Ragweed

The ragweeds (Ambrosia spp.) are a genus of herbaceous or shrubby plants belonging to the Asteraceae family, native to North and South America, from where they have spread to Europe. It comprises about thirty species of annual or perennial plants, which grow especially in flat, slightly humid and sandy regions. Several of the Ambrosia species produce large amounts of pollen, which, due to its anemochoric diffusion, is one of the main causes of hay fever.

Features

The Ambrosia species are low-growing herbs or shrubs, although in some species they reach 4 m. They have erect and hispid stems, which occur in dense tufts up to half a meter in diameter, with basal ramifications. The root tends to be conical and deep, making eradication difficult; some are rhizomatic. The leaves are bipinnatifid, lobed, with winged petioles, greyish-green to silvery on the upper side and underside, opposite at the base and alternate on the upper branches.

The plants are monoecious, producing spike-shaped inflorescences supported by fused bracts for the male flowers, yellowish green in color, discoidal in shape and about 3 mm in diameter. The female flowers are whitish, simple, axillary, located lower than the male ones on the stem; they lack dado

Sexual fertilization is carried out by the wind, spreading pollen grains—of which a single plant can produce up to 1 billion in a season—especially in the wet season and in mid-summer. The fruit is an achene covered with thorns, ovoid in shape, containing a single small brown arrowhead-shaped seed.

Habitat

Ambrosia occurs throughout the temperate regions of the northern hemisphere and in northern South America. They prefer sandy, infertile, slightly alkaline soils, and are strongly photophilous. They occur spontaneously on the side of the roads, in ruderales and on the banks of plain rivers.

Taxonomy

The genus was described by Carlos Linnaeus and published in Species Plantarum 2: 987–988. 1753. The type species is: Ambrosia maritima

Etymology

Ambrosia: generic name derived from the Greek “ἀμβροσία” (= ambrosìa), the name of the food that gave the gods immortality, derived from the Greek άμβροτος (àmbrotos ), which means immortal.

Species