Goldenrod in full bloom.
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Ragweed, Genus Ambrosia, is comprised of 50 species and cause up to half the symptoms collectively called seasonal allergies aka hay fever.
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Goldenrod, Genus Solidago, with double or 100 species typically flower yellow about the same time in the fall that ragweed pollen
is at its height. Thus Goldenrod gets the blame! But Goldenrod pollen is too heavy and sticky to be blown by the wind unlike Ragweed which is
easily dispersed by wind. Goldenrod is mostly polinated by insects as in the adjacent crop http://www.pbase.com/jtsmall/image/157398510
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Both are from the same Family, Asteraceae. See http://en.wikipedia.org/wiki/Asteraceae
For Ragweed http://en.wikipedia.org/wiki/Ragweed and Goldenrod http://en.wikipedia.org/wiki/Goldenrod