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What is ragweed?

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Ragweed is a North American plant in the genus Ambrosia which has spread to almost every region of the Earth. Plants in this genus famously produce huge amounts of pollen, much to the dismay of people who suffer from allergies, and ragweed pollen contributes significantly to hay fever and other allergies all over the world. It is essentially impossible to control ragweed, since it is tenacious, hardy, and quite sneaky, for a plant. Ragweeds vary considerably in appearance. They have lobed to toothed leaves which can be silvery to gray-green, with rough stems and small clusters of greenish flowers. The pollen of ragweed plants is very lightweight, designed for dissemination on the wind, and these plants produce a lot of pollen each year, so the air can be heavily laden with pollen in some regions of the world on a windy day. Ragweed pollen can even enter the jet stream, traveling considerable distances before it returns to earth. Depending on the species and the climate, ragweed can ... more
wisegeek.com
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The most important cause of allergenic rhinitis and pollen asthma in North America is pollen from ragweed, Ambrosia, a widespread genus in the Asteraceae. Ragweeds are annual or perennial herbs with lobed or divided leaves ranging from small plants such as Ambrosia artemisiifolia (short or common ragweed) 30 cm to 1.5 m in height to A. trida, giant ragweed, which can reach 4 to 5 m tall . Although 21 species of ragweed occur in North America, most allergy problems are caused by A. artemisiifolia and A. trifida, two species that account for more hay fever than all other plants together. Both are pioneer plants that are well adapted to invading disturbed soils. In natural habitats these species are restricted; however, in areas where human intervention has cleared existing vegetation, ragweed quickly becomes established. Ragweed Flowers and Flowering Ragweed is monoecious with male and female flowers produced on the same plant. The small male flowers (each with five stamens) are ... more
pollen.utulsa.edu
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Ragweed, a yellow flowering weed, is a member of the Aster Family that often flourishes in disturbed vacant soils which can not support other vegetation. It flourishes during dry hot spells which promote growth and pollen formation. If these conditions continue through late summer then pollen dispersal is very high. The only deterrent to making it difficult for the plant to release its pollen is relative humidity that exceeds 70 percent. When Is Ragweed Pollen Season? The pollination of ragweed occurs nearly the same time each year and lasts between six and eight weeks. In north central Texas area (Dallas Fort Worth), the season ranges from mid-August through November or until the first frost. In many areas, however, ragweed pollen levels usually peak in mid September. One ragweed plant is capable of producing over a billion grains of pollen per season!! In the United States, it is estimated that ragweed produces 100 million tons of pollen each year. No wonder allergies symptoms run ... more
entdocs.org

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