Cirsium arvense (L.) Scop. - Canada Thistle

Cirsium arvense plant

Family - Asteraceae

Stems - To +1m tall, from horizontal rhizomes (deep), glabrous or very sparse pubescent at base, striate, herbaceous, erect, branching near apex, hollow.

Cirsium arvense stem

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.

Cirsium arvense leaf

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.

Cirsium arvense involucreInvolucre.

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.

Cirsium arvense flowers

Cirsium arvense fruitsFruiting head.

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.
C. arvense is dioecious and the flower heads are quite variable depending on if they are staminate or pistillate. The species can also vary on pubescence, flower color, and leaf shape. A white-flowered form, form albiflorum (Rand & Redf.), has not yet been reported from Missouri.

Cirsium arvense white flowers

Photographs taken in Spencer, WI., 8-6-04.


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