Scotch Thistle

Scotch Thistle

(Onopordum acanthium L.)

Asteraceae (Sunflower Family)

▲ ▼ scotch thistle seedlings

▲ ▼ scotch thistle seedlings, young plants

▲ ▼ first year plants with woolly leaves

▲ center of first year plant rosette showing dense white hairs

▲ ▼ second-year plant beginning to elongate flowering stem; lower leaves on above plant are nearly 2 feet long

▲ second-year plant beginning to elongate flowering stem

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

▲ ▼ wide, continuous, spine-edged wings on stem

▲ wide, continuous, spine-edged wings on stem help even up to base of inflorescences

▲ hollow stems

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

▲ mature, flowering plant in western Nebraska

▲ mature, flowering plant in Warnemunde, Germany

▲ ▼ mature, flowering plant

▲ mature, flowering plant, again showing spiny, winged stems

▲ inflorescence in bud

▲ ▼ inflorescences, showing round-spine-tipped bracts below inflorescence

▲ ▼ inflorescences

▲ flattened receptacle during flowering senesecence

▲ ▼ inflorescence receptacle, showing pits that hold seeds/achenes

▲ seeds

Scotch Thistle

noxious weed in Missouri, but not very common; mostly in eastern Missouri
biennial
strongly winged stems with long spines on "wings"
strongly branched, 2-8' tall, usually larger and coarser appearing than other thistles
basal rosette leaves have 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