Halimolobos jaegeri (Rock mustard)
Harpagonella palmeri (Palmer's grappling hook)
Hazardia
    brickellioides (Holly-leaved hazardia)
    detonsa (Island hazardia)
    orcuttii (Orcutt's hazardia)
    squarrosa var. grindelioides (Sawtooth goldenbush)
Hecastocleis shockleyi (Shockley's prickleleaf)
Hedera helix (English ivy)*
Hedeoma
    drummondii (Drummond's false pennyroyal)
    nana ssp. californica (California false pennyroyal)
Hedypnois cretica (Crete weed)*
Helenium
    bigelovii (Bigelow's sneezeweed)
    puberulum (Sneezeweed)
Helianthella californica var. nevadensis (Sierra helianthella)
Helianthemum scoparium, see Crocanthemum scoparium var. vulgare
Helianthus
    annuus (Common sunflower)
    californicus (California sunflower)
    gracilentus (Slender-leaved sunflower)
    niveus ssp. tephrodes (Algodones Dunes sunflower)
    petiolaris ssp. canescens (Dune sunflower)
Heliomeris multiflora var. nevadensis (Nevada goldeneye)
Heliotropium curassavicum var. oculatum (Wild heliotrope)
Helminthotheca echioides (Bristly ox-tongue)*
Hemizonella minima (Pygmy madia)
Hemizonia
    arida, see Deinandra arida
    clementina, see Deinandra clementina
    conjugens, see Deinandra conjugens
    fasciculata, see Deinandra fasciculata
    minthornii, see Deinandra minthornii
    mohavensis, see Deinandra mohavensis
    paniculata, see Deinandra paniculata
    parryi ssp. australis, see Centromadia parryi ssp. australis
    pungens, see Centromadia pungens
Heracleum lanatum, see Heracleum maximum
Heracleum maximum (Cow parsnip)
Herniaria hirsuta var. cinerea (Gray herniaria)*
Hesperevax acaulis var. ambusticola (Stemless evax)
Hesperocallis undulata (Desert lily)
Hesperochiron nanus (California hesperochiron)
Hesperocnide tenella (Black hair nettle)
Hesperocyparis
    stephensonii (Cuyamaca cypress)
    forbesii (Tecate cypress)
Hesperolinon micranthum (Small-flowered dwarf flax)
Hesperoyucca whipplei (Chaparral yucca)
Heterocodon rariflorum (Few-flowered heterocodon)
Heteromeles arbutifolia (Toyon)
Heterotheca
    grandiflora (Telegraph weed)
    sessiliflora ssp. echioides (Bristly golden-aster)
    sessiliflora ssp. fastigiata (Erect golden-aster)
    villosa var. scabra (Rough hairy golden-aster)
Heuchera
    abramsii (Abrams' alumroot)
    brevistaminea (Mount Laguna alumroot)
    caespitosa (Elegant alumroot)
    hirsutissima (Shaggy-haired alumroot)
    maxima (Island alumroot)
    parishii (San Bernardino Mountains alumroot)
    rubescens (Pink alumroot)
Hibiscus denudatus (Rock hibiscus)
Hieracium
    albiflorum (White-flowered hawkweed)
    argutum (Hawkweed)
    horridum (Shaggy hawkweed)
Hilaria rigida (Big galleta grass)
Hirschfeldia incana (Shortpod mustard)*
Hoffmannseggia
    glauca (Hog potato)
    microphylla (Hoffmannseggia)
Hoita
    macrostachya (Leather root)
    orbicularis (Roundleaf leather root)
Hollisteria lanata (False-spineflower)
Holmgrenanthe petrophila (Rock lady)
Holocarpha
    heermannii (Heermann's tarplant)
    virgata ssp. elongata (Graceful tarplant)
Holodiscus
    discolor var. discolor (Cream bush)
    discolor var. microphyllus (Small-leaved cream bush)
    microphyllus var. microphyllus, see Holodiscus discolor var. microphyllus
Hordeum
    brachyantherum ssp. californicum (Meadow barley)
    intercedens (Bobtail barley)
    murinum ssp. glaucum (Foxtail barley)*
    murinum ssp. leporinum (Hare barley)*
Horkelia
    clevelandii var. clevelandii (Cleveland's horkelia)
    cuneata var. cuneata (Wedge-leaf horkelia)
    hispidula (White Mountains horkelia)
    rydbergii (Rydberg's horkelia)
    truncata (Ramona horkelia)
    wilderae (Barton Flats horkelia)
Hornungia procumbens (Prostrate hutchinsia)
Horsfordia
    alata (Pink velvet-mallow)
