• Sambucus nigra Elder
  
• Samolus valerandi Brookweed
  
• Sanguisorba minor Salad Burnet
  
• Sanicula europaea Wood Sanicle
  
• Saponaria officinalis Soapwort
  
• Saxifraga hypnoides Saxifrage, Mossy
  
• Saxifraga rosacea Irish Saxifrage
  
• Saxifraga spathularis St. Patrick's-cabbage
  
• Saxifraga tridactylites Rue-leaved Saxifrage
  
• Scrophularia auriculata Figwort, Water
  
• Scrophularia nodosa Common Figwort
  
• Scutellaria galericulata Scullcap
  
• Scutellaria minor Scullcap
  
• Sedum acre Biting Stonecrop
  
• Sedum album Stonecrop, White
  
• Sedum anglicum English Stonecrop
  
• Sedum forsterianum Rock Stonecrop
  
• Sedum rosea Roseroot
  
• Senecio aquaticus Ragweed, Marsh
  
• Senecio jacobaea Ragweed
  
• Senecio sylvaticus
• Senecio vulgaris Groundsel
  
• Seriphidium maritimum Sea Wormwood
  
• Sherardia arvensis Field Madder
  
• Silene vulgaris Bladder Campion
  
• Sinapis alba White Mustard
  
• Sinapis arvensis Charlock
  
• Sisymbrium officinale Hedge Mustard
  
• Sium latifolium Greater Water-parsnip
  
• Smyrnium olusatrum Alexanders
  
• Solanum dulcamara Bittersweet
  
• Solanum nigrum Black Nightshade
  
• Soleirolia soleirolii Mind-your-own-business
  
• Solidago virgaurea Golden Rod
  
• Sonchus arvensis Sow-thistle
  
• Sonchus asper Sow-thistle
  
• Sonchus oleraceus Sow-thistle
  
• Sorbus aria Whitebeam
  
• Sorbus aucuparia Rowan
  
• Sorbus hibernica Whitebeam, Irish
  
• Sorbus intermedia Swedish Whitebeam
  
• Sorbus rupicola
• Sparganium angustifolium Floating Bur-reed
  
• Sparganium emersum Unbranched Bur-reed
  
• Sparganium erectum Bur-reed, Branched
  
• Sparganium natans Least Bur-reed
  
• Spergula arvensis Corn Spurrey
  
• Spergularia marina Lesser Sea-spurry
  
• Spergularia media Sea-spurry, Greater
  
• Spergularia rupicola Rock Sea-spurry