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Lactuca serriola (prickly lettuce, compass plant) is a winter or summer annual, introduced to Canada from Eurasia, and found in all provinces except Newfoundland. It occupies a variety of disturbed sites, and is becoming an increasing problem in crops grown with reduced tillage. Most seedlings emerge in autumn and form overwintering rosettes, with a smaller peak of emergence in spring. Flowering occurs from July through September. Seed production is proportional to stem height, and ranges up to 200 000 seeds per plant. The wind-dispersed seeds have no primary dormancy and form only a short-term seed bank (I to 3 yr). Many populations in the western United States and southern Australia have developed resistance to Group 2 herbicides, which inhibit the enzyme acetolactate synthase. Prickly lettuce is closely related to, and inter-fertile with, cultivated lettuce, Lactuca sativa L.