Ragweed spp.

Common ragweed, Ambrosia artemisiifolia L., [AMBEL, petite herbe à poux, Short ragweed, ambrosie à feuilles d'armoise] Annual, reproducing only by seed. Stems erect, 15 - 150cm (6-60 in.) high, usually much-branched, hairless or hairy throughout; lower leaves opposite (2 per node) but becoming alternate (1 per node) higher on the plant, bright green to slightly yellowish-green on young plants, becoming greyish-green on older plants, compound and finely divided, the final divisions usually coarsely toothed; flower heads not showy, individually small, 2 - 5 mm (1/12 - 1/5 in.) across, green and inconspicuous but very numerous and forming distinctive inflorescences; individual florets either male or female, but never both; all  flowers within one flower head either only male or female, but both male flower heads and female flower heads usually present on the same plant; heads of male (pollen-producing) flowers in raceme-like elongated clusters at ends of branches, each male head hanging downwards on a short stalk like a tiny inverted umbrella; female (seed-producing) flower heads in axils of short, narrow, green bracts near the base of each long cluster of male flower heads, each female head with only a single flower and producing a single, hard, somewhat triangular or diamond-shaped seed with several, short, sharp spines around the upper shoulder, the whole seed 3 - 5 mm (1/8 - 1/5in.) long. Flowers from August to October.

Common ragweed is one of the most abundant weeds of cultivated land throughout southern Ontario, but is rare or absent in northern and northwestern parts of the province. It also occurs in gardens, flower borders, poorly kept lawns, edges of sidewalks, roadsides, fencelines, waste places, and in disturbed areas in pastures and meadows. It is distinguished by its finely divided leaves, which are opposite in the lower part and alternate in the upper part of the plant, these being yellow-green at first, later gray-green with age, and its very numerous, tiny, non-showy, greenish male flower heads clustered along slender branches in the upper part of the plant.

Common ragweed is the most important cause of hay fever during August and September. Although inconspicuous and not recognized by most people, the tiny male flower heads hanging on their slender stalks produce huge quantities of very light pollen. As the pollen falls from these hanging flowers, it is caught by the wind and may he carried for distances greater than 200 km (125 miles). Hay fever sufferers, therefore, may be affected by pollen from ragweed plants far away. It is essential that Goldenrod, Solidago spp., [verge dor, solidage], not be confused with Common ragweed. Several species of Goldenrod occur throughout Ontario in meadows, pastures, woodland, river flats and roadsides, and have very conspicuous bright yellow inflorescences during the ragweed hay-fever season of late summer and autumn. Goldenrods do produce pollen but only in small quantities, and their pollen is heavy and sticky. It is not carried on the wind and the plants are pollinated by insects. Because Goldenrod pollen is not carried on the wind, it must not be blamed as the source of irritation for ragweed hay fever sufferers.