Sanguinaria canadensis - Bloodroot
Sanicula odorata - Clustered snakeroot
Sanicula marilandica - Black snakeroot
Schizachyrium scoparium - Little bluestem
Scirpus cyperinus - Woolgrass
Scrophularia marilandica - Figwort
Senna hebecarpa - Wild senna
Silene regia - Royal catchfly
Silene stellata - Starry campion
Silphium integrifolium - Rosinweed
Silphium lacinatum - Compass plant
Silphium perfoliatum - Cup plant
Silphium terebinthinaceum - Prairie dock
Sisyrinchium campestre - Blue-eyed grass
Smilax herbacea - Common Carrion Flower
Solidago canadensis - Canada goldenrod
Solidago flexicaulis - Zigzag goldenrod
Solidago gigantea - Giant goldenrod
Solidago missouriensis - Missouri goldenrod
Solidago nemoralis - Gray goldenrod
Solidago ohiensis - Ohio goldenrod
Solidago rigida - Stiff goldenrod
Solidago speciosa - Showy goldenrod
Solidago ulmifolia - Elm leaf goldenrod
Sorghastrum nutans - Indian Grass
Spiranthes casei - Case's lady tresses
Spiranthes ovalis - October Lady's tresses
Sporobolus compositus - Rough dropseed
Sporobolus heterolepis - Prairie dropseed