Parthenium integrifolium L.

Parthenium integrifolium plant

Family - Asteraceae

Stems - To 90cm tall, from thick root, multiple from base, herbaceous, erect, branching above, scabrous, antrorse strigose (dense above in inflorescence).

Parthenium integrifolium stem

Leaves - Alternate, petiolate to sessile, scabrous above and below, crenate-dentate to crenate-serrate, strigose. Lowest (basal) leaves with winged petiole, to +30cm long, +12cm broad. Cauline leaves becoming reduced above, sessile, clasping, somewhat auriculate, lanceolate.

Parthenium integrifolium leafBasal leaf. Parthenium integrifolium leavesCauline leaves.

Inflorescence - More or less flat-topped dense corymbiform arrangement of flower heads terminating stems. Peduncles densely pubescent (the hairs multicellular). Small bracts present at divisions of inflorescence and often subtending involucre.

Parthenium integrifolium inflorescence

Involucre - 4-5mm tall (long), 6-7mm broad. Phyllaries to 4mm long, 3mm broad, elliptic-orbicular, densely covered with multicellular pubescence.

Ray flowers - 5, fertile, pistillate. Ligule tubular, 1.5mm long. Style bifurcate, brownish-black at stigmas. Achenes black, obovate, 3mm long, lenticular.

Disk flowers - Corollas 5-lobed, 1.4mm long. Achenes(in flower) green, 1mm long, pubescent at apex, becoming black and 3mm long in fruit. Receptacle conic. Chaff with pubescence at the apex, 3.5mm long. Pappus of 2-3 short awns.

Parthenium integrifolium flowersFlowers close-up.

Flowering - May - October.

Habitat - Rocky prairies, limestone glades, open woods, thickets.

Origin - Native to U.S.

Other info. - This species is common throughout nearly all of Missouri but is apparently absent from the extreme northwestern corner of the state. This is a fairly easy (see below) species to identify in the field because of its big clusters of flower heads and big scratchy leaves.
Another species, P. hispidum Raf., is similar but has hispid hairs on its stems and leaves. This species grows in the Ozark region of Missouri only.
Some authors group the 3 similar species of Parthenium in the U.S. (P. auriculatum Britt. being the third) into P. integrifolium and varieties. I tend to agree with this manner of thought.

Photographs taken off Hwy 106, Shannon County, MO., 5-26-03, and on Taum Sauk Mountain, MO., 5-31-03.


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