Sonchus arvensis L.
Field Sow Thistle
Family - Asteraceae
Stems - To 1.5m tall, erect, herbaceous, glabrous, glaucous, hollow, with milky sap, branching above in inflorescence, single from stout taproot or stolons.
Leaves - Alternate, sessile, clasping, glabrous, dentate (the teeth with minute prickle tips), to +30cm long, reduced above. Lower leaves lyrate pinnatifid. Upper leaves oblong-linear.
Inflorescence - Loose corymbiform arrangement of flower heads terminating branches of inflorescence. Peduncles glabrous or with glandular pubescence. Each division of inflorescence subtended by small scalelike attenuate bract.
Involucre - Cylindrical to urceolate, to 1.4cm tall (long) in flower, 8-9mm in diameter. Phyllaries in two or three series. Outer most phyllaries small. Inner phyllaries to 1.6mm long, 2mm broad, linear, with scarious margins, spreading to slightly recurved at apex, glabrous or with glandular pubescence, with darker green midvein, appressed.
Involucre in flower.
Involucre in fruit.
Ray flowers - Head to 5cm broad. Flowers fertile. Ligules yellow, -3cm long, 3-4 toothed at apex, glabrous. Corolla tube whitish, pubescent. Pappus of numerous capillary bristles to 1.2cm long. Achene 1mm long in flower(+3mm long in fruit), pubescent. Receptacle slightly concave.
Disk flowers - Absent.
Flowering - June - October.
Habitat - Waste ground, roadsides, railroads.
Origin - Native to Eurasia.
Other info. - This is definitely the most showy member of the genus in Missouri. The large flower heads are unmistakable. Unfortunately, the plant is introduced and should not be willingly spread. It is very aggressive and is becoming quite widespread.
Photographs taken in Wausau, WI., 8-6-04.