    newberryi (Newberry's velvet-mallow)
Hosackia
    crassifolia var. crassifolia (Broad-leaved lotus)
    oblongifolia var. oblongifolia (Streambank lotus)
Hulsea
    algida (Alpine gold hulsea)
    californica (San Diego hulsea)
    heterochroma (Red-rayed hulsea)
    vestita ssp. callicarpha (Beautiful hulsea)
    vestita ssp. gabrielensis (San Gabriel Mountains hulsea)
    vestita ssp. parryi (Parry's hulsea)
    vestita ssp. pygmaea (Pygmy hulsea)
Hutchinsia procumbens, see Hornungia procumbens
Hydrocotyle verticillata (Water pennywort)
Hymenoclea salsola var. salsola, see Ambrosia salsola var. salsola
Hymenopappus
    filifolius var. lugens (Southern mountain cutleaf)
    filifolius var. megacephalus (Fineleaf hymenopappus)
Hymenothrix
    dissecta (Ragged-leaf bahia)
    wrightii (Wright's thimblehead)
Hymenoxys
    acaulis var. arizonica, see Tetraneuris acaulis var. arizonica
    cooperi (Cooper's goldflower)
Hypericum
    anagalloides (Tinker's penny)
    formosum var. scouleri, see Hypericum scouleri
    scouleri (Scouler's St. John's wort)
Hypochaeris
    glabra (Smooth cat's ear)*
    radicata (Rough cat's ear)*
Imperata brevifolia (California satintail)
Ipomopsis
    arizonica (Arizona ipomopsis)
    congesta ssp. montana (Ballhead ipomopsis)
    polycladon (Spreading gilia)
Iris
    douglasiana (Douglas's iris)
    hartwegii ssp. australis (Hartweg's iris )
    missouriensis (Western blue flag)
Isocoma
    acradenia var. bracteosa (Bracted alkali goldenbush)
    acradenia var. eremophila (Solitary-leaved alkali goldenbush)
    menziesii var. decumbens (Decumbent goldenbush)
    menziesii var. menziesii (Menzies' goldenbush)
    menziesii var. sedoides (Prostrate coast goldenbush)
    menziesii var. vernonioides (Coast goldenbush)
Isoetes bolanderi (Bolander's quillwort)
Isomeris arborea, see Peritoma arborea
Iva
    axillaris (Poverty weed)
    hayesiana (Poor man's timothy)
Ivesia
    argyrocoma var. argyrocoma (Silver-haired ivesia)
    callida (Tahquitz ivesia)
    santolinoides (Mousetail ivesia)
    saxosa (Rock ivesia)
Jaumea carnosa (Marsh jaumea)
Jepsonia parryi (Parry's jepsonia)
Johnstonella 
    angustifolia (Narrow-leaved cryptantha)
    costata (Ribbed cryptantha)
    holoptera (Winged cryptantha)
    racemosa (Bushy cryptantha)
Juglans californica (California black walnut)
Juncus
    acutus ssp. leopoldii (Spiny rush)
    balticus (Wire rush)
    bryoides (Moss rush)
    bufonius var. bufonius (Common toad rush)
    bufonius var. congestus (Clustered toad rush)
    bufonius var. occidentalis (Western toad rush)
    cooperi (Cooper's rush)
    covillei (Coville's rush)
    dubius (Mariposa rush)
    effusus ssp. pacificus (Pacific rush)
    macrandrus (Long-anthered rush) 
    macrophyllus (Long-leaved rush)
    mexicanus (Mexican rush)
    orthophyllus (Straightleaf rush)
    patens (Spreading rush)
    phaeocephalus var. paniculatus (Brown-headed rush)
    rugulosus (Wrinkled rush)
    saximontanus (Rocky Mountain rush)
    textilis (Basket rush)
    tiehmii (Tiehm's rush)
    xiphioides (Iris-leaved rush)
Juniperus
    californica (California juniper)
    grandis (Grand juniper)
    occidentalis var. australis, see Juniperus grandis
    osteosperma (Utah juniper)
Justicia californica (Chuparosa)
Kallstroemia
    californica (California kallstroemia)
    parviflora (Few-flowered caltrop)
Keckiella
    antirrhinoides var. antirrhinoides (Yellow penstemon)
    antirrhinoides var. microphylla (Little-leaved yellow penstemon)
    breviflora var. breviflora (Gaping penstemon)
    cordifolia (Heartleaf penstemon)
    rothrockii var. jacintensis (San Jacinto Mountains keckiella)
    rothrockii var. rothrockii (Rothrock's keckiella)
