Eupatorium purpureum L. - Green-Stemmed Joe-Pye Weed

Eupatorium purpureum plant

Family - Asteraceae

Stems - To +2m tall, glabrous, glaucous, erect, simple, herbaceous, solid to subhollow, green with purple at nodes only.

Eupatorium purpureum stemStem with purple node.

Leaves - In whorls of 4 or 5, petiolate, Petioles to +/-2cm long, green. glabrous. Blades to 30cm long, +9cm broad, elliptic-lanceolate, tapering at base, coarse serrate, acuminate, very sparse pubescent above, tomentoulose below. Teeth with strigillose margins and with prickle tip from vascular tissue extending beyond leaf tissue.

Eupatorium purpureum leaves

Inflorescence - Flowers in a terminal paniculate cyme to 30cm tall (long), +/-20 cm in diameter. Branches of inflorescence tomentoulose to tomentose, green.

Involucre - 8-9mm tall (long), 2-2.3mm in diameter, greenish white at base, lilac above. Phyllaries imbricate. Outer phyllaries tomentoulose externally. Innermost phyllaries 8mm long, 1mm broad, with scarious margins and few cilia at apex. Flowers 4-6 per head.

Eupatorium purpureum involucre

Ray flowers - Absent.

Disk flowers - Corolla tube 5-6mm long, 5-lobed, appressed pubescent, lilac. Lobes .9mm long, acute. Stamens 5, adnate about 1/2 way up corolla tube. Anthers whitish-pink, 2mm long, connate around style. Style bifurcate. Stigmas 5-6mm long. Achenes blackish, angled, to 3.5mm long, glabrous. Pappus of capillary bristles to 6.5mm long.

Flowering - July - September.

Habitat - Moist ground, wooded slopes, wet meadows and thickets, streambanks.

Origin - Native to U.S.

Other info. - This species can be distinguished from the similar E. fistulosum Barratt by its green solid stems with purple nodes. E. fistulosum is much less common in the wild( in Missouri) and has completely purple stems which are hollow. Both species are popular in cultivation and often misnamed.
The flowers of E. purpureum appear white at first but become more purplish with age. The species name of the plant, purpureum, means "becoming purple".

Photographs taken at the Kansas City Zoo, 7-23-00 and at Cave Spring, Shannon County, MO., 7-17-03.


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