Perennial Sow Thistle
General: Despite the name, Perennial Sow Thistle more closely resembles giant dandelions than it does true thistles.
Flowers: Strap-shaped with bright yellow petals, resembling those of dandelions. Flowers are 1.5 – 3 cm in diameter and head bracts are covered with sticky hairs. Flowers are grouped in loose clusters at the end of stalks. A plant may have up to 20 flower heads, with only a few blooming at once. Flowers open two to three hours after sunrise and close around noon.
Stems: 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: Colour varies from light to dark green. 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: Small, up to 3.5 mm long, with a single brown seed. They are long and ribbed with a tuft of white, hair-like bristles (pappus).
Roots: Deep (up to 3 m, but most roots grow 5 – 15 cm from the surface), vertical and fleshy with creeping horizontal roots and extensive rhizomes that produce new shoots and break off easily, regenerating into new plants.
- Annual Sow Thistle: Annual Sow Thistle leaves are more deeply lobed and it reproduces only by seed. Unlike its perennial counterpart, Annual Sow Thistle has a short taproot system that makes it easier to remove.
Habitat and Origin
Perennial Sow Thistle is native to Western Europe and Asia. It’s believed to have been introduced as a contaminant in seed.
Perennial Sow Thistle thrives in various environments, including saline soils. It does best in moist, fertile soils with full sunlight. It often grows in disturbed areas, along roadsides, cultivated fields and riparian areas.
Propagation & Vectors of Spread
Perennial Sow Thistle reproduces both by seed and underground stems that produce creeping roots and shoots from its nodes (rhizomes). Each plant can produce 4,000 to 13,000 seeds, germinating in the spring or fall. Fall seedlings overwinter as rosettes. When cut during cultivation, root pieces as small as 1 cm can produce a flowering plant within a year.
The tall stalks, lightweight seeds, and parachute-like pappus (cluster of hairs) help spread the seeds in the wind. Seeds can also spread through water or attach to fur, feathers or fabrics.
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 may survive 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.
- Heavy grazing by cattle and sheep may control some populations.
- Cystiphora sonchi, a leaf-gall fly, has been approved for biological control in BC.
- 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/fact-sheet/sow-thistle-perennial/
- 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, https://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), https://www.ontario.ca/document/weed-identification-guide-ontario-crops/perennial-sow-thistle
- Yukon Invasive Species Council, Perennial Sow Thistle, https://www.yukoninvasives.com/plants/Perennial_Sow_Thistle.pdf