Eupatorium altissimum L.

Tall Thoroughwort


CC = 3
CW = 5
MOC = 74

© DETenaglia

Family - Asteraceae/Eupatorieae

Stems - To +1.5m tall, erect, herbaceous, terete, simple below, branching in the apical 1/3, glabrescent below, tomentose above, green to tan.


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Leaves - Opposite, sessile, decussate, often with a smaller pair of leaves in the axils of the larger main leaves, to +10cm long, to 2cm broad, lanceolate to oblong-lanceolate to linear-lanceolate, coarse serrate in the apical 1/2 (at least on the lower leaves), green on both surfaces, with antrorsely bent pubescence, much more pubescent abaxially, with 3 main veins arising at the very base of the leaf blade. Other veins of the leaves anastomosing before the margin of the blade. Margins antrorse ciliolate.


© DETenaglia

Inflorescence - Multiple flowerheads in a corymbose arrangement terminating the stem. Each division of the inflorescence subtended by a pair of reduced bracts. Bracts foliaceous.

Eupatorium_altissimum_inflorescence.jpg A portion of the inflorescence.

© DETenaglia

Involucre - 5mm long (tall), +/-2mm in diameter, cylindric. Outer phyllaries smaller than the inner. Inner phyllaries to 5mm long, 1.4mm broad, tomentose externally glabrous internally, with a scarious-white margin, with 3 darker green veins (use a lens to see), with a solid green apex, linear to oblanceolate, rounded at the apex.

Eupatorium_altissimum_involucre.jpg Involucre.

© DETenaglia

Ray flowers - Absent.

Disk flowers - Flowerheads with 4-7 flowers. Corolla white in the apical 1/2, green basally, 4mm long, glabrous internally and externally, expanded in the apical 1/2, 5-lobed. Lobes acute, 1mm long, .5mm broad at the base, erect to spreading. Stamens 5, adnate at the apex of the contracted portion of the corolla tube. Filaments white, glabrous, compressed, .7mm long. Anthers purplish, to 1.2mm long, connate around the style, partially exserted. Style glabrous, green basally, white apically, to +/-6mm long, bifurcate in the apical 3mm (the divisions erect). Achenes green in flower, 2-2.2mm long, glabrous, terete or 5-sided. Pappus of capillary bristles. Bristles antrorse barbellate, white, 4-5mm long. Receptacle naked.


© DETenaglia

Flowering - August - October.

Habitat - Prairies, glades, thickets, open rocky wood, waste ground, fallow fields.

Origin - Native to U.S.

Other info. - This species can be found throughout much of Missouri but is mostly absent in the southeastern corner of the state. This is a non-distinct plant and many would consider it a weed. "altissimum" means "tallest" but this is not the tallest species of the genus, there are many which generally exceed its height. The plant is not difficult to identify but it can be confused with the similar members of the genus. The tomentose stems, 3 main leaf veins, and rounded phyllaries help to identify it correctly. Plants growing in full sun are much more stout, have more leaves, and have bigger inflorescences than do plants growing in the shade or in wooded areas.

Photographs taken in Eminence, MO., 9-21-03, and at Tall Grass Prairie National Preserve, KS., 9-20-06 .