Parthenium integrifolium L.
Family - Asteraceae
Stems - To 90cm tall, from thick root, multiple from base, herbaceous, erect, branching above, scabrous, antrorse strigose (dense above in inflorescence).
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.
Basal leaf. Cauline 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.
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.
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.
Photographs taken off Hwy 106, Shannon County, MO., 5-26-03, and on Taum Sauk Mountain, MO., 5-31-03.