Giant ragweed, Ambrosia trifida L., [AMBTR, grande herbe à poux, Great ragweed, Kinghead, Tall ragweed, ambrosie trifide ] Annual, reproducing only by seed. Stems erect, 0.4 -4 m (16 in. - 13 ft) tall, usually much-branched and somewhat hairy; leaves opposite (2 per node) throughout the plant except towards the ends of the smaller branches at flowering time where they may be alternate (1 per node); leaves large, rounded in outline and 3- to 5-lobed, the lobes smooth or coarsely toothed; uppermost small leaves usually not lobed; in some plants most or all leaves not lobed but ovate with acuminate tips and only shallowly toothed; leaf surfaces usually rough (like medium sandpaper); flower heads unisexual as in Common ragweed; male (pollen-producing) flower heads and female (seed-producing) flower heads similar to those described for Common ragweed except that the female head is much larger and usually 2 to 4 are grouped together; seed much larger than Common ragweed, 5-10 mm (1/5 - 2/5 in.) long with several, prominent, lengthwise ridges ending in short blunt spines around the upper shoulder of the seed. Flowers from August to October.

Giant ragweed occurs in river valleys, meadows, roadsides and occasionally in cultivated fields in the southwestern portion of southern Ontario. It is distinguished by its very tall stature (up to 4 m or 13 ft), its large, lobed but not divided leaves, its long, slender spikes of pollen-producing flower heads and its large, angular seeds with spines around the upper shoulder. The pollen of Giant ragweed is an important cause of hay fever during August and September, especially in southwestern Ontario.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)