Annual Sow Thistle
Sonchus oleraceus L.
Common Sow Thistle, Spiny-leaved Sow Thistle, Spiny Annual Sow Thistle, Spiny Milk Thistle, Prickly Sow Thistle, Sharp-fringed Sow Thistle.
Flowers: Bold, yellow, flat-or-round-topped flowers grow in clusters of 2 – 4 at the peak of each branch. For reference, their blooms are highly similar to that of a Dandelion!
Stem: Hollow, glabrous stems contain a milky sap, and can grow up to 1 m tall!
Leaves: Stalked basal leaves are deeply lobes, and contain thinly toothed margins. Higher up on the stem, though, leaves become stalkless and may have thin, purple veins.
Fruit: Annual Sow Thistle forms a ribbed, reddish-brown, roughly 3 mm long seed with a parachute-like, white pappus of hairs extending outwards.
Roots: This weed’s short taproot system makes it much easier for field workers to manually remove!
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.
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 Aster Family.
Annual Sow Thistle thrives in fertile, moist soils and full sun. Often times, you can spot this thorny invasive growing along roadsides, logged areas, marshlands, agricultural fields, rangelands, gravel pits, and even gardens!
Propagation & Vectors of Spread
Fortunately, as the name might imply, this troublesome thistle is an annual, dying off entirely each fall and creating new seedlings in the spring. It spreads exclusively by seed, eliminating any chance of rhizomatous spreading.
Annual Sow Thistle’s main mode of transportation is the wind, which easily catches its light pappus and carries the seeds to new areas. There, they 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.
Ecological, Economic, & Health Impacts
- With wind dispersal and rapid seed set, Annual Sow Thistle spreads aggressively among new areas and out-competes native flora in their natural environment.
- 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.
- Sow Thistles contains a unique chemical compound, used in industrial pharmaceutical contexts.
What Can I Do?
Annual 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 Annual Sow Thistle: use the images presented in this profile page to learn how to identify this thorny invasive!
What to do if you spot it: You can report any Annual Sow Thistle sighting by clicking here.
- 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 mow weeds that have gone to seed set.
- 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.
Mowing weeds that haven’t yet gone to seed can prevent infestations from getting out of control. This, combined with manual hand-pulling of smaller populations, provides the opportunity for an integrated control program.
Pre-emptive applications of the herbicide glyphosate have been effective at controlling young populations of Annual Sow Thistle. Mature plants that receive foliar applications of MCPA, and 2,4-D, become much more manageable, as well.
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.
Research is ongoing for Annual Sow Thistle biocontrol agents. There are currently no active biocontrol methods being implemented in B.C. for this species.
Turns out, this troublesome thistle is palatable to both cattle and sheep! Extensive, regular grazing has been able to suppress infestations by preventing the plants from going to seed.
- E-Flora BC, Sonchus oleraceus L., https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Sonchus%20oleraceus
- Government of B.C., Annual Sow Thistle, https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/alerts/annual_sow.pdf
- Government of Manitoba, Agriculture & Resource Development, Annual Sowthistle, https://www.gov.mb.ca/agriculture/crops/weeds/annual-sowthistle.html
- Invasive Species Council of BC and Government of BC, Field Guide of Noxious Weeds and Invasive Plants in BC, https://bcinvasives.ca/documents/Field_Guide_to_Noxious_Weeds_Final_WEB_09-25-2014.pdf
- Ontario’s Ministry of Agriculture, Food & Rural Affairs, Spiny Annual Sow Thistle, http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/spiny_sowthistle.htm#habitat
- Ontario’s Ministry of Agriculture, Food & Rural Affairs, Annual Sow-Thistle, http://www.omafra.gov.on.ca/english/crops/field/weeds/annual_sowthistle.htm
- University of California Agriculture & Natural Resources, Annual Sowthistle, http://ipm.ucanr.edu/PMG/WEEDS/annual_sowthistle.html