Prickly Lettuce vs Canada Lettuce
These two species, one native and one introduced and invasive, are sometimes confused. Although they are usually easy to distinguish, it can be hard to learn how to do so because both are highly variable in leaf shape and growth habit. Although there is some overlap in habitat, L. serriola is more likely in drier, sunnier, more barren and more disturbed habitats, whereas L. canadensis is more likely in moister, shadier habitats with less disturbance and more competition from other vegetation.
Prickly Lettuce (Lactuca serriola) | Canada Lettuce (Lactuca canadensis) |
A widely-distributed plant, native to Europe, Asia, and North Africa, introduced to North America where it is widely considered invasive. | A tall biennial native to eastern North America, having expanded its range westward as well. |
 Flowerheads average larger (to 1/2 inch across, about 13mm), showier, and average a paler color of yellow, with less variation in color. Photo © Alina Martin, CC BY 4.0. |  Flowerheads are smaller (1/4 to 1/3 inch across, about 6-8mm), less conspicuous, and average a richer orangeish-yellow, but with more variation in color, sometimes tinged with red or orange. Very rarely, flowers may be bluish. Photo © Blake Bringhurst, Public Domain. |
 Leaf margins are usually (not always) prickly, midribs underneath leaves are usually prickly, only rarely smooth. Photo © Lexi Amico, CC BY 4.0. |  Leaf margins may have teeth, but never prickles. Midribs underneath leaves are usually smooth, occasionally have long hairs, but hairs are not prickly. Photo © Alex Zorach, CC BY 4.0. |
 Stems are sometimes prickly. Photo © Nikokin, CC BY 4.0. |  Stems are never prickly. Photo © Ken Kneidel, Public Domain. |
 Plants average much shorter, commonly 30-70cm (1-2.3 feet), rarely to 100cm / 3.25 feet or higher. Photo © Austin R. Kelly, CC BY 4.0. |  Plants average much taller, commonly 40–200cm (1.3-6.5 feet), occasionally to 450cm (14.75 ft) or even higher. Photo © Mike Tilley, CC BY 4.0. |
Leaves average much narrower and maximum width is much narrower. | Leaves average wider, and may be up to twice as wide. |
References & External Resources
These short lists show only links helpful for ID. For a complete list of references and resources also covering other aspects of ecology, visit the links section of the full article on each plant, which is the first entry here.
