From Academic Kids
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Ragweeds (Ambrosia) is a genus of flowering plants from the sunflower family (Asteraceae).
The name of this genus is derived from the Greek word for "food of the gods"
They occur in temperate regions of the northern hemisphere and South America. They prefer dry, sunny grassy plains, sandy soils, along river banks, along roadsides, disturbed soils, vacant lots and ruderal sites.
There are about 30 species of noxious weeds 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.
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 amonoecious 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 pollination, the female flowers develops into an 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 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-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.
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, ie. 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.
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.
- Ambrosia acanthicarpa : Flatspine Burr Ragweed, Annual Bur-Sage
- Ambrosia ambrosioides : Ambrosia Burr Ragweed, Canyon Ragweed
- Ambrosia ambrosioides ssp. septentrionale
- Ambrosia artemisiifolia : Annual Ragweed, Bitterweed, Blackweed, American Wormwood
- 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 Burr Sage.
- Ambrosia confertiflora : Weakleaf Burr Ragweed
- Ambrosia cordifolia : Tucson Burr Ragweed
- Ambrosia coronopifolia
- Ambrosia deltoidea : Triangle Burr Ragweed, Triangleleaf Bur-sage, Rabbitbush.
- Ambrosia dumosa : Burrobush, White Bursage.
- Ambrosia elatior (synonym of A. artemisiifolia): Carrotweed, Annual Ragweed
- 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 : type species
- Ambrosia palustris
- Ambrosia pannosa
- Ambrosia parvifolia
- Ambrosia peruviana : Peruvian Ragweed
- Ambrosia psilostachya : Cuman Ragweed, Western Ragweed, Perennial Ragweed
- Ambrosia pumila : Dwarf Burr Ragweed, San Diego Ambrosia
- Ambrosia sandersonii
- Ambrosia scabra
- 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, Bitterweed, Bloodweed
- Ambrosia trifida texana : Texan great Ragweed
- Ambrosia trifolia : Bitterweed, Bloodweed, Great Ragweed, Buffalo Weed
- Ambrosia velutina