Quercus
    acutidens (Torrey's scrub oak)
    agrifolia var. agrifolia (Coast live oak)
    agrifolia var. oxyadenia (Southern coast live oak)
    berberidifolia (Inland scrub oak)
    chrysolepis (Canyon live oak)
    cornelius-mulleri (Muller's oak)
    douglasii (Blue oak)
    dumosa (Coastal scrub oak)
    durata var. gabrielensis (San Gabriel Mountains leather oak)
    engelmannii (Engelmann oak)
    garryana var. breweri (Brewer's oak)
    john-tuckeri (Tucker's oak)
    kelloggii (California black oak)
    lobata (Valley oak)
    Xmorehus (Oracle oak)
    pacifica (Channel Island scrub oak)
    palmeri (Palmer's oak)
    tomentella (Island oak)
    turbinella (Shrub live oak)
    wislizeni var. frutescens (Shrubby interior live oak)
    wislizeni var. wislizeni (Interior live oak)
Rafinesquia
    californica (California chicory)
    neomexicana (Desert chicory)
Raillardella argentea (Silky raillardella)
Ranunculus
    alismifolius var. alismellus (Small water-plantain buttercup)
    aquatilis var. diffusus (Water buttercup)
    californicus var. californicus (California buttercup)
    cymbalaria (Alkali buttercup)
    eschscholtzii var. oxynotus (Eschscholtz's buttercup)
    hebecarpus (Delicate buttercup)
    occidentalis (Western buttercup)
    sceleratus (Cursed crowfoot)
    testiculatus (Tubercled crowfoot)*
Raphanus sativus (Wild radish)*
Rhamnus
    californica ssp. californica, see Frangula californica ssp. californica
    crocea (Littleleaf redberry)
    ilicifolia (Hollyleaf redberry)
    pirifolia (Island redberry)
    tomentella ssp. cuspidata, see Frangula californica ssp. cuspidata
    tomentella ssp. tomentella, see Frangula californica ssp. tomentella
Rhaponticum repens (Russian knapweed)*
Rhinotropis
    acanthoclada (Desert polygala)
    cornuta var. fishiae (Fish's milkwort)
Rhododendron occidentale (Western azalea)
Rhus
    aromatica (Skunkbrush)
    integrifolia (Lemonade berry)
    ovata (Sugar bush)
    trilobata, see Rhus aromatica
Ribes
    amarum (Bitter gooseberry)
    aureum var. gracillimum (Golden currant)
    californicum var. hesperium (Hillside gooseberry)
    cereum var. cereum (Wax currant)
    indecorum (White-flowering currant)
    malvaceum var. viridifolium (Chaparral currant)
    montigenum (Mountain gooseberry)
    nevadense (Mountain pink currant)
    quercetorum (Oak gooseberry)
    roezlii var. roezlii (Sierra gooseberry)
    speciosum (Fuchsia-flowered gooseberry)
    thacherianum (Santa Cruz Island gooseberry)
    velutinum (Desert gooseberry)
    viburnifolium (Evergreen currant)
Ricinus communis (Castor bean)*
Rigiopappus leptocladus (Wireweed)
Robinia
    neomexicana (Desert locust)
    pseudoacacia (Black locust)*
Romneya
    coulteri (Matilija poppy)
    trichocalyx (Bristly matilija poppy)
Rorippa
    curvipes (Bluntleaf yellowcress)
    curvisiliqua (Curvepod yellowcress)
    nasturtium-aquaticum, see Nasturtium officinale
    palustris ssp. palustris (Western bog yellowcress)
    sphaerocarpa (Round-fruit yellowcress)
Rosa
    californica (Wild rose)
    gymnocarpa var. gymnocarpa (Wood rose)
    minutifolia (Small-leaved rose)
    woodsii ssp. gratissima (Interior rose)
Rubus
    armeniacus (Himalaya-berry)*
    discolor, see Rubus armeniacus
    leucodermis (Western raspberry)
    parviflorus (Thimbleberry)
