|Ambrosia subsp. var.|
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 (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.
Ragweeds are annuals, perennials, and shrubs and subshrubs (called bursages), with erect, hispid stems growing in large clumps to a height of usually Template:Nowrap. 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 (Template:Nowrap) 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 Template:Nowrap 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.
Pests and diseases
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.
- Ambrosia acanthicarpa – Flatspine Burr Ragweed, Annual Bursage, Annual Burrweed
- Ambrosia ambrosioides – Ambrosia Burr Ragweed, Canyon Ragweed, chicura
- Ambrosia ambrosioides ssp. septentrionale
- Ambrosia artemisiifolia – Common Ragweed, Annual Ragweed, American Wormwood, Blackweed, Carrotweed
- Ambrosia aspera
- Ambrosia bidentata – Camphor Weed, Lanceleaf Ragweed
- Ambrosia camphorata
- Ambrosia canescens – Hairy Ragweed
- Ambrosia carduacea – Baja California Ragweed
- Ambrosia chamissonis – Silver Burr Ragweed, Silver Beachweed, Silver Beach Burr, Beach Bur
- Ambrosia cheirnathifolia – Rio Grande Ragweed
- Ambrosia chenopodiifolia – San Diego Burr Ragweed, San Diego Bursage
- 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 eriocentra – Woolly Bursage, Woollyfruit Burr Ragweed
- Ambrosia grayi – Woollyleaf Burr Ragweed
- Ambrosia helenae
- Ambrosia hispida – Coastal Ragweed
- Ambrosia ilicifolia – 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 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 trifolia – Greater Ragweed
- Ambrosia velutina
Ambrosia mexicana is actually the Jerusalem Oak Goosefoot (Chenopodium botrys), an entirely unrelated plant.
Common Ragweed Ambrosia artemisiifolia
- Standard Cyclopedia of Horticulture, by L. H. Bailey, MacMillan Co., 1963