Ragweed
Ragweeds (Ambrosia), also called bitterweeds and bloodweeds, are a genus of flowering plants from the sunflower family (Asteraceae).
The scientific name of this genus is sometimes claimed to be derived from the Ancient Greek term for the perfumed nourishment of the gods, ambrosia () which would be ironic since the genus is best known for one fact: its pollen produces severe and widespread allergies. However, the generic name is actually cognate with the name of the divine dish, both being derived from ambrotos (), "immortal". In the case of the plants, this aptly refers to their tenaciousness, which makes it hard to rid an area of them if they occur as invasive weeds.
Ragweeds occur in temperate regions of the northern hemisphere and South America. Ragweeds prefer dry, sunny grassy plains, sandy soils, and to grow along river banks, along roadsides, disturbed soils, vacant lots and ruderal sites.
There are c.41 species worldwide. Many are adapted to the arid climates of the desert. Burrobush (A. dumosa) is one of the most arid-adapted perennials in North America. About 10 species occur in the Sonoran Desert.

# Description and Ecology
Ragweeds are annuals, perennials, and shrubs and subshrubs (called bursages), with erect, hispid stems growing in large clumps to a height of usually 75-90 cm. The stems are basally branched. They form a slender taproot or a creeping rhizome. Common Ragweed (A. artemisifolia) is the most widespread of this genus in North America. It attains a height of about a meter. Great Ragweed (Giant Ragweed, "Horseweed"; A. trifida), may grow to four meters (13 feet) or more.
The foliage is grayish to silvery green with bipinnatifid, deeply lobed leaves with winged petioles; in the case of Ambrosia coronopifolia, the leaves are simple. The leaf arrangement is opposite at the base, but becomes alternate higher on the stem.
Ambrosia is a monoecious plant, i.e. it produces separate male and female flower heads on the same plant. The numerous tiny male inflorescences are yellowish-green disc flowers about 3 mm in diameter. They grow in a terminal spike, subtended by joined bracts. The whitish-green single female flowers are inconspicuously situated below the male ones, in the leaf axils. A pappus is lacking.
After wind pollination, the female flowers develops into a prickly, ovoid burr with 9-18 straight spines. It contains one arrowhead-shaped seed, brown when mature, and smaller than a wheat grain. This burr gets dispersed by clinging to the fur or feathers of animals passing by.
The seeds are an important winter food for many bird species. Ragweed plants are used as food by the larvae of a number of Lepidoptera (butterflies and moths); see list of Lepidoptera that feed on ragweeds.

# Ragweed pollen as an allergen
Each plant is reputed to be able to produce about a billion grains of pollen over a season, and the plant is anemophilous (wind-pollinated). It is highly allergenic, generally considered the greatest allergen of all pollens, and the prime cause of hayfever in North America. Common Ragweed (A. artemisiifolia) and Western Ragweed A. psilostachya are considered the most noxious to those prone to hay fever. Ragweeds bloom in the northern hemisphere from about mid-August until cooler weather arrives.
A plant usually produces pollen more copiously in wet years. When the humidity rises above 70 percent, however, the pollen tends to clump and is not so likely to become airborne. Ragweed is a plant of concern in the global warming issue, because tests have shown that higher levels of carbon dioxide will greatly increase pollen production. On dry windy days, the pollen will travel many kilometers.
Goldenrod is frequently blamed for hayfever, but simply happens to have a showy flower that blooms about the same time. Goldenrod is entomophilous, i.e. insect pollinated. Its pollen is heavy and sticky, and cannot 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. Ragweeds were accidentally introduced to Europe during World War I; they thrived and have greatly spread since the 1950s. Hungary is currently the most heavily affected country in Europe (and possibly the entire world), especially since the early 1990s, when abandonment of communist-style collective agriculture left vast fields uncultivated, which were promptly invaded by ragweed.
Anecdotal claims are made of honey giving some relief for ragweed pollen allergies, which is noteworthy because honeybees very rarely visit ragweed flowers, and even then only for pollen. 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.
The major allergenic protein has been identified as Amb a 1, a 38 kDa nonglycosylated protein composed of two subunits. Other allergens widespread among pollen - profilin and calcium-binding proteins - are also present.

