This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Eupatorium Torreyanum Short, 2nd Suppl. Cat. Pl.
Ky. 5. 1836. Eupatorium hyssopifolium var. laciniatum A. Gray, Syn. Fl. 1: Part 2, 98. 1884.
Rootstock tuberous-thickened; stem usually densely puberulent, mostly branched above, 1 1/2'°-3° high. Leaves opposite, or sometimes in 3's, commonly with short leafy branches in their axils, often drooping, lanceolate, acute at the apex, narrowed at the base, sessile, 1'-3' long, 2"-6" wide, usually distinctly 3-nerved, distantly serrate, the upper and those of the branches much smaller, linear, entire; inflorescence mostly loose; heads about 3" high, generally 5-flowered; bracts of the campanulate involucre linear-oblong, obtuse, pubescent, or puberulent, imbricated in about 3 series, the outer shorter; flowers white.
In dry soil, Pennsylvania to North Carolina, Kentucky, Florida and Texas. July-Sept.
E. semiserratum DC. Prodr. 5: 177. 1836.
Pubescent or puberulent, loosely branched above, 2°-3° high. Leaves rather thin, short-petioled, oblong-lanceolate to spatulate, acute or obtusish at the apex, narrowed at the base, sharply serrate, at least above the middle, 2'-4' long, 5"-12" wide, 3-nerved, usually with short branches in their axils; inflorescence cymose-paniculate; heads 2"-3" high, about 5-flowered; involucre campanulate, its bracts linear-oblong, obtuse, canescent, imbricated in 2 or 3 series, the outer shorter, flowers white.
In dry soil, Virginia to Florida, Missouri, Arkansas and Texas. Aug.-Sept.
Eupatorium altissimum L. Sp. Pl. 837. 1753- Densely and finely pubescent, corymbosely much-branched above, 4°-8° high. Leaves lanceolate, acuminate at the apex, tapering below into a short petiole, roughish, rather thick, sparingly dentate above the middle, or some of them entire, strongly 3-ribbed, 2'-5' long, 5"-12" wide; inflorescence densely cymose-paniculate; heads about 5-flow-ered, 3"-4" high; involucre campanulate, its bracts oblong, obtuse or truncate, densely pubescent, imbricated in about 3 series, the outer shorter; flowers white.
In dry open places, Pennsylvania to North Carolina, Alabama, Illinois, Minnesota, Nebraska and Texas. Sept.-Oct.
Eupatorium sessilifolium L. Sp. Pl. 837. 1753.
Glabrous throughout, or pubescent above, branched near the summit, 2°-6° high. Leaves opposite, or the lower rarely in 3's, closely sessile, lanceolate or ovate-lanceolate, long-acuminate, thin, sharply serrate, 3'-6' long, 1/2'-1 1/2' wide, inflorescence cymose-paniculate; heads 4"-5" high, about 5-flowered; involucre campanulate, its bracts linear-oblong, imbricated in about 3 series, the inner obtuse, the outer acut-ish, shorter; flowers white.
In dry woods, Vermont to Massachusetts, Pennsylvania, Georgia, Alabama and Missouri. Aug.-Oct. A related plant, described under the name Eupatorium sessilifolium Brittonianum Porter, has leaves ovate or oblong-ovate, sparingly and finely serrate, firm, rounded at the base, acute, the upper 1'-2' long, 6"-9" wide, pinnately veined, dark green; cymes dense, their branches puberulent. It is known only from Budd's Lake, N. J.