| Common Name | Latin Name |
|---|---|
| Saint Johnswort | Hypericum perforatum |
| Saltwort | see Russian Thistle |
| Sandbar Willow | Salix exigua |
| Sassafras | Sassafras albidum |
| Sassy Jack | see Caper Spurge |
| Saw Greenbrier | see Greenbrier |
| Sawtooth Blackberry | see Highbush Blackberry |
| Scarlet Indian Paintbrush | Castilleja coccinea |
| Scarlet Pimpernel | Anagallis arvensis |
| Scarlet Toothcup | see Valley Redstem |
| Scotch Thistle | Onopordum acanthoides |
| Scouring Rush | see Field Horsetail |
| Scouringrush | Equisetum hyemale |
| Sensitive Brier | Mimosa quadrivalvis var. nuttallii |
| Sericea Lespedeza | Lespedeza cuneata |
| Shagbark Hickory | Carya ovata |
| Sharp-Toothed Blackberry | see Highbush Blackberry |
| Shattercane | Sorghum bicolor |
| Sheep-Foot | see Birdsfoot Trefoil |
| Shepherd's-purse | Capsella bursa-pastoris |
| Shingle Oak | Quercus imbricaria |
| Shoo Fly | see Apple-of-Peru |
| Showy Crotalaria | Crotalaria spectabilis |
| Showy Evening-primrose | Oenothera speciosa |
| Showy Rattlebox | see Showy Crotalaria |
| Showy Ticktrefoil | Desmodium canadense |
| Sicklepod | Cassia obtusifolius |
| Silver Wormwood | see White Sage |
| Silverleaf Nightshade | Solanum elaeagnifolium |
| Slender Amaranth | Amaranthus viridis |
| Slender Mountain Mint | Pycnanthemum tenuifolium |
| Slender Naiad | Najas minor |
| Slender Pigweed | see Slender Amaranth |
| Slender Pondweed | Potamogeton pusillus |
| Small Pondweed | see Slender Pondweed |
| Smallflower Geranium | Geranium molle |
| Smallflower Morningglory | Jacquemontia tamnifolia |
| Smallflowered Bittercress | Cardamine parviflora |
| Smallflowered Phacelia | Phacelia dubia |
| Smooth Bedstraw | Galium mollugo |
| Smooth Bedstraw | see Bedstraw |
| Smooth Blackberry | Rubus allegheniensis |
| Smooth Blackberry | see Highbush Blackberry |
| Smooth Brome | Bromus inermis |
| Smooth Bromegrass | see Smooth Brome |
| Smooth Crabgrass | Digitaria ischaemum |
| Smooth Groundcherry | Physalis longifolia var. subglabrata |
| Smooth Hawksbeard | Crepis capillaris |
| Smooth Pigweed | Amaranthus hybridus |
| Smooth Sumac | Rhus glabra |
| Smooth Vetch | Vicia dasycarpa |
| Snow-on-the-Mountain | Euphorbia marginata |
| Snowy spurge | see Snow-on-the-Mountain |
| Soapwort | see Bouncing-bet |
| Southern Cottonwood | see Cottonwood |
| Southern Sneezeweed | see Purple-Headed Sneezeweed |
| Spanish Needles | Bidens bipinnata |
| Spanish Thistle | see Spiny Cocklebur |
| Spiked loosestrife | see Purple Loosestrife |
| Spiny Amaranth | Amaranthus spinosus |
| Spiny Burweed | see Spiny Cocklebur |
| Spiny Clotbur | see Spiny Cocklebur |
| Spiny Cocklebur | Xanthium spinosum |
| Spiny Pigweed | see Spiny Amaranth |
| Spiny Sowthistle | Sonchus asper |
| Spotted Knapweed | Centaurea maculosa |
| Spotted Spurge | Chamaesyce maculata |
| Spotted St. Johnswort | Hypericum punctatum |
| Spurred Anoda | Anoda cristata |
| Squaw-root | Conopholis americana |
| Squawweed | see Cressleaf Groundsel |
| St. Augustine Grass | Stenotaphrum secundatum |
| Star Thistle | see Spotted Knapweed |
| Star-of-Bethlehem | Ornithogalum umbellatum |
| Starch Grapehyacinth | Muscari racemosum |
| Sticktight | see Common Beggar-ticks |
| Stickweed | Verbesina occidentalis |
| Stinging Nettle | Urtica dioica |
| Stinkgrass | Eragrostis cilianensis |
| Stone Clover | see Rabbitfoot Clover |
| Stonewort | see Chara |
| Straw-colored Flatsedge | Cyperus strigosus |
| Sulfur Cinquefoil | Potentilla recta |
| Summer Spurge | see Sun Spurge |
| Sun Spurge | Euphorbia helioscopia |
| Swamp Milkweed | Asclepias incarnata |
| Swamp Smartweed | Polygonum coccineum |
| Swamp Verbena | see Blue Vervain |
| Sweet Annie | see Annual Wormwood |
| Sweet Betty | see Bouncing-bet |
| Sweet Pea | see Perennial Pea |
| Sweet Vernalgrass | Anthoxanthum odoratum |
| Sweetgum | Liquidambar styraciflua |
| Swine wart cress | see Swinecress |
| Swinecress | Coronopus didymus |
