Perennial Sow Thistle
General: Perennial Sow Thistle is a far-creeping perennial plant that more closely resembles giant dandelions than it does true thistles.
Flowers: Numerous bright yellow petals, resembling those of dandelions, 1.5 – 3cm in diameter, strap-shaped, and grouped in loose clusters at the end of stalks. Head bracts are covered with sticky hairs. A plant may have up to 20 flower heads, with only a few blooming at once.
Stems: Are upright and solitary, leafy at the base, branched at the top, and hairy to bristly. Stems can be up to 2 m tall, and exude a milky sap when cut.
Leaves: Are alternate and waxy, with weakly prickled edges; shape is variable. The lower leaves are stalked, but upper leaves are stalkless and clasp the stem.
Seeds: Are dark brown, long and ribbed, with a tuft of white hair-like bristles (pappus).
Roots: Are deep (up to 15-30cm), vertical and fleshy with creeping horizontal roots and extensive rhizomes which produce new shoots and break off easily, regenerating into new plants.
Habitat and Origin
Originating from Europe, this far-creeping weed is part of the Aster family.
Perennial Sow Thistle thrives in a variety of environments, including saline soils, but does best in moist, fertile soils with full sunlight. It is commonly found in disturbed areas, along roadsides, cultivated fields and riparian areas.
Propagation & Vectors of Spread
Perennial Sow Thistle reproduces both by seed and through an extensive system of creeping roots and shoots (rhizomes). Each plant can produce 4,000 to 13,000 seeds, which germinate in the spring or fall.
Perennial Sow Thistle seeds’ main mode of transportation is the wind, which easily catches its light pappus (tuft of hair) and carries the seeds to new areas. These lightweight seeds can also spread by hitching a ride on humans, animals, and water flow.
Ecological, Economic, & Health Impacts
- Negatively impacts native plant communities, reducing biodiversity.
- Chemicals from the roots and decaying plant residue inhibit seed germination in other plants.
- Can modify or retard the successional establishment of native plants.
- Reduces crop value and harvesting efficiency.
- The sticky, latex sap can clog harvesting equipment.
- Can dramatically reduce water resources.
What Can I Do?
Perennial Sow Thistle is currently under STRATEGIC CONTROL throughout the Sea to Sky Corridor, so the best approach to controlling its spread is by PREVENTION:
Learn to identify Perennial Sow Thistle: use the images presented on this profile page to learn how to identify Perennial Sow Thistle
What to do if you spot it: You can report any Perennial Sow Thistle sightings by clicking here.
- Regularly monitor properties for weed infestations.
- Minimize soil disturbances and promptly revegetate disturbed areas to prevent the growth of Perennial Sow Thistle.
- Ensure all flowering heads or buds are bagged or covered to prevent spread during transport to designated disposal sites.
- Unload, park or store equipment or vehicles in infested areas; remove plant material from any equipment, vehicles, or clothing used in such areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
- Plant Perennial Sow Thistle in a garden, no matter how well-contained its enclosure may seem.
- Compost any flowering heads or buds. Instead, dispose of Perennial Sow Thistle in the general/household waste stream at the landfill as the seeds will be able to persist the composting process.
- Move soil, gravel, or fill that has been contaminated with Perennial Sow Thistle.
- Hand-pull new seedlings.
- However, well-developed root systems may be impossible to dig out and require intensive, long-term cultivation (years) to exhaust.
- Seed production can be controlled by cultivation, pulling, grazing, or mowing before the flowers go to seed, but these methods will not eliminate all plants or root spread.
- Perennial Sow Thistle is relatively resistant to herbicides, and high rates are required to kill the extensive root system.
- Clopyralid, dicamba, 2,4-D, picloram, and glyphosate have been effective when plants are at the pre-bud or bud stage.
- However, picloram is not suitable for wet, coastal soils.
- Perennial Sow Thistle has waxy leaves, so the spray solution must have a good wetting ability.
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.
- Alberta Agriculture and Forestry, Sow Thistle, Perennial, http://www.agric.gov.ab.ca/app107/loadPest?action=display&id=79
- Alberta Invasive Species Council, Perennial Sow-thistle, https://abinvasives.ca/wp-content/uploads/2017/11/FS-PerennialSowthistle.pdf
- BC Ministry of Agriculture, Food and Fisheries, “Perennial Sow Thistle” in Guide to Weeds in British Columbia, https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/guidance-resources/guidetoweeds.pdf
- BC Ministry of Forests, Lands, Natural Resource Operations and Rural Development, Perennial Sow Thistle, https://testwww.for.gov.bc.ca/hfp/noxious/plants/perennial-sow-thistle.htm
- Electronic Atlas of the Flora of BC, Sonchus arvensis, http://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Sonchus%20arvensis
- Government of Manitoba, Perennial Sow Thistle, https://www.gov.mb.ca/agriculture/crops/weeds/perennial-sowthistle.html
- Lincoln County Noxious Weed Control Board, Perennial Sowthistle: Options for Control, https://www.nwcb.wa.gov/images/weeds/PERENNIAL-SOWTHISTLE-BROCHURE_Lincoln.pdf
- Ontario Ministry of Agriculture, Food and Rural Affairs, Sow-Thistle, Perennial (Sonchus arvensis), http://www.omafra.gov.on.ca/english/crops/field/weeds/perennial_sowthistle.htm#:~:text=General%20Control&text=Pre%2Dplant%20glyphosate%20applications%20in,control%20of%20perennial%20sow%2Dthistle
- Yukon Invasive Species Council, Perennial Sow Thistle, https://www.yukoninvasives.com/plants/Perennial_Sow_Thistle.pdf