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
- Hare’s Colwort
Flowers: Bold, yellow, flat-or-round-topped flowers grow in clusters of 2 – 4 at the peak of each branch. The blossoms are relatively small (1.5 – 2.3 cm wide), and open only in the morning. For reference, their blooms are highly similar to that of a Dandelion!
Stem: Are hollow and contain a milky sap. Mature plants are 0.1 – 1 m tall.
Leaves: Stalked basal leaves are deeply lobed with small, weak teeth; they end in large, pointed segments. Higher up on the stem, though, leaves become stalkless and may have thin, purple veins. Leaves are 6 – 30 cm long and 1 – 15 cm wide.
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.
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
As its name might imply, this troublesome plant is an annual, dying off entirely each fall and creating new seedlings in the spring. It spreads exclusively by seed, eliminating any chance of vegetative spreading.
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. 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.
What Can I Do?
Annual Sow Thistle is found in the Sea to Sky region and its distribution is beyond landscape-level control. When Annual Sow Thistle is present at high-priority locations and negatively impacting them, their control is 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.
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.
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.
In order to ensure eradication, the taproot must be removed, as re-sprouting can occur if left in the soil.
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.
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.
Annual Sow Thistle is palatable for cattle and sheet, and intensive grazing can suppress infestations on some sites by preventing seed set. Once the grazing has weakened the plants, other control methods can be more effectively used.
- BC Government, Field Guide to Noxious Weeds and Other Selected Invasive Plants of British Columbia, https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/publications/field_guide_to_noxious_and__other_selected_weeds_of_bc.pdf
- BC Ministry of Agriculture, Food and Fisheries, “Annual Sow Thistle” in Guide to Weeds in BC, https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/guidance-resources/guidetoweeds.pdf
- Electronic Atlas of the Flora of British Columbia, Sonchus oleraceus L., https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Sonchus%20oleraceus
- Ontario Ministry of Agriculture, Food and Rural Affairs, Ontario Weeds: Spiny annual sow thistle, https://www.ontario.ca/document/weed-identification-guide-ontario-crops/annual-sow-thistle
- Ontario Ministry of Agriculture, Food and Rural Affairs, Sow-Thistle, Annual (Sonchus oleraceus L.), https://www.ontario.ca/document/weed-identification-guide-ontario-crops/annual-sow-thistle
- Oregon State University (OSU), Annual Sowthistle, https://horticulture.oregonstate.edu/weed/annual-sowthistle
- Province of Manitoba Agriculture and Resource Development, Annual Sowthistle, https://www.gov.mb.ca/agriculture/crops/weeds/annual-sowthistle.html
- University of California Statewide Integrated Pest Management Program, Annual sowthistle, http://ipm.ucanr.edu/PMG/WEEDS/annual_sowthistle.html
- Washington State University Whitman County Extension, Annual Sowthistle, https://extension.wsu.edu/whitman/2013/11/annual-sowthistle/