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| Common Name | Scientific Name | |
| Calamint | Calamintha nepeta | |
| Calepina | Calepina irregularis | |
| Campion, Bladder | Silene vulgaris | |
| Campion, White | Silene alba | |
| Canada Goldenrod | Solidago canadensis | |
| Canada Thistle | Cirsium arvense | |
| Cancer-root | Conopholis americana | |
| Caper Spurge | Euphorbia lathyris | |
| Carolina Geranium | Geranium carolinianum | |
| Carpetweed | Mollugo verticillata | |
| Carrot, Wild | Daucus carota | |
| Catnip | Nepeta cataria | |
| Catsear, Common | Hypochoeris radicata | |
| Cattails | Typha spp. | |
| Centipedegrass | Eremochloa ophiuroides | |
| Chara | Chara spp. | |
| Chickweed, common | Stellaria media | |
| Chickweed, mouse-ear | Cerastium vulgatum | |
| Chicory | Cichorium intybus | |
| Chinese Yam | Dioscorea batatas | |
| Cinnamon Vine | Dioscorea batatas | |
| Cinquefoil, Sulfur | Potentilla recta | |
| Clammy Groundcherry | Physalis heterophylla | |
| Clover, Japanese | Lespedeza striata | |
| Clover, Rabbitfoot | Trifolium arvense | |
| Clover, Red | Trifolium pratense | |
| Clover, White | Trifolium repens | |
| Clover, White Sweet | Melilotus alba | |
| Cocklebur, Common | Xanthium strumarium | |
| Cocklebur, Spiny | Xanthium spinosum | |
| Coffee Senna | Senna occidentalis | |
| Common Blue Violet | Viola papilionacea | |
| Common Burdock | Arctium minus | |
| Common Buttonweed | Diodia teres | |
| Common Catsear | Hypochoeris radicata | |
| Common Cocklebur | Xanthium strumarium | |
| Common Dayflower | Commelina communis | |
| Common Eveningprimrose | Oenothera biennis | |
| Common Groundsel | Senecio vulgaris | |
| Common Lambsquarters | Chenopodium album | |
| Common Lespedeza | Lespedeza striata | |
| Common Mallow | Malva neglecta | |
| Common Milkweed | Asclepias syriaca | |
| Common Mullein | Verbascum thapsus | |
| Common Pokeweed | Phytolacca americana | |
| Common Purslane | Portulaca oleracea | |
| Common Ragweed | Ambrosia artemisiifolia | |
| Common Reed | Phragmites australis | |
| Common Teasel | Dipsacus fullonum | |
| Common Toadflax | Linaria canadensis | |
| Common Venus' Looking-glass | Triodanis perfoliata | |
| Common Vetch | Vicia sativa | |
| Common Yarrow | Achillea millefolium | |
| Coontail | Ceratophyllum demersum | |
| Copperleaf, Hophornbeam | Acalypha ostryifolia | |
| Copperleaf, Virginia | Acalypha virginica | |
| Coral-Berry | Smphoricarpos orbiculatus | |
| Coreopsis Beggarticks | Bidens polylepis | |
| Corn Buttercup | Ranunculus arvensis | |
| Cornflower | Centaurea cyanus | |
| Corn Salad | Valerianella radiata | |
| Corn Spurry | Spergula arvensis | |
| Cotton Morningglory | Ipomoea cordatotriloba* | |
| Cow-itch | Campsis radicans | |
| Crabgrass, Large | Digitaria sanguinalis | |
| Crabgrass, Smooth | Digitaria ischaemum | |
| Creeping Phlox | Phlox subulata | |
| Creeping Primrose | Ludwigia palustris | |
| Creeping Rush | Juncus repens | |
| Creeping Woodsorrel | Oxalis corniculata | |
| Cress, Mouse-ear | Arabidopsis thaliana | |
| Crossvine | Bignonia capreolata | |
| Croton, Tropic | Croton glandulosus | |
| Crowfootgrass | Dactyloctenium aegyptium | |
| Crowfoot, Small-flowered | Ranunculus abortivus | |
| Crownbeard, Yellow | Verbesina occidentalis | |
| Crownvetch | Coronilla varia | |
| Cudweed, Purple | Gnaphalium purpureum | |
| Curly Dock | Rumex crispus | |
| Curlyleaf Pondweed | Potamogeton crispus | |
| Cutleaf Eveningprimrose | Oenothera laciniata | |
| Cutleaf Geranium | Geranium dissectum | |
| Cypress Spurge | Euphorbia cyparissias | |
| Cypressvine Morningglory | Ipomoea quamoclit | |
| *The full scientific name is Ipomoea cordatotriloba var. torreyana | ||
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