Prickly lettuce flower.

Prickly Lettuce, Lactuca Serriola L., a native to the Mediterranean region is. The plant is sometimes called wild lettuce, China lettuce or 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 a 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 and can germinate within the top centimeter of soil after the smallest of rain events. Prickly lettuce forms only a short-term seed bank, with seeds surviving 1–3 yr in soil. Prickly lettuce (Lactuca serriola) 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.

Control Methods

Hand pulling: Hand pulling is effective for small infestations if it’s done when the ground is moist.

Mature prickly lettuce.
Joseph M. DiTomaso, University of California – Davis,

Mechanical: Seedlings and rosettes of prickly lettuce are easily controlled by cultivation. Mowing of rosettes is not an effective control practice because leaves lie close to the soil surface. If mowing occurs after stem elongation the plant will simply produce new stems and flowers
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, however, populations of prickly lettuce are extremely tolerant of glyphosate herbicide. Plants are difficult to control with herbicides once the flowering stems have begun to elongate. Preemergence applications of products containing atrazine, metribuzin, chlorsulfuron, isoxaben, oxyfluorfen, oxadiazon or terbacil will usually control germinating seedlings. Postemergence applications of 2,4-D, MCPA, dicamba, chlorsulfuron+metsulfuron, pyrosulfotole, bromoxynil, clopyralid, metribuzin, tribenuron 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. In some regions of the Pacific Northwest, prickly lettuce has become resistant to specific synthetic auxins (2,4-D, dicamba and MCPA).
Organic Herbicides: There are several herbicides made from natural ingredients. Those that contain clove oil (eugenol) give the best control of young broadleaf weeds.
Please refer to the Pacific Northwest Weed Management Handbook and the following related publications for more information. Please follow all label guidelines, rotate herbicide mode of action and do not apply herbicides at less than recommended rates, therefore, reducing the possibility of additional herbicide resistance.

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