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Figure from The pollen content in the air-Identification Key prapared by RNSA.
Ragweeds (Ambrosia), also called bitterweeds or 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 (ἀμβροσία), 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 (άμβροτος), "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, river banks, roadsides, and ruderal sites (disturbed soils) such as vacant lots and abandoned fields.
There are 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.
This genus is not to be confused with Kochia scoparia, which also has the common name "ragweed".
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 (Ambrosia 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 flower 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 hay fever 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 early July until mid-August or 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 hay fever 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. 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 out to sea. Today, no area in the United States is free of ragweed pollen, and moving can only offer a degree of relief. Ragweeds native to the Americas were accidentally introduced to Europe starting in the nineteenth century and particularly during World War I; they thrived and have greatly spread since the 1950s. Eastern Europe, particularly Hungary, has been badly affected by ragweed since the early 1990s, when the dismantling of live online casino Communist collective agriculture led to large-scale abandonment of agricultural land, and new building projects also resulted in disturbed, online casino no deposit bonus un-landscaped acreage.
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 play the pokies 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, but control is important to minimize its spread and reduce its effect on the allergic.
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 the plant is controlled by reaping, in populated areas and near delicate plantings that limit herbicide use, mowing should be repeated continuously every three weeks because it is difficult to cut the plant right at the soil level, and it will regrow in two weeks (and often branch into three or four full-sized stems) if more than half an inch remains above the ground.
A previously favored method of controlling ragweed was cutting it, leaving the cuts in the field, then burning them there once the stalks had dried, since standing, live ragweed will not burn. It is less popular today, because the smoke produced is seen as unacceptable pollution, as with the decline in leaf-burning and trash burning. The method has the added benefit of killing off the stems so the plant does not grow back, which is otherwise almost inevitable.
Manually uprooting ragweed, 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 hay fever symptoms in persons with latent ragweed hypersensitivity. That being said, ragweed is best uprooted in late spring, before the flowering season and before a strong root system has developed.
There is evidence that mechanical and chemical control methods are actually no more effective in the long run than leaving the weed alone.
Fungal rusts and especially the leaf-eating beetle Ophraella communa have been proposed for biological control to be used against ragweed, but the latter may be dangerous to sunflowers and there have been problems obtaining permits and funding to test such controls.
|Last Updated on Wednesday, 17 April 2013 10:02|