Oemleria cerasiformis (Oso berry)
Oenanthe sarmentosa (Water-parsley)
Oenothera
    californica ssp. avita (Desert evening primrose)
    californica ssp. californica (California evening primrose)
    californica ssp. eurekensis (Eureka Dunes evening primrose)
    caespitosa ssp. marginata (Tufted evening primrose)
    cavernae (Cave-dwelling evening primrose)
    deltoides ssp. deltoides (Dune primrose)
    elata ssp. hirsutissima (Hairy evening primrose)
    elata ssp. hookeri (Hooker's evening primrose)
    primiveris ssp. bufonis (Large yellow primrose)
    rosea (Rose evening primrose)*
Oligomeris linifolia (Oligomeris)
Olneya tesota (Desert ironwood)
Oncosiphon piluliferum (Globe chamomile)*
Ophioglossum californicum (California adder's-tongue)
Opuntia
    acanthocarpa var. coloradensis (Buckhorn cholla)
    basilaris var. basilaris (Beavertail cactus)
    basilaris var. brachyclada (Short-joint beavertail cactus)
    bigelovii (Jumping cholla)
    chlorotica (Pancake prickly-pear)
    echinocarpa (Silver cholla)
    erinacea var. erinacea (Mojave prickly-pear)
    ficus-indica (Mission cactus)*
    X fosbergii (Fosberg's cholla)
    littoralis (Coast prickly-pear)
    occidentalis (Western prickly-pear)
    oricola (Chaparral prickly-pear)
    parishii (Mat cholla)
    parryi (Valley cholla)
    phaeacantha (Desert prickly-pear)
    prolifera (Coast cholla)
    ramosissima (Pencil cholla)
    vaseyi (Vasey's prickly-pear)
    wolfii (Wolf's cholla)
Oreonana vestita (Woolly mountain parsley)
Ornithostaphylos oppositifolia (Baja birdbush)
Orobanche
    bulbosa (Chaparral broom-rape)
    californica ssp. feudgei (California broom-rape)
    cooperi (Cooper's broom-rape)
    fasciculata (Clustered broom-rape)
    parishii ssp. brachyloba (Short-lobed broom-rape)
    parishii ssp. parishii (Parish's broom-rape)
    uniflora (Naked broom-rape)
    valida ssp. valida (Rock Creek broom-rape)
Osmadenia tenella (Three-spot)
Osmorhiza
    brachypoda (Sweet-cicely)
    chilensis (Mountain sweet-cicely)
Osteospermum ecklonis (African daisy)*
Oxalis
    albicans (Wild oxalis)
    corniculata (Creeping wood-sorrel)*
    pes-caprae (Bermuda buttercup)*
Oxystylis lutea (False-clover)
Oxytheca
    caryophylloides (Chickweed oxytheca)
    emarginata (Whitemargin oxytheca)
    parishii var. parishii (Parish's oxytheca)
    perfoliata (Roundleaf oxytheca)
    trilobata (Threelobe oxytheca)
Paeonia californica (Wild peony)
Palafoxia
    arida var. arida (Spanish needles)
    arida var. gigantea (Giant Spanish needles)
Panicum urvilleanum (Desert panic grass)
Papaver californicum (Fire poppy)
Parapholis incurva (Curved sicklegrass)*
Parietaria hespera var. hespera (Pellitory)
Paspalum
    dilatatum (Dallis grass)*
    distichum (Knot grass)
Pectis papposa var. papposa (Chinchweed)
Pectocarya
    heterocarpa (Hairy-leaved pectocarya)
    linearis ssp. ferocula (Slender pectocarya)
    penicillata (Winged pectocarya)
    platycarpa (Broad-fruited pectocarya)
    recurvata (Curvenut pectocarya)
    setosa (Bristly pectocarya)
Pedicularis
    densiflora (Indian warrior)
    semibarbata (Pine lousewort)
Pellaea
    andromedifolia (Coffee fern)
    mucronata var. californica (California cliff brake)