    ternata var. septentrionalis (Northern blue-stemmed keckiella)
    ternata var. ternata (Whorl-leaf penstemon)
Kelloggia galioides (Kelloggia)
Kickxia elatine (Sharpleaf cancerwort)*
Koeleria macrantha (Junegrass)
Koelreuteria elegans (Flamegold)*
Kopsiopsis strobilacea (California groundcone)
Krameria
    bicolor (White rhatany)
    erecta (Littleleaf rhatany)
    grayi, see Krameria bicolor
Krascheninnikovia lanata (Winterfat)
Lactuca
    saligna (Willowleaf lettuce)*
    serriola (Prickly lettuce)*
Laennecia coulteri (Coulter's horseweed)
Lagophylla ramosissima (Branched hareleaf)
Lamarckia aurea (Goldentop grass)*
Lamium amplexicaule (Henbit)*
Langloisia
    setosissima ssp. punctata (Lilac sunbonnets)
     setosissima ssp. setosissima (Bristly langloisia)
Laphamia megalocephala var. megalocephala (Nevada rock daisy)
Lappula
    redowskii var. cupulata (Stickseed)
    redowskii var. redowskii (Redowski's stickseed)
Larrea tridentata (Creosote bush)
Lastarriaea coriacea (Leather spineflower)
Lasthenia
    californica ssp. californica (California goldfields)
    coronaria (Royal goldfields)
    glaberrima (Rayless goldfields)
    gracilis (Common goldfields)
Lathyrus
    latifolius (Perennial sweet pea)*
    vestitus var. alefeldii (San Diego pea)
    vestitus var. vestitus (Canyon sweet pea)
Lavatera
    assurgentiflora, see Malva assurgentiflora
    cretica, see Malva pseudolavatera
Layia
    glandulosa (White tidy tips)
    hieracioides (Hawkweed layia)
    leucopappa (Comanche Point tidy tips)
    platyglossa (Yellow tidy tips)
Lemna
    minuta (Least duckweed)
    turionifera (Turion duckweed)
Lepechinia
    cardiophylla (Heartleaf pitcher sage)
    fragrans (Fragrant pitcher sage)
Lepidium
    didymum (Wart cress)*
    flavum (Yellow peppergrass)
    fremontii (Desert alyssum)
    lasiocarpum ssp. lasiocarpum (Hairy-podded peppergrass)
    latifolium (Perennial peppergrass)*
    montanum (Mountain peppergrass)
    nitidum (Shiny peppergrass)
    perfoliatum (Clasping pepperweed)*
    virginicum ssp. menziesii (Wild peppergrass)
    virginicum ssp. virginicum (Virginia peppergrass)
Lepidospartum
    latisquamum (Nevada broomsage)
    squamatum (Scalebroom)
Leptodactylon
    californicum, see Linanthus californicus
    jaegeri, see Linanthus jaegeri
    pungens, see Linanthus pungens
Leptosiphon
    bicolor (True babystars)
    chrysanthus ssp. chrysanthus (Golden gilia)
    chrysanthus ssp. decorus (Cream gilia)
    breviculus (Mojave linanthus)
    ciliatus (Whiskerbrush)
    floribundus ssp. floribundus (Many-flowered linanthus)
    floribundus ssp. glaber (Smooth many-flowered linanthus)
    floribundus ssp. hallii (Santa Rosa Mountains linanthus)
    grandiflorus (Large-flowered leptosiphon)
    lemmonii (Lemmon's linanthus)
    liniflorus (Flax-flowered linanthus)
    parviflorus (Common linanthus)
    pygmaeus ssp. continentalis (Pygmy linanthus)
Leptosyne
    bigelovii (Bigelow's coreopsis)
    californica (California tickseed)
    calliopsidea (Leafstem tickseed)
    gigantea (Giant coreopsis)
    maritima (Sea dahlia)
Lesquerella
    kingii ssp. bernardina, see Physaria kingii ssp. bernardina
    kingii ssp. kingii, see Physaria kingii ssp. kingii
    tenella, see Physaria tenella
Lessingia
    filaginifolia var. filaginifolia, see Corethrogyne filaginifolia
    glandulifera var. glandulifera (Sticky lessingia)
    glandulifera var. tomentosa (Warner Springs lessingia)
    lemmonii var. lemmonii, see Lessingia glandulifera var. glandulifera
    tenuis (Spring lessingia)
Leucanthemum vulgare (Ox-eye daisy)*
Leucosyris arida (Silver Lake daisy)
Lewisia
    nevadensis (Sierra lewisia)
    pygmaea (Dwarf lewisia)
    rediviva var. minor (Bitterroot)
Leymus
