Ambrosia is a genus of flowering plants from the sunflower family (Asteraceae), commonly referred to as Ragweeds. The name of this genus is derived from the Greek word for "food of the gods".
These are annuals, perennials and shrubs and subshrubs with erect, hispid stems growing in large clumps to a height of 75 – 90 cm. The stems are basally branched. They form a slender taproot or a creeping rhizome.
The foliage is grayish to silvery green with bipinnatifid, deeply lobed leaves with winged petioles. But 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, yellowish-green disc flower are about 3 mm in diameter. They grow in a terminal spike, subtended by joined bracts. The female, whitish-green flowers are 1-flowered and are inconspicuously situated below the male ones, in the leaf axils. The 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 occurs in temperate regions of the northern hemisphere and South America. They prefer dry, sunny grassy plains; sandy soils; and to grow along river banks, along roadsides, disturbed soils, vacant lots and ruderal sites. Ragweed was far less common in the Eastern United States before dense European settlement/agriculture in the late 1700s.
There are 41 species worldwide. They are very ordinary in appearance. Despite being all around, they are easily overlooked. Virtually no animal browses them. 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.
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, as the greatest pollen allergen of all pollens, and 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. Two species, Ambrosia artemisiifolia and A. psilostachya are considered among the most noxious to those prone to hay fever.
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 remedy (e.g. beetle or worm) 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 than leaving the weed alone, in the long run .
The act of manually uprooting ragweeds, sometimes shown in the media for public awareness and propaganda purposes, is best avoided. It is ineffective, and skin contact may cause the onset of full-blown hayfever symptoms in persons with latent ragweed hyper-sensitivity.
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 herbicide 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 gliphosat-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 (equal to app. 0.2 to 0.7 US gallons per acre).
One especially effective method of controlling ragweed; cutting it, then burning it once the stalks have dried  (standing, live ragweed won't burn) has become less popular because the smoke produced is seen as unacceptable pollution, as with the decline in leaf-burning and trash burning. But this 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.
Pests and Diseases
- Aster Leafhopper: Macrosteles facifrons
- Leaf Beetle: Zygogramma bicolorata
- Ragweed Leaf Beetle: Zygogramma suturalis
- Leaf Beetle: Ophraella communa
- Palestriped Flea Beetle: Systena blanda
- Cocklebur Weevil: Rhodobaenus quinquedecimpunctatus
- Stalk Borer: Papaipema nebris
- European Corn Borer: Ostrinia nubilalis
- Ragweed Borer: Epiblema strenuana
- Bordered Patch: Chlosyne lacinia
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.
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. When the humidity rises above 70%, the pollen tends to clump and is not so likely to become airborne.
Goldenrod is frequently blamed for hayfever, but simply happens to have a showy flower that blooms about the same time. Goldenrod is innocent, as it 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. The ragweed was accidentally imported to Europe during World War I, it had adapted to the different environment successfully and has 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 allergen protein has been identified  as Amb a 1, a 38 kDa nonglycosylated protein.
- UVSC Herbarium - Ambrosia
- Wopfner N, Gadermaier G, Egger M, Asero R, Ebner C, Jahn-Schmid B, Ferreira F (Dec 2005). "The spectrum of allergens in ragweed and mugwort pollen.". Int Arch Allergy Immunol 138 (4): 337–46. doi:10.1159/000089188. PMID 16254437. http://content.karger.com/produktedb/produkte.asp?typ=fulltext&file=IAA2005138004337.
- Payne WW (Oct 1963). "The Morphology of the Inflorescence of Ragweeds (Ambrosia-Franseria: Compositae)". American Journal of Botany 50 (9): 872–880. http://www.jstor.org/view/00029122/di001617/00p0112h/0.
- [www.btny.purdue.edu/WeedScience/PostSlide/EOtt12-04.pdf Purdue Weed Science: Distribution od Stalk Boring Insects in Giant Ragweed in Indiana and Southern Michigan]