    mucronata var. mucronata (Bird's-foot fern)
    truncata (Cliff brake)
Pennisetum
    clandestinum (Kikuyu grass)*
    setaceum (Fountaingrass)*
Penstemon
    caesius (San Bernardino beardtongue)
    calcareus (Limestone beardtongue)
    centranthifolius (Scarlet bugler)
    clevelandii var. clevelandii (Cleveland's beardtongue)
    clevelandii var. connatus (San Jacinto beardtongue)
    eatonii var. undosus (Firecracker penstemon)
    floridus var. austinii (Austin's beardtongue)
    fruticiformis var. fruticiformis (Desert beardtongue)
    grinnellii var. grinnellii (Grinnell's penstemon)
    grinnellii var. scrophularioides (Grinnell's penstemon)
    heterophyllus var. australis (Foothill penstemon)
    heterophyllus var. heterophyllus (Foothill penstemon)
    labrosus (San Gabriel beardtongue)
    labrosus (yellow form) (San Gabriel beardtongue)
    laetus var. laetus (Mountain blue penstemon)
    palmeri var. palmeri (Palmer's penstemon)
    pseudospectabilis (Desert penstemon)
    rostriflorus (Beaked penstemon)
    speciosus (Royal penstemon)
    spectabilis var. spectabilis (Showy penstemon)
    spectabilis var. subviscosus (Showy penstemon)
    stephensii (Stephens' beardtongue)
    thurberi (Thurber penstemon)
    utahensis (Utah penstemon)
Pentachaeta
    aurea (Golden-rayed pentachaeta)
    lyonii (Lyon's pentachaeta)
Pentagramma
    triangularis ssp. triangularis (Goldback fern)
    triangularis ssp. viscosa (Silverback fern)
Perideridia
    parishii ssp. latifolia (Wide-leaved Parish's yampah)
    parishii ssp. parishii (Parish's yampah)
Perityle emoryi (Rock daisy)
Petalonyx thurberi (Sandpaper plant)
Petrophyton caespitosum (Rock spiraea)
Petrorhagia dubia (Hairypink)*
Peucephyllum schottii (Desert-fir)
Phacelia
    affinis (Limestone phacelia)
    anelsonii (Aven Nelson's phacelia)
    austromontana (Southern mountain phacelia)
    brachyloba (Yellow-throated phacelia)
    calthifolia (Caltha-leaved phacelia)
    campanularia ssp. campanularia (Desert bluebells)
    campanularia ssp. vasiformis (Desert bluebells)
    cicutaria var. hispida (Caterpillar phacelia)
    cicutaria var. hubbyi (Hubby's caterpillar phacelia)
    ciliata (Great Valley phacelia)
    crenulata var. ambigua (Notchleaf phacelia)
    crenulata var. minutiflora (Smallflower notchleaf phacelia)
    cryptantha (Hiddenflower phacelia)
    curvipes (Curved-stem phacelia)
    davidsonii (Davidson's phacelia)
    distans (Common phacelia)
    douglasii (Douglas phacelia)
    egena (Rock phacelia)
    exilis (Lavender windows)
    fremontii (Fremont's phacelia)
    grandiflora (Large-flowered phacelia)
    hastata ssp. hastata (Timberline phacelia)
    humilis var. dudleyi (Low phacelia)
    imbricata ssp. imbricata (Mountain phacelia)
    imbricata ssp. patula (Imbricate phacelia)
    insularis var. insularis (Northern Channel Islands phacelia)
    ivesiana (Ives' phacelia)
    longipes (Longstalk phacelia)
    minor (Canterbury bells)
    mohavensis (Mojave phacelia)
    mutabilis (Changeable phacelia)
    nashiana (Charlotte's phacelia)
    pachyphylla (Thick-leaved phacelia)
    parryi (Parry's phacelia)
    pedicellata (Pedicellate phacelia)
    perityloides var. perityloides (Panamint phacelia)
