Perennials, 50-100+ cm. Stems (from short caudices or rhizomes) single, sparsely branched distally, pubescent throughout. Leaves usually opposite or whorled (distal sometimes alternate, spreading or horizontal); simple, sessile; blades 3-nerved from bases (laterals sometimes weak), lance-linear, lance-oblong, or linear, 20-60 × 2-15 mm (lengths mostly 6-40 times widths), bases cuneate, margins entire, laciniate-serrate, or serrate, apices acute, faces scabrous (at least abaxial). Heads in corymbiform arrays. Phyllaries 8-10 in 2-3 series, elliptic to oblong, 1.5-5 × 1-1.5 mm, apices obtuse to acute (not mucronate), abaxial faces pubescent throughout (more densely distally). Florets 5; corollas 3-3.5 mm. Cypselae 2-3 mm; pappi of 20-30 bristles 3.5-4 mm. Eupatorium hyssopifolium includes both diploid and polyploid cytotypes and presents a complex situation taxonomically. The diploids are placed here in var. hyssopifolium, which also includes polyploids and is characterized by relatively narrow leaves. Variety laciniatum has broader leaves and molecular data suggest that it arose through hybridization between var. hyssopifolium and E. serotinum; the name E. torreyanum has been applied to plants of similar morphology, but these are hybrid derivatives of E. serotinum and E. mohrii. It has also been suggested that var. hyssopifolium has hybridized with E. album var. album to form E. saltuense, and with diploids of E. linearifolium to form polyploid populations included here under E. linearifolium. Eupatorium hyssopifolium is characterized by a tendency for the leaves to be whorled, in 3s and 4s, and for axillary buds to produce leaves without expansion of shoots; the combination of whorled leaves and suppressed axillary shoots gives the nodes a somewhat tufted or verticillate appearance.
Stems solitary (seldom several) from a short crown, strigose or scabrous-puberulent, especially above; lvs verticillate in 3's or 4's, or sometimes merely opposite, or even alternate above, narrow, mostly 6-40 times as long as wide, sessile or tapering to a slender base, spreading or ascending, glandular-punctate, otherwise glabrous or slightly hairy chiefly on the main veins beneath, the principal ones subtending conspicuous axillary fascicles of reduced lvs; invol 4-7 mm, its bracts imbricate, broadly rounded to acute, the inner narrowly scarious-margined; fls 5, the cor white, 3.5-4 mm; 2n=20, 30, 40. Fields and other open places, especially in dry, sandy soil; coastal states from Mass. to n. Fla. and w. to La., and sometimes inland to O., Ky., and Tenn. Aug.-Oct. Two well marked vars.:
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.