    cinereus, see Elymus cinereus
    condensatus, see Elymus condensatus
    pacificus, see Elymus pacificus
Lilaea scilloides, see Triglochin scilloides
Lilium
    humboldtii ssp. ocellatum (Humboldt lily)
    pardalinum (Leopard lily)
    parryi (Lemon lily)
Limnanthes
    alba ssp. parishii (Parish's meadowfoam)
    gracilis ssp. parishii, see Limnanthes alba ssp. parishii
Limonium
    californicum (Sea-lavender)
    sinuatum (Statice)*
Limosella acaulis (Stemless mudwort)
Linanthus
    aureus ssp. aureus, see Leptosiphon aureus ssp. aureus
    aureus ssp. decorus, see Leptosiphon aureus ssp. decorus
    bellus (Desert beauty)
    bigelovii ssp. johnsonii (Bigelow's linanthus)
    breviculus, see Leptosiphon breviculus
    californicus (Prickly phlox)
    campanulatus (Bell gilia)
    ciliatus, see Leptosiphon ciliatus
    concinnus (San Gabriel linanthus)
    demissus (Desert linanthus)
    dianthiflorus (Ground pink)
    dichotomus (Evening snow)
    filiformis (Threadstem gilia)
    floribundus ssp. glaber, see Leptosiphon floribundus ssp. glaber
    floribundus ssp. hallii, see Leptosiphon floribundus ssp. hallii
    floribundus ssp. floribundus, see Leptosiphon floribundus ssp. floribundus
    jaegeri (San Jacinto prickly phlox)
    jonesii (Jones' linanthus)
    killipii (Baldwin Lake linanthus)
    liniflorus, see Leptosiphon liniflorus
    maculatus ssp. maculatus (Little San Bernardino Mountains linanthus)
    parryae (Parry's linanthus)
    parviflorus, see Leptosiphon parviflorus
    pungens (Mountain prickly phlox)
    pygmaeus ssp. continentalis, see Leptosiphon pygmaeus ssp. continentalis
Linaria
    canadensis var. texana, see Nuttallanthus texanus
    dalmatica ssp. dalmatica (Dalmatian toadflax)*
Linum
    lewisii var. lewisii (Western blue flax)
    puberulum (Plains flax)
    usitatissimum (Common flax)*
Listera convallarioides (Twayblade)
Lithocarpus densiflorus var. densiflorus, see Notholithocarpus densiflorus var. densiflorus
Lithophragma
    affine (Woodland star)
    bolanderi (Bolander's woodland star)
    cymbalaria (Mission woodland star)
    heterophyllum (Hillside woodland star)
    parviflorum var. parviflorum (Smallflower woodland star)
    tenellum (Slender woodland star)
Lithospermum incisum (Narrowleaf stoneseed)
Lobelia
    cardinalis var. pseudosplendens (Scarlet lobelia)
    dunnii var. serrata (Dunn's lobelia)
Lobularia maritima (Sweet alyssum)*
Loeflingia squarrosa (Spreading pygmyleaf)
Loeseliastrum
    matthewsii (Desert calico)
    schottii (Schott's calico)
Logfia
    arizonica (Arizona herba impia)
    depressa (Dwarf filago)
    filaginoides (California filago)
    gallica (Narrow-leaf filago)*
Lolium
    multiflorum, see Festuca perennis
    temulentum, see Festuca temulenta
Lomatium
    californicum (California lomatium)
    caruifolium var. caruifolium (Caraway-leaved lomatium)
    dasycarpum ssp. dasycarpum (Woolly lomatium)
    lucidum (Shiny lomatium)
    mohavense (Desert parsley)
    multifidum (Fernleaf lomatium)
    nevadense var. nevadense (Nevada lomatium)
    nevadense var. parishii (Parish's lomatium)
    parryi (Parry's wild parsley)
    utriculatum (Hog fennel)
Lonicera
    hispidula (Hairy honeysuckle)
    interrupta (Chaparral honeysuckle)
    involucrata var. ledebourii (Twinberry)
    subspicata var. denudata (Southern honeysuckle)
Lotus  
    argophyllus var. adsurgens, see Acmispon argophyllus var. adsurgens
    argophyllus var. argophyllus, see Acmispon argophyllus var. argophyllus
    argophyllus var. niveus, see Acmispon argophyllus var. niveus
    argyraeus var. argyraeus, see Acmispon argyraeus var. argyraeus
    argyraeus var. multicaulis, see Acmispon argyraeus var. multicaulis
