SCOTCH THISTLE — Onopordum acanthium L

Asteraceae — (Sunflower family)

Seeds germinate in disturbed soils or areas with few competitive plants.  The first year scotch thistle begins as a spiny rosette growing very large. The leaves are hairy top and bottom which gives the plant a grey cast.  Leaves may be 1 foot across and 2ft long, are deeply lobed and very spiny. 
The second year the thick tap root supports a huge rosette which may be 4ft wide as well as the numerous tall spiny grey-green stems.  The lower leaf edges clasp the stems growing downward as ruffly spines about 1 inch wide. 
The small upper leaves seem almost inconspicuous because every stem is covered by a 1 inch wide spiny ruffle from top to bottom.  In Fremont county Scotch thistle often gets 6 ft tall. Each branching stem supports a 1 to 2 inch purple flower. 

The buds are deep bowl shaped and covered with narrow, pointed spines, having the purple petals extending from the top.

One plant can produce up to 40,000 seeds.  The seeds do not transport well on the wind but are carried far and wide by water, animals, people and their equipment.
The seeds need to be covered in order to germinate.  They have a built in “timed release mechanism” which causes the seeds to germinate in different years almost guaranteeing that some of them will grow.  In areas where there is little competition, the seeds that germinate may produce an impenetrable wall that stops all movement of livestock, wildlife and people.  Since the seed may lie dormant for up to 20 years it is essential to treat the Scotch thistle in the first year before seed is set. 

The following is courtesy of Weeds of the West:
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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