    ursinus (California blackberry)
Rumex
    acetosella (Sheep sorrel)*
    californicus (Willow-leaved dock)
    conglomeratus (Whorled dock)*
    crispus (Curly dock)*
    hymenosepalus (Wild-rhubarb)
    pulcher (Fiddle dock)*
    salicifolius var. denticulatus see Rumex californicus
Rupertia
    physodes (California tea)
    rigida (Parish's psoralea)
Sabulina
    douglasii (Douglas's stitchwort)
    nuttallii var. gracilis (Brittle sandwort)
    pusilla (Annual sandwort)
Sagina
    apetala (Dwarf pearlwort)*
    decumbens ssp. occidentalis (Western pearlwort)
    saginoides (Alpine pearlwort)
Salazaria mexicana, see Scutellaria mexicana
Salicornia
    pacifica (Pacific pickleweed)
    subterminalis, see Arthrocnemum subterminale
    virginica, see Salicornia pacifica
Salix
    exigua var. exigua (Sandbar willow)
    exigua var. hindsiana (Hinds' narrowleaf willow)
    geyeriana (Geyer's willow)
    gooddingii (Goodding's black willow)
    laevigata (Red willow)
    lasiolepis (Arroyo willow)
    lemmonii (Lemmon's willow)
    lasiandra var. lasiandra (Shining willow)
    lucida ssp. lasiandra, see Salix lasiandra var. lasiandra
    scouleriana (Scouler's willow)
Salsola
    paulsenii (Barbwire Russian thistle)*
    tragus (Russian thistle)*
Saltugilia
    australis (Southern woodland-gilia)
    caruifolia (Caraway-leaved gilia)
    latimeri (Latimer's woodland-gilia)
    splendens ssp. grantii (Grant's woodland-gilia)
    splendens ssp. splendens (Splendid woodland-gilia)
Salvia
    apiana (White sage)
    brandegeei (Brandegee's sage)
    carduacea (Thistle sage)
    clevelandii (Cleveland's sage)
    columbariae (Chia)
    dorrii var. dorrii (Blue sage)
    dorrii var. pilosa (Hairy blue sage)
    eremostachya (Desert sage)
    funerea (Death Valley sage)
    leucophylla (Purple sage)
    mellifera (Black sage)
    munzii (Munz's sage)
    pachyphylla (Rose sage)
    sonomensis (Creeping sage)
    spathacea (Crimson pitcher sage)
    vaseyi (Vasey's sage)
Sambucus
    mexicana (Mexican elderberry)
    nigra ssp. caerulea, see Sambucus nigra ssp. caerulea
    racemosa var. racemosa (Red elderberry)
Samolus parviflorus (Water-pimpernel)
Sanicula
    arguta (Snakeroot)
    bipinnata (Poison sanicle)
    bipinnatifida (Purple sanicle)
    crassicaulis (Pacific sanicle)
    graveolens (Sierra sanicle)
    tuberosa (Turkey pea)
Sanvitalia abertii (Abert's sanvitalia)
Sarcodes sanguinea (Snow plant)
Sarcostemma
    cynanchoides ssp. hartwegii, see Funastrum cynanchoides var. hartwegii
    hirtellum, see Funastrum hirtellum
Satureja
    chandleri, see Clinopodium chandleri
    douglasii, see Clinopodium douglasii
    mimuloides, see Clinopodium mimuloides
Saxifraga californica, see Micranthes californica
Schinus
    molle (Peruvian pepper)*
    terebinthifolius (Brazilian pepper)*
Schismus barbatus (Common Mediterranean grass)*
Schizotechium jamesianum (Sticky starwort)
Schkuhria multiflora var. multiflora, see Bahia neomexicana
Schoenoplectus
    americanus (American bulrush)
    californicus (California bulrush)
Schoenus nigricans (Black sedge)
Scirpus
    americanus, see Schoenoplectus americanus
    californicus, see Schoenoplectus californicus
    maritimus, see Bolboschoenus maritimus
    microcarpus (Panicled bulrush)
Sclerocactus
    johnsonii (Johnson's bee-hive cactus)
