy suffers. Much of the yellow pollen found in thin layers on automobiles and in other outdoors areas is produced by the male part of this plant. What is known as ragweed season is the time when most of these plants are sending out great clouds of pollen. Although this creates a beneficial situation of pollination for other plants, in many humans it creates the itchy, red, watery eyes and fits of sneezing. The female heads are borne in tight clusters in the upper leaf axils.
Other species of ragweed includes Giant Ragweed which has similar flowers but is about twice as large as common Ragweed. The leaves on the Giant Ragweed have three lobes. Another species, Western Ragweed, is a perennial plant that spreads. This variety is more common in the west and has hairy divided leaves.
Due to the wide variation in climate and geography in North America, the area and habitat in which this plant can be found is an important clue to its identity. Ragweeds flowering period in most areas usually occurs from August until the first frost. It is possible that knowing when the plant is sending out it's pollen in the area where one lives, might aide one with controlling the hay fever reaction caused by this the plant.