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 (Desert scrub oak)
    kelloggii (California black 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. capillaceus (Water buttercup)
    californicus var. californicus (California buttercup)
    cymbalaria var. saximontanus (Rocky Mountain buttercup)
    eschscholtzii var. oxynotus (Eschscholtz's buttercup)
    hebecarpus (Delicate buttercup)
    occidentalis (Western buttercup)
    sceleratus (Cursed crowfoot)
    testiculatus (Tubercled crowfoot)*
Raphanus sativus (Wild radish)*
Rhamnus
    crocea (Littleleaf redberry)
    ilicifolia (Hollyleaf redberry)
    pirifolia (Island redberry)
Rhododendron occidentale (Western azalea)
Rhus
    aromatica (Skunkbush)
    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)
    lasianthum (Alpine gooseberry)
    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
    curvisiliqua (Curvepod yellowcress)
    nasturtium-aquaticum, see Nasturtium officinale
    officinale (Watercress)*
    palustris var. occidentalis (Western bog yellow-cress)
Rosa
    californica (Wild rose)
    gymnocarpa (Wood rose)
    minutifolia (Small-leaved rose)
    woodsii var. ultramontana (Interior rose)
Rubus
    discolor (Himalaya-berry)*
    leucodermis (Western raspberry)
    parviflorus (Thimbleberry)
    ursinus (California blackberry)
    ursinus fruit (California blackberry)
Rumex
    acetosella (Sheep sorrel)*
    conglomeratus (Whorled dock)*
    crispus (Curly dock)*
    hymenosepalus (Wild rhubarb)
    pulcher (Fiddle dock)*
    salicifolius var. denticulatus (Willow-leaved dock)
Rupertia
    physodes (California tea)
    rigida (Parish's psoralea)
Sagina
    apetala (Dwarf pearlwort)*
    decumbens ssp. occidentalis (Western pearlwort)
    saginoides (Alpine pearlwort)
Salazaria mexicana, see Scutellaria mexicana
Salicornia
    subterminalis (Glasswort)
    virginica (Virginia pickleweed)
Salix
    exigua (Sandbar willow)
    gooddingii (Goodding's black willow)
    laevigata (Red willow)
    lasiolepis (Arroyo willow)
    lemmonii (Lemmon's willow)
    lucida ssp. lasiandra (Shining willow)
    scouleriana (Scouler's willow)
Salsola
    paulsenii (Barbwire Russian thistle)*
    tragus (Russian thistle)*
Saltugilia
    australis (Southern gilia)
    caruifolia (Caraway-leaved gilia)
    splendens ssp. grantii (Grant's gilia)
    splendens ssp. splendens (Splendid gilia)
Salvia
    apiana (White sage)
    brandegeei (Brandegee's sage)
    carduacea (Thistle sage)
    clevelandii (Cleveland sage)
    columbariae (Chia)
    dorrii var. dorrii (Blue sage)
    dorrii var. pilosa (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, see Sambucus nigra ssp. caerulea
    nigra ssp. caerulea (Mexican elderberry)
    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)
Sarcobatus vermiculatus (Greasewood)
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 mimuloides
    mimuloides, see Clinopodium mimuloides
Saxifraga californica (California saxifrage)
Schinus
    molle (Peruvian pepper)*
    molle fruit (Peruvian pepper)*
    terebinthifolius (Brazilian pepper)*
Schismus barbatus (Common Mediterranean grass)*
Schkuhria multiflora var. multiflora (Many-flowered bahia)
Schoenoplectus
    americanus (American bulrush)
    californicus (California bulrush)
Scirpus
    americanus, see Schoenoplectus americanus
    californicus, see Schoenoplectus californicus
    maritimus, see Bolboschoenus maritimus
    microcarpus (Panicled bulrush)
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)
    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)
    gallica (Windmill 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 (male flowers) (Jojoba)
    chinensis (female flowers) (Jojoba)
Sisymbrium
    altissimum (Tumble mustard)*
    irio (London rocket)*
    officinale (Hedge mustard)*
    orientale (Oriental mustard)*
Sisyrinchium
    bellum (Blue-eyed grass)
    elmeri (Yellow-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)
    xanti fruit (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 ssp. macrotheca (Sticky sand spurrey)
    marina (Salt marsh sand spurrey)
    villosa (Hairy sand spurrey)*
Sphaeralcea
    ambigua var. ambigua (Apricot mallow)
    ambigua var. rosacea (Rosy apricot mallow)
    ambigua var. rugosa (Desert globemallow)
    coulteri (Coulter's globemallow)
    rusbyi var. eremicola (Rusby's desert mallow)
Sphenosciadium capitellatum (Ranger's buttons)
Sporobolus airoides (Alkali sacaton)
Stachys
    ajugoides var. rigida (Rigid hedge nettle)
    albens (White hedge nettle)
    bullata (California hedge nettle)
Stanleya
    elata (Panamint prince's plume)
    pinnata var. inyoensis (Inyo prince's plume)
    pinnata var. pinnata (Prince's plume)
Stebbinsoseris heterocarpa (Brown microseris)
Stellaria
    calycantha (Northern starwort)
    longipes ssp. longipes (Meadow starwort)
    media (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)
    pauciflora var. pauciflora (Desert straw)
    spinosa, see Pleiacanthus spinosus
    virgata ssp. pleurocarpa (Wand wirelettuce)
    virgata ssp. virgata (Twiggy wreathplant)
Stillingia
    linearifolia (Narrow-leaved stillingia)
    spinulosa (Broad-leaved stillingia)
Stipa
    arida (Mormon needlegrass)
    coronata (Giant stipa)
    diegoensis (San Diego County needlegrass)
    hymenoides (Indian ricegrass)
    occidentalis var. occidentalis (Western needlegrass)
    parishii (Parish's needlegrass)
    speciosa (Desert needlegrass)
Streptanthella longirostris (Longbeak streptanthella)
Streptanthus
    bernardinus (Laguna Mountains jewelflower)
    campestris (Southern jewelflower)
    cordatus (Heartleaf jewelflower)
Stylocline
    gnaphaloides (Everlasting neststraw)
    micropoides (Woollyhead neststraw)
Stylomecon heterophylla (Wind poppy)
Styrax
    officinalis var. redivivus, see Styrax redivivus
     redivivus (Snowdrop bush)
Suaeda
    esteroa (Estuary sea-blite)
    moquinii (Rush seepweed)
    taxifolia (Woolly sea-blite)
Swallenia alexandrae (Eureka Valley dune grass)
Swertia
    neglecta (Pine green-gentian)
    parryi (Parry's green-gentian)
Symphoricarpos
    albus var. laevigatus (White snowberry)
    longiflorus (Desert snowberry)
    mollis (Creeping snowberry)
    mollis fruit (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)