    polyancistrus (Mojave fishhook cactus)
Scopulophila rixfordii (Rixford's rockwort)
Scrophularia
    californica (California figwort)
    desertorum (Desert figwort)
Scutellaria
    bolanderi ssp. austromontana (Southern skullcap)
    mexicana (Paperbag bush)
    siphocampyloides (Grayleaf skullcap)
    tuberosa (Common skullcap)
Secale cereale (Cereal rye)*
Sedum
    niveum (Davidson's stonecrop)
    spathulifolium (Broad-leaf stonecrop)
Selaginella
    asprella (Bluish spike-moss)
    bigelovii (Bigelow's spike-moss)
    cinerascens (Ashy spike-moss)
    eremophila (Desert spike-moss)
    hansenii (Hansen's spike-moss)
    watsonii (Watson's spike-moss)
Senecio
    bernardinus, see Packera bernardina
    californicus (California groundsel)
    flaccidus var. douglasii (Bush senecio)
    flaccidus var. monoensis (Mono groundsel)
    fremontii var. occidentalis (Dwarf mountain ragwort)
    ionophyllus, see Packera ionophylla
    mohavensis (Mojave ragwort)
    multilobatus, see Packera multilobata
    spartioides (Broom groundsel)
    triangularis (Arrowleaf butterweed)
    vulgaris (Common groundsel)*
Senegalia greggii (Catclaw acacia)
Senna
    armata (Desert senna)
    artemisioides (Silver senna)*
    covesii (Coues' senna)
Sequoiadendron giganteum (Giant sequoia)*
Shepherdia argentea (Buffalo-berry)
Sherardia arvensis (Field madder)*
Sibbaldia procumbens (Sibbaldia)
Sidalcea
    malviflora ssp. dolosa (Dwarf checkerbloom)
    malviflora ssp. malviflora (Island checkerbloom)
    malviflora ssp. sparsifolia, see Sidalcea sparsifolia
    pedata (Big Bear checkerbloom)
    sparsifolia (Few-leaved checkerbloom)
Sidotheca
    caryophylloides (Chickweed oxytheca)
    emarginata (Whitemargin oxytheca)
    trilobata (Threelobe oxytheca)
Silene
    antirrhina (Sleepy catchfly)
    bernardina (Palmer's catchfly)
    gallica (Windmill pink)*
    krantzii (Krantz's catchfly)
    laciniata ssp. californica (California pink)
    laciniata ssp. laciniata (Mexican pink)
    laciniata ssp. major, see Silene laciniata ssp. laciniata
    lemmonii (Lemmon's catchfly)
    menziesii (Menzies' campion)
    multinervia (Many-nerved catchfly)
    parishii (Parish's catchfly)
    verecunda (White catchfly)
Silybum marianum (Milk thistle)*
Simmondsia chinensis (Jojoba)
Simpsonanthus jonesii (Mojave popcorn flower)
Sisymbrium
    altissimum (Tumble mustard)*
    irio (London rocket)*
    officinale (Hedge mustard)*
    orientale (Oriental mustard)*
Sisyrinchium
    bellum (Blue-eyed grass)
    elmeri (Yellow-eyed grass)
    funereum (Death Valley blue-eyed grass)
    halophilum (Nevada blue-eyed grass)
    idahoense var. occidentale (Idaho blue-eyed grass)
Smilacina
    racemosa, see Maianthemum racemosum
    stellata, see Maianthemum stellatum
Solanum
    americanum (Small-flowered nightshade)
    douglasii (White nightshade)
    elaeagnifolium (Silverleaf horse-nettle)*
    parishii (Parish's nightshade)
    umbelliferum (Blue-witch nightshade)
    xanti (Purple nightshade)
Solidago
    californica, see Solidago velutina ssp. californica
    confinis (Southern goldenrod)
    velutina ssp. californica (California goldenrod)
Sonchus
    asper ssp. asper (Prickly sow-thistle)*
    oleraceus (Sow-thistle)*
Spartina foliosa (Cord grass)
Spartium junceum (Spanish broom)*
Spergula arvensis (Corn spurrey)*
Spergularia
