Cirsium arvense (L.) Scop. - Canada Thistle
Family - Asteraceae
Stems - To +1m tall, from horizontal rhizomes (deep), glabrous or very sparse pubescent at base, striate, herbaceous, erect, branching near apex, hollow.
Leaves - Alternate, to -15cm long, 6-7cm broad, sessile, pinnately lobed, with spines on margins, sparse arachnoid pubescent (mostly on lobes below). Upper leaves less lobed to serrate.
Inflorescence - Single terminal flower heads in loose corymbiform arrangement. Branches of inflorescence (peduncles) arachnoid pubescent.
Involucre - To 1.5cm tall (long), 8-9mm in diameter, suburceolate. Phyllaries imbricate, appressed, with dark purple tips. Tips acute, to 1mm long, slightly spreading. Innermost phyllaries to 1.1cm long, 1.5mm broad, glabrous, with scarious margins. Outer phyllaries with sparse to moderate arachnoid pubescence.
Ray flowers - Absent.
Disk flowers - Corolla pink, 5-lobed. Lobes 2.5-3mm long, linear, thickened at apex with small protuberance. Corolla tube 1.1cm long, lilac-whitish. Achenes to 2mm long, glabrous, white(in flower) terete to weakly 4-angled. Pappus of dense plumose bristles to 1cm long. Receptacle flat.
Flowering - June - October.
Habitat - Fields, waste ground, roadsides, railroads.
Origin - Native to Europe.
Other info. - This is a nasty
little plant. It is not too common in Missouri yet but most certainly will
be soon. It is very common in states to the north and east.
The plant is considered a noxious weed and should not be spread. It is
quite aggressive and because of its deep rhizomes is very hard to eradicate.
Photographs taken in Spencer, WI., 8-6-04.