Prickly lettuce (Lactuca serriola L.) is a common weed in wheat production systems throughout the Inland Pacific Northwest (PNW). It is an annual, winter annual, or occasionally a biennial, that reproduces only by seed. Individual plants can produce from 35 to 2,300 flowers and 700 to 46,000 seeds. Sanitation (the prevention of weed seed production and dispersal) is an important aspect of prickly lettuce management, as is growing a healthy, competitive wheat crop. Herbicides can provide effective control of prickly lettuce in wheat, but many biotypes are now resistant to ALS-inhibitor and synthetic auxin herbicides. An integrated management approach is required for the sustainable, long-term control of this troublesome species.
Prickly lettuce, also known as China lettuce, wild lettuce, and compass plant, is a native of the Mediterranean region. The genus name, Lactuca, incorporates the Latin word “lac” for milk, referring to the milky sap produced by the plant. Prickly lettuce is a member of the sunflower family (Asteraceae). It is an annual, winter annual, or occasionally a biennial that reproduces only by seed. It can be found throughout the US growing in disturbed sites, along roadsides, in orchards, and in cropland.
Prickly lettuce reduces crop yields through competition for water, nutrients, and light. Yield losses in winter wheat are usually less than 5%, but in spring wheat, prickly lettuce can reduce grain yields by up to 25%. The milky sap produced by the plant (Figure 1) contains latex and rubber that can cause problems with harvesting equipment and increase grain moisture content. Flower buds can be difficult to remove from grain. Grain that is contaminated with flower buds may be “docked” or graded at a lower value due to the presence of foreign material.
The cotyledons, also known as the seed leaves, are broad and oval in shape (Figure 2). Subsequent seedling leaves are similar in shape, pale green, papery thin, and cupped or curved upward at the outer ends (Gaines and Swan 1972). The seedling leaves form a basal rosette: outward radiating leaves from a short stem at soil level (Figure 3).