- Cabbage palmetto
- Cabbage Thistle
- Caesar weed
- Calabur tree
- Calico aster
- California Aster
- California Barrel Cactus
- California bay
- California beeplant
- California black oak
- California brittlebush
- California buckeye
- California Buckwheat
- California buttercup
- California Centaury
- California cudweed
- California fetid adderstongue
- California fuchsia
- California Golden Violet
- California incense-cedar
- California juniper
- California Maidenhair Fern
- California manroot
- California mugwort
- California poppy
- California sagebrush
- California Wild Rose
- California yerba santa
- Calla lily
- Callery pear
- Camphor
- Camphorweed
- Canada anemone
- Canada clearweed
- Canada goldenrod
- Canada hawkweed
- Canada mayflower
- Canada mint
- Canada toadflax
- Canada Violet
- Canada yew
- Canadian buffalo-berry
- Canadian bunchberry
- Canadian lousewort
- Canadian Meadow garlic
- Canadian Waterweed
- Canary Island palm
- Candy Flower
- Cannabis
- Canyon live oak
- Cape Aloe
- Cape gooseberry
- Cape Honeysuckle
- Cape Smilax
- Cape-ivy
- Caper spurge
- Capeweed
- Caraway
- Cardinal catchfly
- Cardinal flower
- Carline Thistle
- Carob tree
- Carolina buckthorn
- Carolina crane's-bill
- Carolina horsenettle
- Carolina laurelcherry
- Carolina ponysfoot
- Carolina ruellia
- Carolina snailseed
- Carolina Springbeauty
- Carolina sweetshrub
- Carpet bugle
- Carthusian Pink
- Cascade Oregon-grape
- Cascara
- Castor bean
- Catchweed bedstraw
- Catclaw Acacia
- Caterpillar scorpionweed
- Catherine's moss
- Catnip
- Catsfoot
- Cedar Elm
- Celandine Poppy
- Cenizo
- Chain-fruit Cholla
- Chalk Dudleya
- Chamberbitter
- Chameleon Plant
- Chamise
- Chaparral yucca
- Cheatgrass
- Checker lily
- Checkerbloom
- Cheeseweed mallow
- Cherry Ballart
- Cherry laurel
- Cherry-plum
- Chestnut oak
- Chia
- Chickpea Milkvetch
- Chicory
- Chilicothe
- China knotweed
- Chinaberry
- Chinese banyan
- Chinese bushclover
- Chinese holly
- Chinese pistache
- Chinese privet
- Chinese Tallow
- Chinese violet
- Chinese-pink
- Chinkapin oak
- Chives
- Chocolate lily
- Chokecherry
- Christmas cholla
- Christmas fern
- Chuparosa
- Cinnamon fern
- Citronella horse balm
- Clasping Venus's looking glass
- Climbing fig
- Climbing hempvine
- Cloudberry
- Clustered bellflower
- Clustered Tarweed
- Coast Cholla
- Coast live oak
- Coast morning glory
- Coast redwood
- Coastal Banksia
- Coastal bush lupine
- Coastal Goldenbush
- Coastal manroot
- Coastal woodfern
- Cobaea beardtongue
- Cobwebby Thistle
- Coconut palm
- Cocoplum
- Coffee Fern
- Coffeeberry
- Cogon Grass
- Colorado Pinyon
- Colt's-foot
- Columbia lily
- Columbian Monkshood
- Common agrimony
- Common alder
- Common alkanet
- Common arrowgrass
- Common beargrass
- Common bilberry
- Common bistort
- Common Blue Wood Aster
- Common boneset
- Common bracken
- Common buckthorn
- Common Butterbur
- Common Butterwort
- Common Cat's-ear
- Common centaury
- Common chickweed
- Common cinquefoil
- Common cocklebur
- Common columbine
- Common comfrey
- Common Correa
- Common Cottongrass
- Common Cotula
- Common Cow-wheat
- Common cowparsnip
- Common dandelion
- Common dog-violet
- Common dogwood
- Common Duckweed
- Common evening-primrose
- Common fig
- Common Figwort
- Common Fleabane
- Common Fumitory
- Common groundsel
- Common guava
- Common hackberry
- Common Haircap Moss
- Common Harebell
- Common hawthorn
- Common hazel
- Common Heath
- Common heather
- Common hedge parsley
- Common hedge-nettle
- Common hemp-nettle
- Common hibiscus
- Common Honeysuckle
- Common hops
- Common hoptree
- Common ivy
- Common jewelweed
- Common juniper
- Common Kidney-Vetch
- Common Lady Fern
- Common Lambsquarters
- Common lantana
- Common lilac
- Common Liverwort
- Common Lousewort
- Common Mallow
- Common Mare's Tail
- Common Marsh-bedstraw
- Common milkweed
- Common morning-glory
- Common motherwort
- Common mouse-ear chickweed
- Common mouse-ear chickweed
- Common mugwort
- Common ninebark
- Common pawpaw
- Common Pear
- Common polypody
- Common poppy
- Common Privet
- Common Purslane
- Common ragweed
- Common reed
- Common Rock-rose
- Common selfheal
- Common silverweed
- Common sneezeweed
- Common Snowberry
- Common snowdrop
- Common soapwort
- Common soft brome
- Common Sorrel
- Common Sow-thistle
- Common Spotted Orchid
- Common St. John's Wort
- Common star-of-Bethlehem
- Common starlily
- Common Sugarbush
- Common Sunflower
- Common Sunshine Conebush
- Common Tansy
- Common toadflax
- Common Toothwort
- Common Twayblade
- Common valerian
- Common vervain
- Common Vetch
- Common water hyacinth
- Common Whitlowgrass
- Common wild petunia
- Common woolly sunflower
- Common wrinkle-leaved goldenrod
- Common yarrow
- Common yew
- Compass plant
- Coontail
- Coral honeysuckle
- Coral Vine
- Coralberry
- Coralberry
- Coralroot
- Cork oak
- Corkystem passionflower
- Corn gromwell
- Corn Lily
- Corn mint
- Corn speedwell
- Cornelian cherry
- Cornflower
- Cotton thistle
- Cow Parsley
- Cowbane
- Cowpen daisy
- Cowslip
- Coyote brush
- Crane-fly orchid
- Cream Scabious
- Cream violet
- Creamcups
- Creeping Bellflower
- Creeping buttercup
- Creeping cinquefoil
- Creeping cucumber
- Creeping Foamflower
- Creeping Jenny
- Creeping juniper
- Creeping mahonia
- Creeping Saltbush
- Creeping snowberry
- Creeping thistle
- Creeping Woodsorrel
- Creosote Bush
- Crescent-cup liverwort
- Crested Wheatgrass
- Cretan mallow
- Cretanweed
- Crimson clover
- Crocus heuffelianus
- Crofton Weed
- Cross Gentian
- Cross vine
- Cross-leaved Heath
- Crossflower
- Crosswort
- Crown flower
- Crowpoison
- Crystalline ice plant
- Cuban jute
- Cuckoo-pint
- Cuckooflower
- Cucumber Root
- Cucumberleaf sunflower
- Culver's root
- Cup plant
- Curled dock
- Curly-leaf Pondweed
- Curlycup Gumweed
- Cursed crowfoot
- Cut-leaved crane's-bill
- Cut-leaved toothwort
- Cutleaf Anemone
- Cutleaf Blackberry
- Cutleaf coneflower
- Cutleaf evening primrose
- Cutleaf groundcherry
- Cutleaf teasel
- Cypress spurge
- Cypress-leaved Plait-moss