    corniculatus (Bird's-foot trefoil)*
    crassifolius var. crassifolius, see Hosackia crassifolia var. crassifolia
    dendroideus var. dendroideus, see Acmispon dendroideus var. dendroideus
    dendroideus var. veatchii, see Acmispon dendroideus var. veatchii
    denticulatus, see Acmispon denticulatus
    grandiflorus var. grandiflorus, see Acmispon grandiflorus var. grandiflorus
    hamatus, see Acmispon micranthus
    heermannii var. heermannii, see Acmispon heermannii var. heermannii
    humistratus, see Acmispon brachycarpus
    nevadensis var. davidsonii, see Acmispon nevadensis var. davidsonii
    nevadensis var. nevadensis, see Acmispon nevadensis var. nevadensis
    oblongifolius var. oblongifolius, see Hosackia oblongifolia var. oblongifolia
    procumbens var. procumbens, see Acmispon procumbens var. procumbens
    purshianus var. purshianus, see Acmispon americanus var. americanus
    rigidus, see Acmispon rigidus
    salsuginosus var. brevivexillus, see Acmispon maritimus var. brevivexillus
    salsuginosus var. salsuginosus, see Acmispon maritimus var. maritimus
    scoparius var. brevialatus, see Acmispon glaber var. brevialatus
    scoparius var. scoparius, see Acmispon glaber var. glaber
    strigosus, see Acmispon strigosus
    wrangelianus, see Acmispon wrangelianus
Ludwigia peploides (Yellow water-weed)*
Lunaria annua (Annual moonwort)*
Lupinus
    adsurgens (Drew's silky lupine)
    albifrons var. albifrons (Silver bush lupine)
    albifrons var. douglasii (Douglas's silver bush lupine)
    andersonii (Anderson's lupine)
    arboreus (Coast bush lupine)
    arbustus (Long-spurred lupine)
    argenteus var. argenteus (Silvery lupine)
    arizonicus (Arizona lupine)
    benthamii (Bentham lupine)
    bicolor (Miniature lupine)
    brevicaulis (Sand lupine)
    breweri var. breweri (Brewer's lupine)
    breweri var. bryoides (Matted lupine)
    breweri var. grandiflorus (Showy Brewer's lupine)
    chamissonis (Dune lupine)
    concinnus (Bajada lupine)
    elatus (Silky lupine)
    excubitus var. austromontanus (Southern mountain lupine)
    excubitus var. excubitus (Grape soda lupine)
    excubitus var. hallii (Hall's bush lupine)
    excubitus var. johnstonii (Johnston's bush lupine)
    flavoculatus (Yellow-eyed lupine)
    formosus var. formosus (Summer lupine)
    hirsutissimus (Stinging lupine)
    hyacinthinus (San Jacinto lupine)
    latifolius var. latifolius (Broad-leaved lupine)
    latifolius var. parishii (Parish's lupine)
    lepidus var. confertus (Clustered tidy lupine)
    longifolius (Bush lupine)
    luteolus (Pale yellow lupine)
    luteus (European yellow lupine)*
    microcarpus var. densiflorus (White-whorled lupine)
    microcarpus var. horizontalis (Chick lupine)
    microcarpus var. microcarpus (Valley lupine)
    nanus (Sky lupine)
    odoratus (Royal desert lupine)
    peirsonii (Peirson's lupine)
    polyphyllus var. burkei (Meadow lupine)
    shockleyi (Desert lupine)
    sparsiflorus (Coulter's lupine)
    stiversii (Harlequin lupine)
    succulentus (Arroyo lupine)
    truncatus (Collar lupine)
Luzula comosa (Hairy wood rush)
Lycium
    andersonii (Anderson's thornbush)
    brevipes var. brevipes (Desert thorn)
    californicum (California boxthorn)
    cooperi (Cooper's boxthorn)
    fremontii (Fremont's thornbush)
    pallidum var. oligospermum (Rabbit thorn)
Lycopersicon esculentum (Common tomato)*
Lyonothamnus
    floribundus ssp. aspleniifolius (Fernleaf ironwood)
    floribundus ssp. floribundus (Santa Catalina Island ironwood)
Lyrocarpa coulteri (Coulter's lyrepod)
Lysimachia
    arvensis (Scarlet pimpernel)*
    minima (Common chaffweed)
Lythrum
    californicum (California loosestrife)
    hyssopifolia (Hyssop loosestrife)