    bocconi (Boccone's sand-spurrey)*
    macrotheca var. macrotheca (Sticky sand-spurrey)
    marina (Salt marsh sand-spurrey)
    rubra (Red sand-spurrey)*
    villosa (Hairy sand-spurrey)*
Sphaeralcea
    ambigua var. ambigua (Apricot mallow)
    ambigua var. rosacea (Rosy apricot mallow)
    ambigua var. rugosa (Desert globemallow)
    angustifolia (Narrow-leaved globemallow)
    coulteri (Coulter's globemallow)
    rusbyi var. eremicola (Rusby's desert mallow)
Sphaeromeria cana (Gray chickensage)
Sphenosciadium capitellatum, see Angelica capitellata
Sporobolus airoides (Alkali sacaton)
Stachys
    albens (White hedge-nettle)
    bullata (California hedge-nettle)
    rigida (Rigid hedge-nettle)
Stanleya
    elata (Panamint prince's plume)
    pinnata var. pinnata (Prince's plume)
Stebbinsoseris heterocarpa (Brown microseris)
Stellaria
    calycantha (Northern starwort)
    longipes ssp. longipes (Meadow starwort)
    media (Chickweed)*
    nitens (Shining chickweed)
Stephanomeria
    cichoriacea (Chicory-leaved stephanomeria)
    diegensis (San Diego wreathplant)
    exigua ssp. coronaria (Whiteplume wirelettuce)
    exigua ssp. deanei (Deane's wirelettuce)
    exigua ssp. exigua (Small wirelettuce)
    parryi (Parry's wirelettuce)
    pauciflora (Desert straw)
    spinosa, see Pleiacanthus spinosus
    tenuifolia (Narrow-leaved stephanomeria)
    virgata ssp. pleurocarpa (Wand wirelettuce)
    virgata ssp. virgata (Twiggy wreathplant)
Stillingia
    linearifolia (Narrow-leaved stillingia)
    spinulosa (Broad-leaved stillingia)
Stipa
    arida (Mormon needlegrass)
    cernua (Nodding needlegrass)
    comata var. comata (Needle-and-thread)
    coronata (Giant stipa)
    diegoensis (San Diego County needlegrass)
    hymenoides (Sand ricegrass)
    lepida (Foothill needlegrass)
    miliacea var. miliacea (Smilo grass)*
    occidentalis var. occidentalis (Western needlegrass)
    parishii var. parishii (Parish's needlegrass)
    pulchra (Purple needlegrass)
    speciosa (Desert needlegrass)
Streptanthella longirostris (Longbeak streptanthella)
Streptanthus
    bernardinus (Laguna Mountains jewelflower)
    campestris (Southern jewelflower)
    cordatus var. cordatus (Heartleaf jewelflower)
Strigosella africana (African mustard)*
Stutzia covillei (Arrowscale)
Stylocline
    gnaphaloides (Everlasting neststraw)
    micropoides (Woollyhead neststraw)
Stylomecon heterophylla, see Papaver heterophyllum
Styrax
    officinalis var. redivivus, see Styrax redivivus
     redivivus (Snowdrop bush)
Suaeda
    esteroa (Estuary sea-blite)
    moquinii, see Suaeda nigra
    nigra (Rush seepweed)
    taxifolia (Woolly sea-blite)
Swallenia alexandrae (Eureka Valley dune grass)
Swertia
    neglecta, see Frasera neglecta
    parryi, see Frasera parryi
    puberulenta, see Frasera puberulenta
Symphoricarpos
    albus var. laevigatus (White snowberry)
    longiflorus (Fragrant snowberry)
    mollis (Creeping snowberry)
    rotundifolius var. parishii (Mountain snowberry)
Symphyotrichum
    ascendens (Long-leaved aster)
    chilense (Chile aster)
    defoliatum (San Bernardino aster)
    greatae (Greata's aster)
    lanceolatus ssp. hesperium (Western lanceleaf aster)
    spathulatum var. spathulatum (Western mountain aster)
    subulatum var. parviflorum (Slim aster)
Syntrichopappus fremontii (Fremont's syntrichopappus)