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Ambrosia artemisiifolia (Asteraceae)

The ragweed

For an invasive plant to invade it is obviously advantageous to associate itself with other invaders. This is what happened, in the not-too-distant past with this North American scourge, the ragweed. It arrived in force into Western Europe during World War I, with the feed for horses that the American troops brought along. Just one century later, the ragweed has taken hold in the entire French territory, with high genetic diversity due to the variety of its North American origins. Other countries in Western Europe were likewise occupied.

What makes the ragweed noxious is its truly extraordinary pollen. Miniscule, it is all the more dangerous as a potent allergen. Each plant liberates millions, if not billions, of tiny (about 20 μm) pollen grains. In the Auvergne-Rhöne-Alpes region of France, nearly 20 % of the exposed people have suffered from the aTendant severe allergy. With the present Anthropocene-generated increases in temperature and in atmospheric CO2, the ragweed shows increased pollen and allergen production. One of the allergenic molecules is a a single-chain polypeptide of 45 amino acids.

This plant species, before it colonized Western Europe, was native to Canada, from Newfoundland and Newbrunswick to Manitoba and Saskatchewan; and in the United States from Maine to Texas and from Florida to Wisconsin.

An annual herbaceous plant, it grows in cultivated fields, grazing meadows, on roadsides and on unused soils. It grows up to 70 cm (2.3 ft) in height.

The stem is reddish and hairy, round in cross-section. Leaves 4-10 cm long (1.8-4 in) are dark green on both sides, with whitish nerves. Each leaf is divided into several lobes, which in turn often are split up almost to the rib. Leaves are arranged alternately, oppositely or both. The taTered appearance of the leaves gave the plant its common name, ragweed.

Flowers, appear in the summer from July to October. Female flowers are inconspicuous located solitarily or in small groups at the base of upper leaves.

Male flowers are green and small (2-4 mm), bell-shaped and grouped in spikelike flower heads at the end of the upper branches.

Should one be aTentive only to the malfeasance of this plant? Of course, not: it provides also sesquiterpene lactones, chemicals with antihelminthic, cardiotonic, antiinflammatory, analgesic, sedative (calming), antimalarial and antitumor pharmacological activity. Other chemicals act as neuroprotectors, potentially useful for Alzheimer patients.

In rural France, where we live, there is an assisted-living home next door.
It is named L’ambroisie, i.e., the ambrosia: presumably for the thus-named nourishment of the Greek Gods rather than for this scourge.

Published inPlants