Management Strategy
Squamish
Whistler
Pemberton
Vectors of Spread
Synonyms
- Common sow thistle
- Spiny-leaved sow thistle
- Spiny annual sow thistle
- Spiny milk thistle
- Prickly sow thistle
- Sharp-fringed sow thistle
- Hare’s colwort
ID Characteristics
Flowers: Yellow, dandelion-like flowers that are grouped clusters of 1 – 5. These relatively small blossoms are roughly 1.5 – 2.3 cm wide, and open only in the morning.
Stems: Mature plants are 0.1 – 1.0 m tall, with hollow stems containing a milky sap.
Leaves: The deeply lobed, dull green, stalked basal leaves are lined with small, weak teeth and end with large, pointed segments. Upper leaves are stalkless, clasping the stem and about 6 – 30 cm long and 1 – 15 cm wide.
Fruit: Seeds are 2.5 – 3.5 mm long, reddish brown and ribbed with a parachute-like pappus (cluster of hairs). Seeds are football to egg-shaped.
Roots: This plant’s short taproot system makes it much easier for field workers to manually remove!
Similar Species
Invasive:
Prickly sow-thistle (Sonchus asper)
This spiny weed grows avidly among disturbed soils, dotting roadsides, and waste disposal sites. Although its yellow flowers may cause confusion, Prickly sow-thistle’s large, toothed basal leaves and annual root formation act as key distinguishing features.
Perennial sow thistle (Sonchus arvensis)
Unlike its annual counterpart, perennial sow thistle has an extensive system of creeping roots and shoots (rhizomes) that aids its distribution and makes it more difficult to remove.
Wall lettuce (Lactuca muralis)
This delicate, perennial weed’s fibrous root system and ivy-like tipped leaves are key distinguishing features.
Habitat and Origin
Originating from Europe, this prickly weed is a member of the Asteraceae (daisy) Family.
Annual sow thistle flourishes in a wide range of habitats but prefers fertile, moist soils in full sunlight (such as riparian areas). It is often found among artificial plant communities like cultivated fields, gardens, roadsides and disturbed sites.
How it Spreads
As its name implies, this troublesome plant is an annual, dying off entirely each fall and creating new seedlings in the spring. It spreads exclusively by seed.
Annual sow thistle’s main mode of transportation is the wind, which easily catches its light pappus (cluster of hairs on the seeds) and carries the seeds to new areas. Annual sow thistle seeds can germinate at any point throughout the growing season, and remain viable in the soil for years. It’s important to note, though, that these lightweight seeds can also spread by hitching a ride on humans, animals, and water flow.
Impacts
Ecological:
- With wind dispersal and rapid seed set, annual sow thistle spreads aggressively among new areas and out-competes native flora in its natural environment.
- Can host several plant viruses, nematodes and aphids, bringing such bugs with them when colonizing new areas.
- Especially problematic in marshes, ponds and other riparian environments, where this plant spreads rapidly and out-competes native plants, thus reducing biodiversity.
Economic:
- Annual sow thistle can indirectly impact the agricultural industry, by hosting nematodes, aphids, and viruses.
- It can also contaminate crop yields and reduce property value.
Stop the Spread
Annual sow thistle is found throughout the Sea to Sky region. Its distribution is beyond landscape-level control. Therefore, there is too much annual sow thistle in the Sea to Sky to effectively eradicate it.
Only when annual sow thistle is present at high-priority locations and negatively impacting them, is control considered a high priority. Otherwise, the goal is to prevent annual sow thistle from spreading to new (uninfested) areas and to control it where possible to limit its impact on biodiversity.
To prevent annual sow thistle from spreading into new areas please report sightings on our website.
DO:
- Regularly monitor properties for weed infestations.
- Plant weed-free crop seed for gardens and crop fields.
- Ensure soil and gravel are uncontaminated before transport.
- Check wildflower mixes to ensure that they do not contain annual sow thistle.
- Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.
DO NOT:
- Do not mow weeds that have gone to seed.
- Do not 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.
- Do not plant annual sow thistle in a garden, no matter how well-contained its enclosure may seem.
- Do not move soil that has been contaminated with annual sow thistle.
Control
Mechanical
- Hand-pull small infestations; for larger infestations, mowing repeatedly before seed set will help control annual sow thistle’s spread, but mowing height must be lower than 20 cm to prevent regrowth.
- To ensure eradication, the taproot must be removed, as re-sprouting can occur if left in the soil.
Chemical
- Before flowers bloom, selective herbicides can be applied to prevent seed production; meanwhile, foliar applications of MCPA and 2,4-D offer effective post-emergence control.
- 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
Research is ongoing in Australia about a rust fungus (Miyagia pseudosphaeria) and an eriophyid mite species. Although not yet approved for release in BC, a gall midge species (Cystiphora sonchi) also shows potential.
Cultural
Annual sow thistle is palatable for cattle and sheep, and intensive grazing can suppress infestations at some sites by preventing seed set. Once the grazing has weakened the plants, other control methods can be more effectively used.
Sea to Sky Distribution
Annual Sow Thistle Factsheet
Having trouble viewing the factsheet? Don’t worry, all the information is included on this page. You can also contact us with any questions.
Additional Resources
Annual Vs. Perennial Sow Thistle
Annual Vs. Perennial Sow Thistle
Annual Vs. Perennial Sow Thistle
References
- BC Government, Field Guide to Noxious Weeds and Other Selected Invasive Plants of British Columbia
- BC Ministry of Agriculture, Food and Fisheries, “Annual Sow Thistle” in Guide to Weeds in BC
- Electronic Atlas of the Flora of British Columbia, Sonchus oleraceus L
- iNaturalist, Sonchus oleraceus
- Ontario Ministry of Agriculture, Food and Rural Affairs, Ontario Weeds: Spiny annual sow thistle
- Ontario Ministry of Agriculture, Food and Rural Affairs, Sow-Thistle, Annual (Sonchus oleraceus L.)
- Oregon State University (OSU), Annual Sowthistle
- Province of Manitoba Agriculture and Resource Development, Annual Sowthistle
- Sustainable Agriculture Research and Education (SARE), Annual Sowthistles
- University of California Statewide Integrated Pest Management Program, Annual sowthistle
- Washington State University Whitman County Extension, Annual Sowthistle