Common lousewort 
Common Mare's Tail 
Common Self-heal 
Common snapdragon 
Common Thyme 
Common toothwort 
Common Vervain 
Common yellow monkeyflower 
Corn Speedwell 
Corytoplectus speciosus 
Costarican Justicia 
Cowpea Witchweed 
Crantzia cristata 
Creeping boobialla 
Creeping charlie 
Creeping Charlie 
Creeping Lantana 
Cremosperma castroanum 
Crescentia alata 
Crested flower isodon 
Cretan Plantain 
Crossvine 
Culvers root 
Cusick's speedwell