Native. Annual. Propagates by seeds. Time of bloom: July to September. Seed-time: August to November.
Range: Nova Scotia to Florida, westward to the Northwest Territory, Nebraska, Colorado, and Arkansas.
A huge, coarse plant, occupying so much room and feeding so grossly that crops growing with it are crowded and starved to death. Its usual height is four to ten feet, but on very fertile river bottom-lands it attains to twelve and even fifteen feet.
Stem stout, tough, woody, widely branched and rough with bristly hairs. Leaves also rough-hairy and varying greatly in shape, often more than a foot long, mostly three-parted, but some may have five lobes and yet others may be ovate or lance-shaped; usually they are coarsely toothed but the smaller upper ones are often entire; all are opposite, three-nerved, the petioles stout and margined. Sterile heads in racemes six inches to a foot in length, their involucres three-ribbed on the outer side with scalloped margins. Fertile involucres clustered in the axils of the upper leaves. These form a fruit a quarter-inch or more long, brown, obovoid, five- or six-ribbed, with a conic beak at apex surrounded by five or six shorter spines like the points of a crown; whence its names of Kinghead and Crown weed. These spines, or tubercles, give much trouble in cleaning it from other seeds, as they catch in the screens; also they contain air spaces, which enable the fruits to float on water and in winter to be blown far over crusted snow. (Fig. 317.)
Fig. 317. -Giant Ragweed (Ambrosia trifida). X 1/8.
When young and tender, Great Ragweed succumbs readily to the blighting touch of chemical sprays. But if allowed to approach maturity it pays to hand-pull the weed, for the stout, woody stalks so dull and break the blades of mowers and reapers, cause so much waste of binding twine, and are so clogging to the feed-way of threshing machines, that the earlier handwork is really an economy. In cultivated crops the plant gives little trouble, being killed there while young.