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Ragweeds (Ambrosia) is a mostly American genus of mostly shrubs and subshrubs from the Sunflower family Asteraceae.

They are noxious weeds that grow in disturbed soil in fallow fields, along roadsides, in vacant lots, and along river banks.

There are about 30 species worldwide. They are very ordinary in appearance. Despite being all around, they are easily overlooked. Virtually no animal browses them. They do not host many insects. Many are adapted to the arid climates of the desert. Burrobush (Ambrosia dumosa) is one of the most arid-adapted perennials in North America. About 10 species occur in the Sonoran Desert.

Common Ragweed (Ambrosia artemisiifolia) is the most widespread of this genus in North America. It attains a height of about a meter. Great Ragweed, Giant Ragweed or Horseweed, (Ambrosia trifida), may grow to four meters (13 feet) or more.

The grayish leaves range from small to rather large.

The plants are monoecious, i. e the stamens (male) and pistils (female) are in separate flowers on the same plant. The tiny flowers are lacking a calyx. The light green male flowers grow in spikes. Female flowers are yellow-green and are inconspicuously placed at leaf bases and forks.

The seeds are arrowhead shaped, brown when mature, and smaller than a wheat grain. After pollination by wind, some plants develop a burr. This gets dispersed by clinging to the fur or feathers of animals passing by. The seeds are in important winter food for many bird species.

Each plant is reputed to be able to produce about a billion grains of pollen, and the plant is anemophilous (wind- pollinatedPollination is an important step in the reproduction of seed plants: the transfer of pollen grains (male gametes) to the plant carpel, the structure that contains the ovule (female gamete). The receptive part of the carpel is called a stigma in the flower). It is highly allergenic. It is the cause of much human misery, as the greatest pollen allergenAn allergen is any substance that causes an allergic reaction, most often when eaten or inhaled. No comprehensive list of allergens is possible, because sensitivities vary from one person to another and it is possible to be allergic to literally anything. of all pollens, being the prime cause of hayfever. The plant blooms in the northern hemisphere from about mid August until cooler weather arrives. It usually produces pollen more copiously in wet seasons.

Ragweed is a plant of concern in the global warmingGlobal warming is an increase over time of the average temperature of Earth's atmosphere and oceans. Global warming theories attempt to account for the documented rise in average global temperatures since the late 19th century and assess the extent to whi issue, because tests have shown that higher levels of carbon dioxideCarbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. One of the best known of chemical compounds, it is frequently called by its formula: :CO (pronunciation: "see oh two") Carbon dioxide results from the combustion of organic will greatly increase pollen production. On dry windy days, the pollen will travel many kilometers. When the humidity rises above 70%, the pollen tends to clump and is not so likely to become airborne.

Some high mountain and desert areas of North America used to be refuges for severe hay fever sufferers, who would go to such areas for relief during the pollen season, but increased human activity such as building and other disturbances of the soil, irrigation, and gardening, have encouraged ragweed to spread to these areas as well. Today, no area in the United States is ragweed pollen free, and moving can only offer a degree of relief.

Anecdotal claims are made of honey giving some relief for ragweed pollen allergies, which is noteworthy because honeybees do not visit ragweed flowers. However, during ragweed pollen shed, the pollen dusts every surface, and honeybees, being electrostatically charged, will accumulate some ragweed pollen. The pollen is frequently identified as a component of raw honey.