Opposite-leaved Saltwort 
Opuntia triacantha 
Oregon silene 
Organpipe cactus 
Oval Leaf Knotweed 
Pacific Glasswort 
Pale Smartweed 
Pancake prickly pear 
Paper-Spined Cholla 
Parodia comarapana 
Parodia Magnifica 
Paronychia arabica 
Paronychia argentea 
Paronychia echinulata 
Paronychia palaestina 
Paronychia sinaica 
Parry's silene 
Parsnipflower buckwheat 
Pataconejo comun 
Pennsylvania smartweed 
Perennial Glasswort 
Perennial knawel 
Perez's Sea Lavender 
Persicaria lanigera