Yarrow (Achillea millefolium)
Also called Milfoil, “thousand-leaf,” from the finely divided leaves. A European import that has become a common wildflower all over the East. Still a popular garden flower; in recent years many colors have been bred, but the wild ones are almost always either white or pink. This plant, blooming in mid-October, grew by the side of a country road a little west of Houston, Pennsylvania.
Gray describes the genus and the species:
ACHILLEA [Vaill.] L. YARROW
Heads many-flowered, radiate; the rays few, fertile. Involucral bracts imbricated, with scarious margins. Receptacle chaffy, flattish. Achenes oblong, flattened, margined; pappus none. Perennial herbs, with small corymbose heads. (So named because its virtues are said to have been discovered by Achilles.)
A. millefolium L. (COMMON Y., MILFOIL.) Stem simple or sometimes forked above, 3-10 dm. high, arachnoid or nearly smooth; stem-leaves numerous (8-15), smooth or loosely pubescent; corymbs very compound, 6-20 cm. broad, flat-topped, the branches stiff; involucre 3-5 mm. long, its bracts all pale, or in exposed situations the uppermost becoming dark-margined; rays 5-10, white to crimson, short-oblong, 1.5-2.5 mm. long. Fields and river-banks, common. (Eurasia.)