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| scientific name | common name(s) | image gallery |
| Oenothera jamesii | River Primrose Trumpet Evening Primrose Trumpet evening-primrose | |
| Oenothera macrocarpa | Bigfruit evening-primrose Missouri evening-primrose Fluttermill Big-fruit evening-primrose Missouri Primrose | |
| Oenothera macrocarpa ssp. incana | Bigfruit evening-primrose Silver leaf primrose | |
| Oenothera speciosa | Pink evening primrose Showy evening primrose Mexican evening primrose Showy primrose Pink ladies Buttercups Pink buttercups | |
| Oenothera triloba | Stemless evening-primrose Stemless Evening Primrose | |
| Opuntia ellisiana | Spineless prickly pear Tigertongue South Texas pricklypear | |
| Opuntia engelmannii var. lindheimeri | Texas pricklypear Prickly Pear Texas Prickly Pear | |
| Opuntia macrorhiza | Common prickly-pear Plains prickly pear Prickly Pear Twist-spine prickly-pear Twistspine pricklypear | |
| Opuntia macrocentra | Purple Prickly Pear Purple pricklypear | |
| Oxalis drummondii | Drummond's woodsorrel Wood-sorrel | |
| Oxalis stricta | Common yellow oxalis Yellow Wood-sorrel | |
| Parkinsonia aculeata | Retama Paloverde Mexican Palo Verde Jerusalem thorn Lluvia de Oro | |
| Passiflora affinis | Bracted passionflower Passion flower | |
| Passiflora foetida | Corona de Cristo Scarlet-fruit passion flower Fetid passionflower | |
| Parthenocissus heptaphylla | Sevenleaf creeper Seven-leaf Creeper | |
| Passiflora incarnata | Purple passionflower Purple passion vine Maypop Apricot vine | |
| Pavonia lasiopetala | Rock rose Rose pavonia Rose mallow Pavonia | |
| Passiflora lutea | Yellow passion vine Yellow passion-flower Yellow passionflower | |
| Papaver nudicaule | Icelandic poppy | |
| Parthenocissus quinquefolia | Virginia creeper | |
| Passiflora tenuiloba | Bird Wing Passionflower Birdwing passionflower Slender-lobe Passionflower Spread Lobe Passionflower | |
| Panicum virgatum | Switchgrass Wand panic grass | |
| Paronychia virginica | Whitlow-wort Broom Nailwort Yellow Nailwort Park's Nailwort | |
| Penstemon baccharifolius | Rock penstemon Cut-Leaf Penstemon Baccharis-leaf beardtongue Beard Tongue | |
| Penstemon cobaea | Wild foxglove Prairie penstemon Foxglove penstemon False foxglove Large-flowered beardtongue Large-flowered penstemon Prairie beardtongue Cobaea penstemon | |
| Penstemon laxiflorus | Nodding beardtongue Loose-flowered Penstemon | |
| Pellaea ovata | Ovate-leaf Cliffbrake Ovateleaf cliffbrake | |
| Penstemon triflorus | Hill Country Penstemon Scarlet Penstemon Heller's beardtongue | |
| Phytolacca americana | American pokeweed Pokeweed Pokeberry Red Ink Plant | |
| Phacelia congesta | Blue curls Caterpillars Fiddleneck | |
| Physostegia intermedia | Intermediate false dragonhead Marsh obedient-plant Obedient-plant Slender false dragonhead Spring obedient plant | |
| Phyla nodiflora | Texas frogfruit Turkey tangle fogfruit Frogfruit | |
| Physostegia virginiana | Fall obedient plant Obedient plant False dragonhead | |
| Pistacia mexicana | Mexican pistachio Texas pistachio American pistachio | |
| Platanus occidentalis | American sycamore Buttonwood Plane-tree Sycamore | |
| Pluchea odorata var. odorata | Marsh Fleabane Sweetscent | |
| Polygala alba | White milkwort | |
| Poa arachnifera | Texas bluegrass Texas blue grass | |
| Pontederia cordata | Pickerelweed Pickerel Weed | |
| Populus deltoides | Eastern cottonwood Alamo Carolina poplar Necklace Poplar | |
| Polanisia dodecandra ssp. trachysperma | Clammy-Weed Clammyweed Sandyseed clammyweed | |
| Potamogeton nodosus | Knotty Pondweed Longleaf pondweed | |
| Polytaenia texana | Texas prairie parsley Texas-Parsley | |
| Prosopis glandulosa | Honey mesquite | |
| Prunus minutiflora | Dwarf Plum Texas almond | |
| Prunus texana | Peachbush Sand Plum Peach bush Texas plum Texas peach bush | |
