This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Annual. Propagates by seed.
Time of bloom: July to September.
Seed-time: August to November.
Range: United States and Canada from Nova Scotia to British Columbia, southward to Florida and Texas.
Habitat: Dry soil; cultivated ground, meadows, waste places.
One of the most common of weeds, intruding almost everywhere; it is a pest in meadows and pastures, for, though cattle do not relish its bitter juices, they will sometimes eat it when better forage is scarce and, as a consequence, yield bitter milk with a bad odor. After the removal of a grain crop this plant nearly always springs up in the stubble. When in bloom its abundant pollen is said to cause "hay fever," and it is dreaded and avoided by persons subject to the disease.
Ragweed has rather deep, branching roots, from which the stem rises one to five feet, erect, finely hairy, and branching freely. Leaves alternate, two to four inches long, thin, deep green above, paler beneath, twice pinnatifid, giving the plant an open, feathery appearance. Flowers of two kinds, the staminate heads in crowded spike-like recemes at the summit of the plant and in its upper axils; the involucres top-shaped, formed of five to twelve united bracts, and containing six to twenty small, greenish flowers. Below, in the axils, concealed by clustering bracts, are the fertile involucres, each one containing a single flower, the elongated branches of its style protruding from the closed and pointed crown; when mature these involucres form hard achene-like fruits, about an eighth of an inch long, ovoid, with a beaked crown, surrounded by four to six spiny points. Once in the soil, they survive for years, springing up when opportunity offers; they are a common impurity of grain and grass seed and are also distributed in baled hay. (Fig. 318.)
The thin, softly hairy, and widespread foliage of young Ragweed is very susceptible to injury from chemical sprays, and an application of Copper sulfate or Iron sulfate will kill the plants in multitudes without injury to the grass or grain among which they are growing. In clover fields the crop is slightly injured but recovers from the roots, which the weed-seedlings seldom do. Infested clover fields that are not treated should be cut early before the flower-ing of the weed, as its pollen is extremely bitter and "cuts the quality" of the hay even more than its dried young stalks. Stubbles should have surface cultivation directly after harvest so as to encourage germination of seeds in the soil, when the young plants may be killed with the harrow, or they may be plowed under for humus. In cultivated ground tillage should be continued late, as it is the plants that bloom and fruit after cultivation has ceased which are most certain to foul the soil.
Fig. 318. - Common Ragweed (Ambrosia artemisiifolia). X 1/6.