Scotch Thistle

What is Scotch thistle?

Scotch thistle is a giant biennial thistle from Eurasia that was brought to North America as an ornamental in the 1800s. Scotch thistle identification is mostly about size and color: no other common thistle gets as tall or looks as silvery, thanks to the thick layer of woolly hair on its leaves and stems.

Its spines and bulk make it a real land-use problem. Dense stands crowd out grass, cut the forage large animals can use, and can block livestock and wildlife from grazing and reaching water. A heavy patch can make a campsite unusable or a trail nearly impassable. Because each plant can drop tens of thousands of seeds that stay viable for up to 20 years, one season of neglect can mean years of cleanup.

Scotch Thistle identification: what to look for

In the first year, look for a wide rosette of gray-green, woolly, spiny leaves; rosettes range from a few inches to 5 feet across. In the second year, check the stems. Scotch thistle has broad, spiny wings in vertical rows along the stems right up to the flower heads.

• Leaves: Broad, oblong, shallowly lobed, with sharp yellowish spines and a dense mat of cottony hair on both sides. Wider and paler than other thistles' leaves.
• Stems: Heavily branched, with spiny, ribbon-like wings in vertical rows; older stems can look almost square.
• Flower heads: Upright, globe-shaped, purple to pink, 1 to 2 or more inches across, single or in small groups at branch tips.
• Bracts: Long, stiff, spine-tipped bracts surround the base of each head and stay green.
• Rosette: Flat, gray-green and felt-like, sometimes enormous.
• Roots: A single thick taproot.

Where it grows in the United States

Scotch thistle occurs across most of North America, but it is mainly a pasture and rangeland problem in the West and on the Great Plains. Weed agencies and Extension services in Washington, Utah, Colorado and Nebraska all treat it as a serious weed; in Nebraska it follows the Platte River in the western part of the state and turns up in poorly managed pastures.

It establishes fastest on disturbed ground: gravel pits, new developments, roadsides, burned sites, ditch banks, overgrazed pastures and areas already full of cheatgrass and other annual weeds. From there it moves into neighboring pastures and natural areas. It also grows in wet and dry meadows and along streams, and it can invade healthy, undisturbed rangeland.

Scotch Thistle look-alikes

A few plants get mixed up with it:

• Musk thistle (Carduus nutans): Greener and less woolly, with large flower heads that nod to one side and bracts colored like the flower. Scotch thistle's heads stand upright and its bracts stay green.
• Bull thistle (Cirsium vulgare): Green rather than silvery. Scotch thistle has noticeably broader, paler, woollier leaves with short, shallow lobes.

How to get rid of Scotch thistle

Pulling, digging and mowing: Dig small infestations by cutting the taproot below the soil line with a sharp shovel; young plants pull fairly easily, mature ones do not. Mowing only helps before flowering, and plants often regrow and bloom on shorter stalks, so plan on repeat cuts or a follow-up treatment. Once seed heads have formed, cut them, bag them and put them in the trash, since cut heads can still ripen seed. Tilling works on young plants. Wear heavy gloves and long sleeves. Some native thistles are rare or protected, so confirm the ID before you start.

Cultural control matters most, because the right conditions keep it from coming back.

• Keep pastures dense and don't graze below about 4 inches; Scotch thistle exploits bare, overgrazed ground.
• Reseed disturbed sites with well-adapted perennial grasses and forbs, and use rotational grazing.
• Goats eat the flower heads and sheep eat small rosettes, which can cut seed production; no insect biocontrol is available.
• Use weed-free hay, seed and fill, and feed animals weed-free hay for three days before moving them off an infested property.

Herbicides: Herbicides work best on rosettes in spring or fall, before bolting; once plants are in full bloom, it is usually too late. Fact sheets name aminopyralid, clopyralid, chlorsulfuron, metsulfuron, dicamba, triclopyr, picloram (a restricted-use product), 2,4-D and glyphosate. Nevada research found 2,4-D alone did a poor job on rosettes and maturing plants, and glyphosate kills the surrounding grass too. Many pasture herbicides injure clovers and alfalfa, so check before spraying a grass-legume mix. Expect to retreat for three to five years until the seed bank runs out. Use only products labeled for your site and follow the label; the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Stop seed production at all costs; with seed that can last 20 years, every flower head matters. Scout disturbed ground, field edges and roadsides each spring and fall and remove rosettes while they are small.

Clean seeds and plant parts off vehicles, equipment, clothing, pets and livestock before leaving an infested area, and buy weed-free fill, hay and seed. A thick stand of perennial grass under good grazing management is the cheapest long-term defense.

Questions we get about Scotch thistle

Similar weeds

Musk ThistleCarduus nutansBull ThistleCirsium vulgareBuffaloburSolanum rostratum