Tall Blue Lettuce, Wild Blue Lettuce: Lactuca biennis (Synonym: Sonchus biennis) - Flowers 13-34 in a head. Flowers white to bluish, occasionally yellow. Leaves not prickly margined. Middle and upper stem leaves gradually reduced in size.
Canadian Wild Lettuce, Florida Blue Lettuce: Lactuca canadensis -
Louisiana lettuce, Prairie Lettuce, Western Lettuce: Lactuca ludoviciana -
Least Lettuce, Willow Lettuce: Lactuca saligna - Flowers 8-16 per head. Flowers yellow. Leaves linear and entire, or with slightly toothed lobes. Not prickly-margined. Middle and upper stem leaves gradually reduced in size.
Garden Lettuce: Lactuca sativa - Non-native, and most likely not naturalized.
Prickly Lettuce: Lactuca serriola (Synonyms: Lactuca scariola, Lactuca scariola var. integrata, Lactuca scariola var. scariola) - Flowers 18-24 in a head. Flowers yellowish, sometimes drying to bluish. Leaves prickly margined. Middle and upper stem leaves gradually reduced in size.
Blue Lettuce: Lactuca tatarica ssp. pulchella (Synonyms: Lactuca pulchella, Lactuca tatarica var. pulchella, Mulgedium oblongifolium, Mulgedium pulchellum) -
Wall Lettuce: Mycelis muralis (Synonym: Lactuca muralis) - Flowers 5 to a head. Flowers yellow. Middle and upper leaves much reduced in size.