- Campion, Bladder
- Campion, Starry
- Campion, White
- Canada Lily
- Canada Mayflower
- Cankerweed
- Cardinal Flower
- Carolina Cranesbill
- Carrion Flower
- Catchfly, Sleepy
- Catnip
- Cat's Ear, Spotted
- Celandine
- Chickweed, Common
- Chickweed, Mouse-ear
- Chickweed, Nodding
- Chickweed, Star or Great
- Chicory
- Childing Pink
- Cinquefoil, Dwarf
- Cinquefoil, Rough
- Cinquefoil, Rough-fruited
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- Cinquefoil, Shrubby
- Clammy Cuphea
- Clearweed
- Cleavers
- Cliff Stonecrop
- Climbing False Buckwheat
- Climbing Fumitory
- Clintonia, White
- Clotbur, Beach or Common
- Clover
- Clover Crimson
- Clover, Hop or Golden
- Clover, Kate's Mountain
- Clover, Rabbit's Foot
- Clover, Sweet
- Colicroot
- Coltsfoot
- Columbine
- Common Barberry
- Common Burdock
- Common Cattail
- Common Dittany
- Common Dodder
- Common Greenbrier
- Coneflower, Gray-headed
- Coneflower, Purple
- Coneflower, Tall or Green-headed
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- Coralroot, Crested
- Coralroot, Spotted
- Coreopsis, Greater
- Coreopsis, Lanceleaved
- Coreopsis, Tall
- Coreopsis, Whorled
- Corn Gromwell
- Corn Salad
- Corydalis, Pale
- Corydalis, Yellow
- Cowbane, Stiff
- Cow Parsnip
- Cow Wheat
- Cranefly Orchid
- Cress, Bitter Hairy
- Cress, Creeping Yellow
- Cress, Early Winter
- Cress, Marsh Yellow
- Cress, Small-flowered Bitter
- Cress, Smooth Rock
- Cress, Spreading Rock
- Cress, Spring
- Crowfoot, Cursed
- Crowfoot, Hooked
- Crowfoot, Small-flowered
- Crowfoot, White Water
- Cuckooflower
- Culver's Root
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