- Sacred Datura
- Sage-leaved Rock-rose
- Sagebrush buttercup
- Saguaro
- Saharan Mustard
- Sainfoin
- Salad Burnet
- Salal
- Sallow sedge
- Salmonberry
- Saltgrass
- Santa Maria feverfew
- Saskatoon
- Sassafras
- Saw greenbrier
- Saw palmetto
- Sawbrier
- Scarlet Beeblossom
- Scarlet Gilia
- Scarlet Globemallow
- Scarlet monkeyflower
- Scarlet Pea
- Scarlet pimpernel
- Scarlet spiderling
- Scented Solomon's-seal
- Scentless Feverfew
- Scentless mayweed
- Scotch Broom
- Scots pine
- Scrambling Dayflower
- Sea Almond
- Sea Aster
- Sea Bindweed
- Sea Campion
- Sea Daffodil
- Sea fig
- Sea grape
- Sea hibiscus
- Sea Holly
- Sea kale
- Sea Milkwort
- Sea oats
- Sea ox-eye
- Sea Plantain
- Sea purslane
- Sea Sandwort
- Sea Spurge
- Sea Thrift
- Sea-buckthorn
- Seaside daisy
- Seaside pea
- Seep monkeyflower
- Sensitive fern
- Sensitive pea
- Sensitive Plant
- Sessile bellwort
- Shagbark hickory
- Shaggy portulaca
- Shaggy soldier
- Sharp-lobed hepatica
- Sheep laurel
- Sheep's sorrel
- Sheep's-bit
- Shell ginger
- Shepherd's-purse
- Shining Crane's-bill
- Shining firmoss
- Shining sumac
- Shinleaf
- Shiny-leaved Wild Coffee
- Shortpod Mustard
- Shortspur seablush
- Showy milkweed
- Showy orchis
- Showy tick-trefoil
- Shrubby cinquefoil
- Shrubby false buttonweed
- Shrubby milkwort
- Siam weed
- Siberian Crane's-bill
- Siberian elm
- Siberian hogweed
- Siberian larch
- Siberian peashrub
- Siberian pine
- Siberian spruce
- Siberian squill
- Sickle alfalfa
- Sickleleaf Conebush
- Sickletop Lousewort
- Sideoats grama
- Silver Banksia
- Silver beachweed
- Silver birch
- Silver Cholla
- Silver maple
- Silver Thistle
- Silver wattle
- Silver Wormwood
- Silverberry
- Silverleaf nightshade
- Silverpuffs
- Silverrod
- Silvery Bryum
- Silvery Cinquefoil
- Singleleaf pinyon
- Sitka spruce
- Sitka valerian
- Skeleton fork fern
- Skunk vine
- Sleepy Morning
- Slender leafy spurge
- Slender Path Rush
- Slender speedwell
- Slender stinging nettle
- Slender vetch
- Slender yellow woodsorrel
- Small balsam
- Small bugloss
- Small camas
- Small melilot
- Small-flowered blue-eyed mary
- Small-flowered buttercup
- Small-flowered Catchfly
- Small-flowered Crane's-bill
- Smallflower woodland star
- Smallseed sandmat
- Smokebush
- Smooth blue aster
- Smooth Brome
- Smooth golden fleece
- Smooth Solomon's seal
- Smooth sumac
- Smooth Yellow Violet
- Snakewort
- Snapdragon
- Snapdragon Vine
- Sneezewort
- Snow-on-the-mountain
- Snowbrush Ceanothus
- Snowdrop Anemone
- Snowplant
- Soaptree yucca
- Soft Rush
- Soft shield fern
- Solomon's plume
- Solomon's-seal
- Sorbaria
- Sorrelvine
- Sosnowsky's hogweed
- Sourwood
- Southern blue flag
- Southern bush monkeyflower
- Southern coastal bushmallow
- Southern dewberry
- Southern lady fern
- Southern live oak
- Southern magnolia
- Southern red oak
- Southwestern Ponderosa Pine
- Spanish Broom
- Spanish lavender
- Spanish moss
- Spanish needles
- Sparkleberry
- Spatterdock
- Spiked rampion
- Spiked Speedwell
- Spindle
- Spinulose wood fern
- Spiny Asparagus
- Spiny hackberry
- Spiny phlox
- Spiny restharrow
- Spiny Starwort
- Spiny-headed Mat-rush
- Spinystar
- Splendid Mariposa Lily
- Spoonleaf sundew
- Spotted coralroot
- Spotted deadnettle
- Spotted horsemint
- Spotted Joe-Pye weed
- Spotted knapweed
- Spotted lady's thumb
- Spotted medick
- Spotted spurge
- Spreading Bellflower
- Spreading dogbane
- Spreading wood fern
- Spring Adonis
- Spring gentian
- Spring Heath
- Spring Snowflake
- Spring starflower
- Spring vetch
- Spurge nettle
- Spurge-laurel
- Square Gooseneck Moss
- Squashberry
- Squirrel corn
- Squirting cucumber
- St Lucie Cherry
- St. Andrew's cross
- Stag's-horn clubmoss
- Staghorn sumac
- Stairstep moss
- Star chickweed
- Star of Bethlehem
- Star-flowered lily-of-the-valley
- Starry campion
- Steeplebush
- Sticky Catchfly
- Sticky geranium
- Sticky mouse-ear chickweed
- Sticky Sage
- Stiff greenthread
- Stiff-leaved goldenrod
- Stinking hellebore
- Stinking iris
- Stinknet
- Stinkwort
- Stone Bramble
- Stone pine
- Straggler daisy
- Strawberry bush
- Strawberry clover
- Strawberry tree
- Stream violet
- Stretchberry
- Stringy Stonecrop
- Striped maple
- Striped wintergreen
- Subalpine fir
- Subterranean Clover
- Suffolk Lungwort
- Sugar bush
- Sugar hackberry
- Sugar maple
- Sulfur buckwheat
- Sulphur cinquefoil
- Summer bluet
- Summer snowflake
- Summer-cypress
- Sun Cup
- Sun spurge
- Sundial Lupine
- Swallow-wort
- Swamp candles
- Swamp Laurel
- Swamp loosestrife
- Swamp milkweed
- Swamp rose
- Swamp rose mallow
- Swedish Cornel
- Sweet acacia
- Sweet acorn oak
- Sweet alyssum
- Sweet birch
- Sweet Chestnut
- Sweet everlasting
- Sweet Pepperbush
- Sweet scabious
- Sweet Thorn
- Sweet vernal grass
- Sweet violet
- Sweet Woodruff
- Sweet-William
- Sweetbay magnolia
- Sweetbush
- Sweetfern
- Switchgrass
- Sycamore maple