Prickly Lettuce

Prickly Lettuce (Lactuca serriola)

Report

Photo credit: R. Mueller

Management Strategy

Overview

Prickly Lettuce is an invasive plant within British Columbia. This invasive is a biennial weed from the daisy family (Asteraceae) that can grow up to 1.8 m tall.

Synonyms

Prickly Lettuce is also known as:

China Lettuce
Compass Plant
Wild Lettuce

ID Characteristics

Flowers: A ray of 12-20 strap-shaped yellow petals. The flowers close before midday and are very small (about 3 – 4 mm in diameter). Individual flowers are similar to dandelions.

Stems: Erect, branched, and hairy stems. Every part of the plant exudes a milky sap when cut.

Leaves: Prickly Lettuce emerges in autumn as a rosette of basal leaves (on the ground). In summer, a flowering stem grows with narrow, lobed, arrow-shaped leaves that grow in an alternate pattern. These leaves have spiny margins, painful to handle. Most notably, leaves have a white mid-vein, which is prickly on the underside.

Roots: Thin, branched taproot.

Fruits: Small, greyish-brown single-seed, about 3 mm long. They are thin and flattened, with a pappus (tuft of hairs).

Similar Species

Invasive:

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.

Photo credit: S. Dewey, Utah State University, Bugwood.org (Perennial Sow Thistle)

Exotic:

Cultivated Lettuce (Lactuca sativa L.): Although this lettuce is not native to British Columbia, it is not competitive with the native plant community. Therefore, it is classed as ‘exotic’. The key difference between Cultivated and Prickly Lettuce is in the leaves. Cultivated lettuce does not have prickly leaves, whereas Prickly Lettuce (of course) does!

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 agricultural seed.

Prickly Lettuce prefers dry conditions. It thrives along dry roadsides, thicket clearings, agricultural areas, abandoned fields and other disturbed areas. It is also commonly found at waste disposal sites.

How it Spreads

Prickly Lettuce is a biennial weed (two-year lifespan). In the first year, it forms a rosette, which will bolt (grow a flowering stem) and go to seed the following year.

These seeds are predominately self-propagated and their pappus (tuft of hairs) allow for wind dispersal. The 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.

Impacts

Ecological

Causes habitat loss for native flora, especially for Vancouver Island Beggarsticks (Bidens amplissima), a provincially-designated, Special Concern species.

Economic

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.

Stop the Spread

Prickly Lettuce is found in the Sea to Sky region and its distribution is beyond landscape-level control. When Prickly Lettuce is present at high-priority locations and negatively impacting them, their control is considered a high priority.

Otherwise, the goal is to prevent it from spreading to new (uninfested) areas, and to control it where possible to limit its impact on biodiversity.

Learn to identify Prickly Lettuce: use the images presented on this profile page.

What to do if you spot it: you can report any Prickly Lettuce sighting by visiting our reporting page.

Do:

Regularly monitor properties for weed infestations.
Remove plant material from any equipment, vehicles or clothing used in infested areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
Ensure soil and gravel are uncontaminated before transport.
Minimize soil disturbances (e.g., use grazing plants that prevent soil exposure from overgrazing), and use seed mixes with dense, early colonization (e.g., alfalfa or barley) to re-vegetate exposed soil and resist invasion.
Ensure invasive plant (particularly flowering heads or root fragments) are bagged or covered to prevent spread during transport to designated disposal sites (e.g., landfill).

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.
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.
Compost Prickly Lettuce!

Control

Mechanical

Prickly Lettuce is easily controlled by cultivation or pulling.
Mowing, on the other hand, is not an effective control method 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.

Chemical

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.

Biological

There is no biocontrol available for this plant.