- St. Andrew's Cross
- St. Johnswort, Common
- St. Johnswort, Dense-flowered
- St. Johnswort, Dwarf
- St. Johnswort, False Spotted
- St. Johnswort, Marsh
- St. Johnswort, Shrubby
- Sarsaparilla, Wild
- Sassafras
- Sedge, Fraser's
- Sedge, Pennsylvania
- Sedge, Seersucker or Plantainleaf
- Seedbox
- Self-heal
- Seneca Snakeroot
- Senna, Maryland
- Senna, Wild
- Shale Barren Buckwheat
- Shale Barren Rockcress
- Shepherd's Purse
- Showy Fly or Bush Honeysuckle
- Shrubby Lesbedeza
- Sicklepod
- Silverrod
- Skullcap, Hairy
- Skullcap, Heart-leaved
- Skullcap, Hyssop
- Skullcap, Mad-dog
- Skullcap, Marsh
- Skullcap, Showy
- Skullcap, Veiny or Veined
- Skunk Cabbage
- Small Bugloss
- Small-flowered Cranesbill
- Small-flowered Phacelia
- Smartweed, Common
- Smooth Sumac
- Snakeroot, Black
- Snakeroot, White
- Sneezeweed
- Solomon's Seal
- Solomon's Seal, False
- Solomon's Seal, Great
- Spanish Needles
- Spatterdock or Southern Pond Lily
- Spearmint
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- Speedwell, American
- Speedwell, Bird's-eye
- Speedwell, Common
- Speedwell, Corn
- Speedwell, Ivy-leaved
- Speedwell, Thyme-leaved
- Speedwell, Water
- Spicebush
- Spiderwort, Virginia
- Spikenard
- Spiraea, Corymbed
- Spiraea, Japanese
- Spreading Dogbane
- Spring Beauty, Carolina
- Spring Beauty, Virginia
- Spurge, Caper
- Spurge, Cypress
- Spurge, Falcate or Sickle
- Spurge, Flowering
- Spurge, Sun
- Spurge, Toothed
- Spurge, Upright Spotted
- Spurge, Wood
- Squawroot
- Squirrel Corn
- Starflower
- Star-of-Bethlehem
- Steeplebush
- Stitchwort, Lesser
- Stitchwort, Long-leaved
- Stonecrop Allegheny
- Storksbill
- Sundew, Round-leaved
- Sundrops
- Sundrops, Small
- Sunflower, Maximilian's
- Sunflower, Pale-leaved
- Sunflower, Small Wood
- Sunflower, Tall
- Sunflower, Thin-leaved
- Sunflower, Midwestern Tickseed
- Sunflower, Paleleaf Woodland
- Sunflower, Woodland
- Swamp Candles
- Swamp Lousewort
- Sweet Cecily
- Sweetfern
- Sweet William
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