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Biological controls for SCOTCH THISTLE

 

 

 

 

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Scotch Thistle —Onopordum acanthium L

Asteraceae — (Sunflower family)

Scotch thistle is a biennial that grows up to twelve feet tall. Stems have broad, spiny wings. Leaves are large, spiny and covered with fine dense hair, giving a grayish or blue-green color in appearance. Upper leaves are alternate, coarsely lobed; basal leaves may be up to two feet long and one foot wide. Flower heads are numerous, one to two inches in diameter, bracts, spine-tipped. Flowers of violet to reddish are produced the second year of this biennial’s growth cycle. Fruits are about 3/16 inch long, tipped with slender bristles.

Scotch thistle is a native of Europe and Eastern Asia and is now sparsely naturalized over much of the United States. It can be found along waste areas and roadsides. It is an aggressive plant and may form stands so dense that they are impenetrable to livestock. Rosettes two feet across are not uncommon for this hearty plant. Scotch thistle is best controlled in the rosette stage.

Nonstandard name: cotton thistle.
Growth Habit: Biennial, sometimes annual, erect, up to 8` tall. Rosette formed first year, flowering stem elongates second year.
Leaves: Large, coarsely lobed, hairy on both sides, velvety gray appearance. Margins lined with sharp conspicuous spines. Basal leaves up to 2` long and 1` wide.
Stems: Erect, branching, spiny leaf wings extend down onto stem, covered with dense fine hairs.
Flower: Solitary, terminal, 1 to 2" in diameter, violet to reddish colored. Bracts spine tipped.
Roots: Large fleshy taproot.
Seeds: Deep brown to black, distinctly wrinkled, 3/16" long.
Other: Reproduce by seed only. Dense stands may be impenetrable to recreationists, livestock or wildlife.
(Courtesy of Weeds of the West)

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