Scotch thistle

Identification & Biology
Alias' : cotton thistle
Latin Name : Onopordum acanthium
Category : Terrestrial Plants
Description :
  • Biennial or perennial; reaches heights of 1.5 – 3 m
  • Flowers are pink to violet, 2.5-5 cm in diameter, with spine-tipped bracts.
  • Stems are numerous and branched with broad, spiny wings and woolly hairs.
  • Normally forms dense stands
  • Leaves are alternate, very large, and irregularly lobed with sharp yellow spikes.
  • Foliage has grey-green colour due to a thick mat of cotton-like or woolly hairs covering upper and lower surfaces.

Scotch thistle produces a large, ground level rosette in the first year and bolts into a tall, spiny plant during its second year. Flowering occurs from mid-June to September, depending on location. Individual plants can produce 70-100 flowering heads, each containing 100 – 140 seeds. Seeds may remain viable in the soil for over 30 years. Look alikes: Scotch thistle closely resembles the native wavy-leafed thistle (Cirsium undulatum), but the native thistle is typically smaller (it does not exceed 1.2 m), and basal leaves are considerably smaller (3.0-7.0 cm long).

Look Alikes

In British Columbia, Scotch thistle is found at lower elevations along roadsides and disturbed areas. It also grows in irrigation ditches and on rangelands.

Impact & Risks

Scotch thistle can be competitive with desirable forage species when it forms dense stands that are impenetrable to livestock. This thistle species can occasionally invade field crops.

Prevention & Mitigation

The most effective way to ensure that your lands do not become infested with Scoth thistle is by prevention. Here are some recommendations to prevent invasion on your property:

  • Maintain your crops and natural lands in a healthy, vigorous condition to ensure a competitive plant community; competitive perennial grasses and forbs utilize water and nutrients that would otherwise be readily available to thistle.
  • Follow a well-designed grazing management plan; excessive livestock grazing reduces competition and favors invasive plants.
  • Regularly patrol your property for Scotch thistle plants and immediately manage new infestations.
  • Cooperate with adjacent landowners and encourage them to prevent invasive thistle spread.
  • Immediately re-vegetate disturbed, bare soils with a suitable seed mixture that provides dense, early colonization to prevent weed invasion.
  • Clean your vehicles and machinery of plant material and soil before leaving a Scotch thistle infestation, or any other invasive plant infestation.
Treatment & Disposal

Mowing or hand-cutting are usually most effective in combination with other methods. Plants can re-grow from severed roots, and cut stems may still produce viable seed. Chemical control is also an option. Before applying herbicides, read the label for full use and precautionary instructions. Consult the most recent edition of the Alberta Agriculture and Forestry Crop Production Guide for the selection and application of chemicals to protect your crop.

Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.