    ramosissima var. latifolia (Branching phacelia)
    ramossisima var. ramossisima (Branching phacelia)
    rotundifolia (Round-leaved phacelia)
    tanacetifolia (Tansy phacelia)
    vallis-mortae (Death Valley phacelia)
    viscida (Sticky phacelia)
Phalaris aquatica (Harding grass)*
Philadelphus microphyllus (Little-leaved mock orange)
Phleum alpinum (Mountain timothy)
Phlox
    austromontana (Southern mountain phlox)
    condensata (Condensed phlox)
    diffusa (Spreading phlox)
    dolichantha (Big Bear Valley phlox)
    gracilis [ssp. gracilis] (Common slender phlox)
    gracilis [ssp. humilis] (Laguna slender phlox)
    stansburyi (Stansbury phlox)
Pholisma
    arenarium (Scaly-stemmed sand plant)
    sonorae (Sand food)
Pholistoma
    auritum var. auritum (Fiesta flower)
    membranaceum (White fiesta flower)
Phoradendron
    californicum (Desert mistletoe)
    densum (Dense mistletoe)
    juniperinum (Juniper mistletoe)
    libocedri (Incense-cedar mistletoe)
    macrophyllum (Big leaf mistletoe)
    pauciflorum (Fir mistletoe)
    villosum (Oak mistletoe)
Phyllodoce breweri (Mountain heather)
Physalis
    crassifolia (Thick-leaved ground-cherry)
    hederifolia var. fendleri (Fendler's ground-cherry)
    lobata (Lobed ground-cherry)
Physocarpus capitatus (Pacific ninebark)
Phytolacca americana (Pokeweed)*
Pickeringia
    montana var. montana (Chaparral pea)
    montana var. tomentosa (Woolly chaparral pea)
Picris echioides (Bristly ox-tongue)*
Pilostyles thurberi (Thurber's stemsucker)
Pinus
    attenuata (Knobcone pine)
    contorta ssp. murrayana (Lodgepole pine)
    coulteri (Coulter pine)
    flexilis (Limber pine)
    halepensis (Aleppo pine)*
    jeffreyi (Jeffrey pine)
    lambertiana (Sugar pine)
    longaeva (Bristlecone pine)
    monophylla (Singleleaf pinyon pine)
    muricata (Bishop pine)
    ponderosa (Yellow pine)
    quadrifolia (Four-needle pinyon pine)
    sabiniana (Gray pine)
    torreyana (Torrey pine)
Piperia
    cooperi (Rein orchid)
    elongata (Dense-flowered rein orchid)
    transversa (Royal rein orchid)
    unalascensis (Alaska rein orchid)
Piptatherum miliaceum (Smilo grass)*
Plagiobothrys
    acanthocarpus (Adobe popcorn flower)
    arizonicus (Arizona popcorn flower)
    canescens (Valley popcorn flower)
    collinus var. fulvescens (California popcorn flower)
    fulvus (Tawny popcorn flower)
    jonesii (Mojave popcorn flower)
    nothofulvus (Rusty-haired popcorn flo
    undulatus (Hooked popcorn flower)
Plantago
    coronopus (Cut-leaf plantain)*
    elongata (Longleaf plantain)
    erecta (California plantain)
    lanceolata (English plantain)*
    major (Broadleaf plantain)*
    ovata (Woolly plantain)
    patagonica (Patagonia plantain)
Platanthera
    leucostachys (White-flowered bog orchid)
    sparsiflora (Sparse-flowered bog orchid)
Platanus racemosa (Western sycamore)
Platystemon californicus (Cream cups)
Plectritis ciliosa ssp. insignis (Long-spurred plectritis)
Pleuraphis rigida (Big galleta grass)
Pleurocoronis pluriseta (Arrowleaf)
Pluchea
    odorata (Salt marsh fleabane)
    sericea (Arrowweed)
Poa
    atropurpurea (San Bernardino bluegrass)
    bulbosa (Bulbous bluegrass)*
    douglasii (Sand-dune bluegrass)
