Cirsium horridulum var. horridulum, Yellow Thistle
Usually yellow, sometimes purple. Long, spiny leaves surround the flower head.

Cirsium discolor, Field Thistle
Lower surface of leaves "densely white-wooly".

Cirsium pumilum, Pasture Thistle
Stem is densely covered with long hairs, but not prickly. Lower surface of leaf is green.

Cirsium vulgare, Bull Thistle
Prickly "wings" between the leaves on the upper part of the stem.
Alien.

Cirsium arvense, Canada Thistle
Flower heads less than an inch high.
Invasive alien.

Also see Carduus nutans, the musk thistle.

These species are no longer found in Delaware.

Cirsium muticum, Swamp Thistle

Cirsium altissimum, Tall Thistle
Formerly found in the Piedmont.

Cirsium virginianum, Virginia Thistle
Formerly found on the coastal plain.

Sources: The Flora of Delaware, Herbaceous Plants of Maryland.