Compass Plant, Wild Lettuce, China Lettuce
Prickly Lettuce, Lactuca Serriola L., a native to the Mediterranean region is also called wild lettuce, China lettuce or compass plant. The plant is sometimes called the compass plant, because the leaves on the main stem are held vertically in a north-south plane, perpendicular to direct sunlight.
Prickly lettuce may be mistaken for dandelion, at the rosette stage, or for sow-thistles at any stage. All of these species are members of the sunflower family, contain milky latex, and produce numerous wind-dispersed seeds. Prickly lettuce is an annual weed growing from 1 to 5′ in height. Prickly lettuce has a deep tap root which will exude a milky sap. The leaves are alternate, clasp the stem, and are deeply lobed with spiny margins. The leaves have a row of spines along the mid-vein of the lower surface which is a distinguishing characteristic of this plant.
The flowers of prickly lettuce are yellow in color and approximately one-third of an inch in diameter. Flowers are produced in late spring to early summer. Individual plants can produce from 35 to 2,300 flowers. Each flower head contains an average of 20 seeds, giving an estimated seed production of 700 and 46,000 seeds per plant, respectively. Most seeds are viable and ready to germinate immediately after dispersal. Prickly lettuce forms only a short-term seed bank, with seeds surviving 1–3 years in soil. Prickly lettuce seeds travel with the help of their downy, white plumes. Prickly lettuce is most commonly a weed of nurseries, orchards, roadsides, and agronomic crops, and is found throughout the United States.
Prickly lettuce is drought tolerant and can have detrimental effects on crop value and harvesting efficiency of wheat. The flower buds can be difficult to screen out of grain, resulting in discounted prices. The sticky white latex in the stems can clog harvesting equipment and raise the moisture content of the grain.
Cultural: Hand pulling is effective for small infestations, if it’s done when the ground is moist.
Biological: Sheep and goats enjoy feeding on prickly lettuce and will devour whole fields of this weed.
Chemical: Rosettes of prickly lettuce can be controlled in the fall or spring by non-selective herbicides containing glyphosate, glufosinate, or paraquat. Plants are difficult to control with herbicides one the flowering stems have begun to elongate. Pre-emergence application of products containing atrazine, metribuzin, chlorsulfuron, isoxaben, oxyfluorfen, oxadiazon or terbacil will usually control germinating seedling. Post-emergence application of 2,4-D, MCPA, dicamba, clopyralid, metribuzin, or thifensulfuron-methyl/tribenuron-methyl, can control prickly lettuce rosettes in a variety of crops.
Prickly lettuce is resistant to Group II herbicides in the northwestern United States (ID, OR, WA) and Australia.