    fendleriana ssp. longiligula (Fendler's bluegrass)
    secunda ssp. juncifolia (Rush bluegrass)
    secunda ssp. secunda (One-sided bluegrass)
Pogogyne nudiuscula (Otay Mesa mint)
Polemonium occidentale (Western sky pilot)
Polycarpon tetraphyllum (Four-leaf allseed)*
Polygala cornuta var. fishiae (Fish's milkwort)
Polygonum
    amphibium var. emersum (Swamp knotweed)
    arenastrum (Common knotweed)*
    bistortoides (Western bistort)
    capitatum (Himalayan smartweed)*
    douglasii ssp. douglasii (Douglas's knotweed)
    douglasii ssp. johnstonii (Douglas's knotweed)
    lapathifolium (Willow smartweed)
    punctatum (Water smartweed)
Polypodium
    californicum (California polypody)
    hesperium (Western polypody)
Polypogon
    interruptus (Ditch beard grass)*
    monspeliensis (Rabbitsfoot grass)*
Polystichum
    imbricans ssp. curtum (Cliff sword fern)
    munitum (Western sword fern)
   scopulinum (Mountain holly fern)
Populus
    balsamifera ssp. trichocarpa (Black cottonwood)
    fremontii ssp. fremontii (Fremont's cottonwood)
    tremuloides (Quaking aspen)
Porophyllum gracile (Slender poreleaf)
Portulaca
    halimoides (Desert portulaca)
    oleracea (Purslane)*
Potamogeton diversifolius (Diverse-leaved pondweed)
Potentilla
    anserina ssp. anserina (Silverleaf cinquefoil)
    anserina ssp. pacifica (Silverweed)
    glandulosa ssp. ewanii (Ewan's cinquefoil)
    glandulosa ssp. glandulosa (Sticky cinquefoil)
    glandulosa ssp. nevadensis (Nevada cinquefoil)
    glandulosa ssp. reflexa (Mountain sticky cinquefoil)
    gracilis var. fastigiata (Slender cinquefoil)
    wheeleri (Wheeler's cinquefoil)
Prenanthella exigua (Bright white)
Prosopis
    glandulosa var. torreyana (Honey mesquite)
    pubescens (Screwbean mesquite)
Prunella vulgaris var. lanceolata (Self-heal)
Prunus
    andersonii (Desert peach)
    emarginata (Bitter cherry)
    eremophila (Desert prunus)
    fasciculata var. fasciculata (Desert almond)
    fremontii (Desert apricot)
    ilicifolia ssp. ilicifolia (Hollyleaf cherry)
    ilicifolia ssp. ilicifolia fruit (Hollyleaf cherry)
    ilicifolia ssp. lyonii (Catalina cherry)
    virginiana var. demissus (Western chokecherry)
Psathyrotes
    annua (Fan leaf)
    ramosissima (Turtleback)
Pseudostellaria jamesiana (Sticky starwort)
Pseudotsuga macrocarpa (Bigcone spruce)
Psilocarphus
    brevissimus var. brevissimus (Dwarf woollyheads)
    tenellus var. tenellus (Slender woollyheads)
Psilostrophe cooperi (Paperflower)
Psorothamnus
    arborescens var. minutifolius (Little-leaved Mojave indigo bush)
    arborescens var. simplicifolius (California indigo bush)
    emoryi (Emory indigo bush)
    polydenius (Dotted indigo bush)
    schottii (Schott's indigo bush)
    spinosus (Smoke tree)
Pteridium aquilinum var. pubescens (Bracken fern)
Pteris vittata (Ladder brake)*
Pterospora andromedea (Pinedrops)
Pterostegia drymarioides (Threadstem)
Pulicaria paludosa (Pulicaria)*
Purshia
    mexicana var. stansburyana (Cliff rose)
    tridentata var. glandulosa (Antelope bush)
Pycnanthemum californicum (Mountain mint)
Pyrola
    asarifolia ssp. asarifolia (Bog wintergreen)
    picta (White-veined wintergreen)
Pyrrocoma uniflora var. gossypina (Bear Valley pyrrocoma)