Scotch thistle Onopordum acanthium L

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Scotch thistle

Onopordum acanthium L. Sunflower Family

Key identifying traits

  • Very large thistle growing up to 8 feet tall

  • Stems have broad spiny wings

  • Leaves are spiny and covered with fine dense hair

  • Hairy leaf surface causes a grayish appearance

  • Flower heads numerous, 1 to 2 inches in diameter

  • Flowers violet to reddish; bracts taper to a spine

  • Upper leaves alternate and coarsely lobed

  • Basal leaves up to 2 feet long and 1 foot wide

Biology and ecology

  • Tap-rooted biennial that reproduces by seed

  • Invades roadsides, range and disturbed areas

  • Thrives in sunny sites – but can tolerate shade

  • Up to 50,000 seeds per plant; seeds viable 6-15 years

  • Repeated branching may result in plants 5’ wide

  • Dense stands can create barriers that restrict livestock

  • Drought tolerant but can flourish along open streams


Prevention- Learn to identify the plant; know your property; beware of fill dirt, hay, manure and seed from outside your area

Biological – No known biological controls

Cultural – Competitive vegetation helps reduce open spaces for invasion

Mechanical – Mowing can stop seed production in short term, but plants can regrow; digging cutting and cultivation are effective; monitor for new growth season long; seed bank will remain for years

Chemical – Several effective at label rates; best timing is in early spring when plants are in rosette stage and again in fall when more rosettes germinate

a yard stick on a Scotch thistle rosette
Where found- Scattered small infestations, particularly in old barnyards as well as some sites covering several acres over pasture/range ground in the northern half of the county.

Created by Stevens County Noxious Weed Control Board, March 2000; Updated Jan 2006

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