Scotch Thistle

General: Scotch thistle is considered a biennial herb, but it can behave like an annual or a short-lived perennial depending on the conditions. It emerges as a rosette, before bolting and producing seeds in a subsequent season. Scotch thistle plants die after flowering.

Flowers: The vibrant purple flowers grow at the end of leafy stalks, either as a single flower or as a cluster of flowers. Flowers are globe-shaped and 2 – 8 cm in diameter.

Stem: Erect and branched. Scotch thistle stems have broad, spiny wings and woolly hairs. The plant can grow quite large, up to 1.5 – 3 m tall and 1.8 m wide.

Leaves: Alternate, large, and irregularly lobed, with sharp yellow spikes. Leaves are covered in a thick mat of cotton-like or woolly hairs, giving them a grey-green appearance. Rosette leaves will grow to be 60 cm long and 30 cm wide.

Seeds: Brown to greyish black, wrinkled, and slender with hairs at one end.

Roots: Fibrous roots.

Similar Species

Invasive

Bull thistle(Cirsium vulgare): Scotch thistle has larger, yet fewer flower heads than bull thistle, and is woolly all over.

Bull thistle

Milk thistle (Silybum marianum) has distinctive marbling patterns on its leaves.

Photo credit: J. Leekie

Habitat and Origin

Scotch thistle is native to Europe and eastern Asia. It was introduced to North America during the 19th century as a garden ornamental.

While Scotch thistle is drought tolerant, it tends to prefer moist soils. It typically grows along roadsides and in disturbed areas, as well as in irrigation ditches, riverbanks, pasturelands and construction sites.

How it Spreads

Scotch thistle reproduces by seed. Each plant can produce 8,400 – 40,000 seeds, which can remain viable for over 40 years. It is considered a biennial herb, but it can behave like an annual or a short-lived perennial depending on the conditions. As such, it emerges as a rosette before bolting and producing seeds in a subsequent season.

Scotch thistle seeds are spread primarily by the wind, but they also disperse in hay and water, as well as by attaching to clothing, wildlife, livestock, or the undercarriage of vehicles. Lastly, Scotch thistle is spread through the horticulture trade, as it is still sometimes grown as a garden ornamental.

Impacts

Ecological:

Outcompetes native species, which decreases habitat availability.
Dense stands become impassable and limit animals’ access to grazing land and water.
If eaten, the spines can cause injury to animals, particularly around the mouth.

Economic:

Reduces forage production and virtually prohibits land utilization for livestock.
Forms impenetrable thickets for ranchers, cattle and recreationists.

Stop the Spread

Scotch thistle is found in the Sea to Sky region, but with a limited distribution. The goal is to eradicate this species from the region, and to prevent new introductions.

Learn to identify Scotch thistle: use the images presented in this profile page to learn how to identify Scotch thistle.

What to do if you spot it: You can report any Scotch thistle sighting by visiting our reporting page.

DO:

Regularly monitor properties for weed infestations.
Ensure soil and gravel are uncontaminated before transport.
Check wildflower mixes to ensure that they do not contain Scotch thistle.
Remove plant material from any equipment, vehicles, or clothing used in infested areas and wash equipment and vehicles at designated cleaning sites before leaving such areas.
Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.

DO NOT:

Plant Scotch thistle in a garden, no matter how well-contained its enclosure may seem.
Move soil that has been contaminated with Scotch thistle.
Compost invasive plant material.

Control

Mechanical

Hand-pull small infestations, but ensure you are wearing appropriate protection (heavy gloves, a long-sleeved shirt and pants, and eyewear) to protect yourself from the plant’s spines.
Since plants can regrow from severed roots, and cut stems may still produce viable seeds, mowing or hand-pulling is recommended in combination with chemical control.
Moreover, ensure mechanical control is performed before the plant goes to seed, to avoid further spread.

Chemical

Dicamba, 2,4-D, aminopyralid, clopyralid, metsulfuron, and glyphosate are effective on Scotch thistle.
Picloram has also proven effective, however, it is not suitable for wet, coastal soils.
Herbicides should be applied in the spring before the plant bolts, or in the autumn to rosettes.
We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Biological

There is no biocontrol agent available for Scotch thistle.