# Control and eradication
Total eradication of ragweed is considered impossible, owing to the plant's frugality and tremendous seed-producing capability. As of 2005, there is no known safe biological control to be used against ragweed in the open. Mechanical and chemical methods are available and can be used to control its spread, although there is evidence that these are actually no more effective in the long run than leaving the weed alone.
The act of manually uprooting ragweeds, sometimes shown in the media for public awareness purposes, promises more than it can deliver. It is ineffective, and skin contact may cause the onset of full-blown hayfever symptoms in persons with latent ragweed hyper-sensitivity. That being said, ragweed is best uprooted in late spring, before the flowering season and before a strong root system has developed.
Although the scythe and its motorized descendants have a reduced efficiency against ragweed, they remain indispensable tools, especially in populated areas and near delicate plantation, where herbicides use must be limited. Fighting ragweed with the scythe is a continuous process, because it is difficult to cut the plant right at the soil level, and the plant will regrow in two weeks (and often branch into three or four full-sized stems) if more than half an inch of the plant remains above the ground. Areas where ragweed has been reaped should be mowed down every three weeks to prevent regrowth.
It is considered important to control the spread of ragweed in large abandoned or uncultivated areas. Ragweed pollen can remain airborne for days and travel great distances, affecting people hundreds of miles away. One efficient method for large-scale ragweed extermination is chemical spraying. 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. Some proven effective active ingredients include those that are glyphosate-based (Roundup, Gliphogan, Glialka), sulphosat-based (Medallon) and gluphosinat-ammonia based (Finale14SL). 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).
One favored method of controlling ragweed in the past was cutting it, leaving the cuts in the field, then burning them there once the stalks have dried since standing, live ragweed won't burn. It has become less popular today because the smoke produced is seen as unacceptable pollution, as with the decline in leaf-burning and trash burning. But the method has the added benefit of killing off the stems so the plant does not grow back, which (as noted above) is otherwise almost inevitable.

# Species
- Ambrosia acanthicarpa  Flatspine Burr Ragweed, Annual Bursage
- Ambrosia ambrosioides  Ambrosia Burr Ragweed, Canyon Ragweed, chicura
Ambrosia ambrosioides ssp. septentrionale
- Ambrosia ambrosioides ssp. septentrionale
- Ambrosia artemisiifolia  Common Ragweed, Annual Ragweed, American Wormwood, Blackweed, Carrotweed
- Ambrosia aspera
- Ambrosia bidentata  Camphor Weed, Lanceleaf Ragweed
- Ambrosia canescens  Hairy Ragweed
- Ambrosia carduacea  Baja California Ragweed
- Ambrosia chamissonis  Silver Burr Ragweed, Silver Beachweed, Silver Beach Burr
- Ambrosia cheirnathifolia  Rio Grande Ragweed
- Ambrosia chenopodiifolia  San Diego Burr Ragweed, San Diego Burrsage
- Ambrosia confertiflora  Weakleaf Burr Ragweed
- Ambrosia cordifolia  Tucson Burr Ragweed
- Ambrosia coronopifolia
- Ambrosia deltoidea  Triangle Burr Ragweed, Triangleleaf Bursage, Rabbitbush
- Ambrosia dumosa  Burrobush, Burroweed, White Bursage
- Ambrosia grayi  Woollyleaf Burr Ragweed
- Ambrosia helenae
- Ambrosia hispida  Coastal Ragweed
- Ambrosia ilicfolia  Hollyleaf Burr Ragweed, Hollyleaf Bursage
- Ambrosia intergradiens
- Ambrosia johnstoniorum
- Ambrosia linearis  Streaked Burr Ragweed
- Ambrosia maritima  (the type species)
- Ambrosia palustris
- Ambrosia pannosa
- Ambrosia parvifolia
- Ambrosia peruviana  Peruvian Ragweed
- Ambrosia psilostachya  Western Ragweed, Cuman Ragweed, Perennial Ragweed
- Ambrosia pumila  Dwarf Burr Ragweed, San Diego Ambrosia
- Ambrosia sandersonii
- Ambrosia scabra
Ambrosia scabra var. robusta
Ambrosia scabra var. tenuior
- Ambrosia scabra var. robusta
- Ambrosia scabra var. tenuior
- Ambrosia tarapacana
- Ambrosia tenuifolia  Slimleaf Burr Ragweed
- Ambrosia tomentosa  Skeletonleaf Burr Ragweed
- Ambrosia trifida  Great Ragweed, Giant Ragweed, Buffalo Weed
Ambrosia trifida texana  Texan Great Ragweed
- Ambrosia trifida texana  Texan Great Ragweed
- Ambrosia trifolia  Greater Ragweed
- Ambrosia velutina
Ambrosia mexicana is actually the Jerusalem Oak Goosefoot (Chenopodium botrys), an entirely unrelated plant.

# Footnotes
-  Payne (1963)
-  Mainly Common (A. artemisiifolia), Western (A. psilostachya) and Great Ragweed (A. trifida)
-  Wopfner et al. (2005)
-  Jump up to: 4.0 4.1 Lewis (1973)