Lactuca floridana, Woodland Lettuce
Blue flowers; pappus white; up to 8' tall.

Lactuca biennis, Tall Blue Lettuce
Blue or whitish flowers; pappus tawny; up to 12' tall.

Lactuca canadensis, Canada Lettuce
Yellow flowers; leaf edges smooth; up to 9' tall.

Lactuca serriola, Prickly Lettuce
Yellow flowers; leaf edges prickly; up to 5' tall.  Alien.

Lactuca saligna, Willow-leaf Lettuce
Yellow flowers in a "narrow, straight, erect panicle"; smooth, narrow leaves; up to 3' tall. Alien; no photo.

Lactuca sativa, Garden Lettuce
Similar to L. serriola, but "Phyllaries usually erect in fruit; midribs of leaves usually smooth". Alien; no photo.

Lactuca is from the Latin for milk, referring to the plants' milky sap.

Sources: The Flora of Delaware, Weakley's Flora.