Scotch Thistle

(Onopordum acanthium L.)

Asteraceae (Sunflower Family)

 

▲ seedling plants with woolly leaves, and slightly indented leaf margins

▲ ▼ mature, flowering plants (late July) in Colorado, about 6 feet tall and almost as wide

▲ smaller flowering plant showing "Hershey's Kisses" shaped buds

▲ wide, continuous, spine-edged wings on stem help distinguish Scotch Thistle from other large thistles

▲ ▼ flower heads have stiff, narrow bracts that stand out from the base of the head

 

Scotch Thistle  (pp. 166-167, Weeds of the Great Plains—not in Weeds of the Northeast)

• *    noxious weed in Missouri, but not very common; mostly in eastern Missouri

• *    biennial

• *    strongly winged stems with long spines on "wings"

• *    strongly branched, 2-6' tall, usually larger and coarser appearing than other thistles

• *    basal rosette fleshy, greyish with whitish-cottony appearance from hairs

• mature leaves, stems covered with dense white hairs, giving plant a grayish appearance

• *    receptacle at base of head inflorescence is fleshy and pitted once seeds are removed

• *    can distinguish from tall thistle, bull thistle, musk thistle and field thistle by the pitted receptacle, usually white-woolly leaf and stem appearance and winged stems on scotch thistle

• *    can distinguish from canada thistle by the presence of creeping roots and much small flower head on canada thistle

• *    Initiating a biological control evaluation program in California