Scotch Thistle

Information

Onopordum acanthium - Asteraceae Family - Biennial

Other common name: Cotton Thistle

scotch_thistle_1_tile scotch_thistle_2_tile scotch_thistle_3_tile scotch_thistle_4_tile

Identification

Flowers: Plants are covered with 1” spiny purple flowers.
Seeds: Plants produce up to 6,000 seeds annually with seeds remaining viable in the soil up to 5 years.
Leaves: Up to 2 ft long and covered in cottony/ woolly hairs with spines on the margins.
Flowering Time: June - September.
Life cycle/ other: Biennial growing up to 8 feet tall and 3 feet wide.

Impacts

Scotch thistle primarily invades disturbed lands.
Its rapid growth and large size reduce available resources for smaller plants. It competes with and decreases desirable forage and can form a dense monoculture stand.
It can act as a living barbed-wire fence, preventing livestock and wildlife access to feed and water.

Most effective control methods

Prevention of this plant's invasion is the best management. Control plants before they flower and set seed.
Control of these plants must include preventing new seed dispersal for 6 years.
For small infestations the best method for removal is digging plants out by hand.
For larger infestations the use of herbicide on young plants is effective in the rosette stage before flowering prevents seed set. Picloram and metsulfuron offer excellent control.
In both small and large infestations, plant competition by seeding disturbed areas with desirable grass species that will compete for resources but not be affected by the broad leaf herbicides is best for long-term management.

Control methods and timing

March	April	May	June	July	Aug	Sept	Oct
Prev	Prev Chem	Prev Chem	Mech	Mech	Mech	Mech Chem	Mech Chem

March

April

May

June

July

Aug

Sept

Oct

Chem

Mech

Chem

Mech

Chem

Prevention (Prev) Monitor and destroy new plants before seed production.

Mechanical (Mech) Hand pulling, digging, cutting, mowing and tilling.

Cultural (Cult) Biological control agents, livestock grazing, and revegetation practices.

Chemical (Chem) Selective herbicides based on the plant and the specific location. Check our weed fact sheets for specific control information.

Scotch thistle: flower

Scotch thistle

Scotch thistle: flower

Scotch thistle: infestation

Scotch thistle: rosettes

Scotch thistle: foliage

Scotch thistle: rosette

Scotch thistle: seeds