Ragweeds are annual and perennial herbs and shrubs. Species may grow just a few centimeters tall or well exceed four meters in height. The stems are erect, decumbent or prostrate, and many grow from rhizomes. The leaves may be arranged alternately, oppositely, or both. The leaf blades come in many shapes, sometimes divided pinnately or palmately into lobes. The edges are smooth or toothed. Some are hairy, and most are glandular.
Ragweeds are monoecious, most producing inflorescences that contain both staminate and pistillate flowers. Inflorescences are often in the form of a spike or raceme made up mostly of staminate flowers with some pistillate clusters around the base. Staminate flower heads have stamens surrounded by whitish or purplish florets. Pistillate flower heads have fruit-yielding ovules surrounded by many phyllaries and fewer, smaller florets. The pistillate flowers are wind pollinated, and the fruits develop. They are burs, sometimes adorned with knobs, wings, or spines.
Many Ambrosia species occur in desert and semi-desert areas, and many are ruderal species that grow in disturbed habitat types.
Ragweed pollen is a common allergen. A single plant may produce about a billion grains of pollen per season, and the pollen is transported on the wind. It causes about half of all cases of pollen-associated allergic rhinitis in North America, where ragweeds are most abundant and diverse. Common culprits are common ragweed (A. artemisiifolia) and great ragweed (A. trifida).
Ragweed pollen can remain airborne for days and travel great distances, affecting people hundreds of miles away. It can even be carried 300 to 400 miles (640 km) out to sea. Ragweeds native to the Americas have been introduced to Europe starting in the nineteenth century and especially during World War I, and have spread rapidly there since the 1950s. Eastern Europe, particularly Hungary, has been badly affected by ragweed since the early 1990s, when the dismantling of Communist collective agriculture led to large-scale abandonment of agricultural land, and new building projects also resulted in disturbed, un-landscaped acreage.
The major allergenic compound in the pollen has been identified as Amb a 1, a 38 kDa nonglycosylated protein composed of two subunits. It also contains other allergenic components, such as profilin and calcium-binding proteins.
Ragweed allergy sufferers may show signs of oral allergy syndrome, a food allergy classified by a cluster of allergic reactions in the mouth in response to the consumption certain fruits, vegetables, and nuts. Foods commonly involved include beans, celery, cumin, hazelnuts, kiwifruit, parsley, potatoes, bananas, melons, cucumbers, and zucchini. Because cooking usually denatures the proteins that cause the reaction, the foods are more allergenic when eaten raw; exceptions are celery and nuts, which may not be safe even when cooked. Signs of reaction can include itching, burning, and swelling of the mouth and throat, runny eyes and nose, hives, and, less commonly, vomiting, diarrhea, asthma, and anaphylaxis.
Merck & Co, under license from allergy immunotherapy (AIT) company ALK, has launched a ragweed allergy immunotherapy treatment in sublingual tablet form in the US and Canada. Allergy immunotherapy treatment involves administering doses of the allergen to accustom the body to induce specific long-term tolerance.
Chemical spraying is effective for control in large areas. Because ragweed only reacts to some of the more aggressive herbicides, it is highly recommended to consult professionals when deciding on dosage and methodology, especially near urban areas. Effective active ingredients include those that are glyphosate-based (Roundup, Glyphogan, Glialka), sulfosate-based (Medallon), and glufosinate ammonium-based (Finale 14SL). In badly infested areas usually 2 to 6.5 liters of herbicides are dispersed per hectare (approx. 0.2 to 0.7 US gallons per acre).
Where herbicides cannot be used, mowing may be repeated about every three weeks, as it grows back rapidly. In the past, ragweed was usually cut down, left to dry, and then burned. This method is used less often now, because of the pollution caused by smoke. Manually uprooting ragweed is generally ineffective, and skin contact can cause allergic reaction. If uprooting is the method of choice, it should be performed before flowering. There is evidence that mechanical and chemical control methods are actually no more effective in the long run than leaving the weed in place.
^Payne, Willard W. (October 1963). "The Morphology of the Inflorescence of Ragweeds (Ambrosia-Franseria: Compositae)". American Journal of Botany 50 (9): 872–80. JSTOR2439774.
^Samter, M. and D. W. Talmage. Immunological Diseases 3rd ed. Volume 2. Boston: Little Brown. 1978. pg. 788. ISBN 0-316-76985-1 "It is estimated that a single plant produces 1 billion grains of pollen, or that 1 square mile of ragweed plants produces 16 tons of pollen".
^Rees, A. M. Consumer Health USA: Essential Information from the Federal Health Network 2nd ed. Volume 2. Westwood, Connecticut: Greenwood, 1997. pg. 32. ISBN 1-57356-068-5 "Each ragweed plant produces about one billion pollen grains during an average allergy season".
^Kiss, L. "Spread of Common Ragweed in Europe: An Example for Biological Invasion Caused by an Alien Weed Introduced to a New Environment." In: Vincent, C., et al. Biological Control: A Global Perspective. Wallingford, Oxon.: CABI. 2007. pg. 81. ISBN 1-84593-265-X
^Wopfner, Nicole; Gadermaier, Gabriele; Egger, Matthias; Asero, Riccardo; Ebner, Christof; Jahn-Schmid, Beatrice; Ferreira, Fatima (2005). "The Spectrum of Allergens in Ragweed and Mugwort Pollen". International Archives of Allergy and Immunology 138 (4): 337–46. doi:10.1159/000089188. PMID16254437.
Annual herb (in ours). Leaves alternate, petiolate, 2-pinnatisect. Capitula small, unisexual. Male capitula: racemose, many-flowered; phyllaries connate into a funnel-shaped involucre; receptacular scales filiform or 0; corolla 5-lobed, whitish. Female capitula: sessile or clustered in axils of leaves below the male; 1-flowered; corolla 0. Achenes tightly enclosed in the female involucre which bears spines or tubercles. Pappus 0. Another genus, which is atypical for Asteraceae.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:70 Specimens with Sequences:76 Specimens with Barcodes:61 Species:14 Species With Barcodes:13 Public Records:19 Public Species:2 Public BINs:0