Prickly Lettuce is also known as China lettuce, Compass plant, or Wild lettuce.
General: Prickly Lettuce is a biennial weed from the daisy family (Asteraceae).
Flowers: Are strap-shaped with yellow petals. Flowers close before midday, and are very small – about 3-4 mm in diameter. Individual flowers are similar to those of dandelions.
Stem: Are erect, branched, and glabrous (hairy). Every part of the plant exudes a milky sap when cut. Prickly Lettuce grows to be about 0.3-1.5 m tall.
Leaves: The most distinguishing factor is the white mid-vein, which is prickly on the underside (hence the plant’s name). Leaves are narrow, lobed and arrow-shaped, with spiny leaf margins, which can make the plant painful to handle. The leaf arrangement is alternate.
Fruit: Are small, about 3 mm long, with a single seed. They are thin and flattened, with a pappus (tuft of hairs).
Roots: Prickly Lettuce has a thin, branched taproot.
Annual Sow Thistle (Sonchus oleraceus) is also invasive; it has larger flowers than Prickly Lettuce.
Perennial Sow Thistle (Sonchus arvensis) is another invasive plant that has larger flowers than Prickly Lettuce; it is also distinguishable by its rhizomes.
Habitat and Origin
Prickly Lettuce was introduced from Europe. Its presence dates back to the late 19th century; it is speculated that it was introduced as a contaminant in seed.
Prickly Lettuce tends to prefer dry conditions. It occurs along dry roadsides, thicket clearings, agricultural areas, abandoned fields and other disturbed areas. It is also commonly found at waste disposal sites.
Propagation & Vectors of Spread
Prickly Lettuce is a biennial weed; in its first year, it forms a rosette, which will bolt and go to seed the following year.
Prickly Lettuce is propagated by seed, and is predominantly self-pollinated. Its seeds germinate with the onset of winter rains.
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 or feathers.
Ecological and Economic Impacts
- Causes habitat loss for native flora, especially for the provincially-designated, special concern species Vancouver Island Beggarsticks (Bidens amplissima).
- Reduces crop value and harvesting efficiency.
- The sticky, latex sap can clog harvesting equipment.
- Cattle that exclusively graze on young Prickly Lettuce have been reported to develop pulmonary emphysema. However, mature or dried younger plants appear to be non-toxic.
What Can I Do?
Prickly Lettuce is currently found throughout the Sea to Sky Corridor, so the best approach to controlling its spread is by PREVENTION.
Learn to identify Prickly Lettuce: use the images presented in this profile page to learn how to identify Prickly Lettuce.
What to do if you spot it: You can report any Prickly Lettuce sighting by clicking here.
- Regularly monitor properties for weed infestations.
- Ensure soil and gravel are uncontaminated before transport.
- Check wildflower mixes to ensure that they do not contain Prickly Lettuce.
- Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.
- 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 Prickly Lettuce in a garden, no matter how well-contained its enclosure may seem.
- Move soil that has been contaminated with Prickly Lettuce.
- Do not compost!
- Prickly Lettuce is easily controlled by cultivation or pulling.
- Mowing is not effective because the plants will readily produce new stems or flowering branches.
- It is recommended that you wear gloves and long sleeves to remove Prickly Lettuce, as the leaves are spiked.
- Rosettes can be killed with non-selective herbicides containing glyphosate, with larger rosettes requiring higher concentrations.
- Other effective herbicides include 2,4-D, MCPA, dicamba and clopyralid.
- Plants are difficult to control with herbicides once the flowering stems have begun to elongate.
- 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.
There is no biocontrol available for this plant.
- Electronic Atlas of the Flora of BC, Lactuca serriola, https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Lactuca%20serriola
- Global Biodiversity Information Facility, Lactuca serriola, https://www.gbif.org/species/3140490
- Government of Canada, Management Plan for the Vancouver Island Beggarsticks (Bidens amplissima) in Canada, https://www.registrelep-sararegistry.gc.ca/virtual_sara/files/plans/mp_vancouver_island_beggarticks_e_proposed.pdf Section 8.1 (pp. 17-18)
- Invasive Plant Atlas of the United States, Prickly Lettuce, https://www.invasiveplantatlas.org/subject.html?sub=5918
- Seattle Times, WSU Wants To Turn Goo From Weed Prickly Lettuce Into Rubber, https://www.seattletimes.com/seattle-news/wsu-envisions-prickly-lettuce-becoming-rubber-source/
- S. E. Weaver and M. P. Downs, The Biology of Canadian Weeds 122: Lactuca serriola, Canadian Journal of Plant Science, https://www.nrcresearchpress.com/doi/pdfplus/10.4141/P02-059
- University of California Agriculture and Natural Resources Integrated Pest Management Program, Prickly Lettuce, http://ipm.ucanr.edu/PMG/WEEDS/prickly_lettuce.html