| Prunella vulgaris | Common selfheal | |
| Ptelea trifoliata | Wafer ash Common hoptree Hop tree | |
| Ratibida columnifera | Mexican hat Prairie coneflower Upright prairie coneflower Red-spike mexican-hat Long-headed coneflower Thimbleflower | |
| Ranunculus macranthus | Large buttercup | |
| Rhus aromatica | Fragrant sumac Aromatic sumac Lemon sumac Polecat bush | |
| Rhus aromatica var. serotina | Fragrant sumac | |
| Rhynchospora colorata | Starrush whitetop Star sedge White-topped sedge Whitetop sedge | |
| Rhus lanceolata | Prairie flameleaf sumac Flame-leaf sumac Prairie sumac Lance-leaf sumac | |
| Rhus virens | Evergreen sumac Tobacco sumac | |
| Rivina humilis | Pigeonberry Rouge plant Baby peppers | |
| Ruellia drummondiana | Drummond's ruellia Drummond's wild petunia | |
| Rudbeckia hirta | Black-eyed Susan Common black-eyed Susan Brown-eyed Susan | |
| Ruellia metziae | Metz Wild Petunia Metz's wild petunia Wild White Petunia | |
| Ruellia nudiflora | Violet ruellia Common wild petunia Violet wild petunia Wild petunia | |
| Ruellia occidentalis | Western wild petunia | |
| Rubus trivialis | Dewberry Southern dewberry | |
| Salvia azurea var. grandiflora | Pitcher sage Blue sage | |
| Salvia ballotiflora | Shrubby blue sage Blue Shrub Sage Mejorana | |
| Saururus cernuus | Lizard's tail Lizard's-tail Breast weed | |
| Salvia coccinea | Scarlet Sage Tropical Sage Blood sage | |
| Samolus ebracteatus ssp. cuneatus | Limerock Brookweed Limewater brookweed | |
| Salvia engelmannii | Engelmann's sage Engelmann’s salvia | |
| Salvia farinacea | Mealy blue sage Mealy sage Mealycup sage | |
| Salvia greggii | Autumn sage Cherry sage Gregg salvia | |
| Sagittaria lancifolia | Bulltongue arrowhead Lanceleaf Arrowhead | |
| Sagittaria lancifolia ssp. media | Bulltongue arrowhead Scythe-fruit Arrowhead | |
| Sagittaria longiloba | Flecha de Agua Long-barb Arrowhead Long-lobe Arrowhead Longbarb arrowhead | |
| Salvia lycioides | Canyon sage | |
| Salvia lyrata | Lyreleaf sage Cancer weed | |
| Sabal mexicana | Mexican palm Palma Di Micharos Rio Grande palmetto Texas palm Texas palmetto | |
| Sabal minor | Dwarf palmetto Palmetto Bush palmetto | |
| Salvia pentstemonoides | Big red sage Penstemon sage | |
| Salvia regla | Mountain sage Royal sage | |
| Salvia roemeriana | Cedar sage | |
| Sapindus saponaria var. drummondii | Western soapberry Soapberry | |
| Salvia texana | Texas sage Blue sage | |
| Samolus valerandi ssp. parviflorus | Seaside brookweed Water Pimpernel Thin-leaf Brookweed | |
| Schoenoplectus californicus | California bulrush California club-rush Giant Bulrush | |
| Scutellaria drummondii | Drummond's skullcap | |
| Scutellaria ovata | Heartleaf skullcap Heart-leaf Skullcap | |
| Scutellaria ovata ssp. bracteata | Heartleaf skullcap Heart-leaf skullcap | |
| Scutellaria resinosa | Resin-dot skullcap Resinous skullcap Shrubby skullcap Sticky skullcap | |
| Schizachyrium scoparium | Little bluestem | |
| Scutellaria wrightii | Wright's skullcap Bushy skullcap Shrubby skullcap | |
| Senna lindheimeriana | Lindheimer's senna Velvet leaf senna Velvetleaf senna Velvet-leaf wild sensitive-plant Puppy-dog ears | |
| Senna roemeriana | Twoleaf senna Two leaved senna | |
| Silphium albiflorum | White rosinweed White-flowered rosinweed Compass Plant | |
| Simsia calva | Awnless bush sunflower Bush sunflower | |
| Silphium simpsonii | Simpson's rosinweed Tall Rosinweed | |
| Solanum elaeagnifolium | Silverleaf nightshade Silver-leaf nightshade White horse nettle Trompillo Tomato weed | |
| Solidago nemoralis | Gray goldenrod Prairie goldenrod Old field goldenrod | |
| Sorghastrum nutans | Indiangrass Yellow indian grass | |
| Solanum rostratum | Buffalobur nightshade Buffalo bur | |
| Sophora secundiflora | Texas mountain laurel Mountain laurel Mescal bean Mescal bean sophora Frijolillo Frijolito | |
| scientific name | common name(s) | image gallery |