Theodore Payne Foundation

With its friendly, vaguely gemütlich, hippie vibe, this is the premier resource for California native plant gardening in Los Angeles. Great nursery with knowledgeable staff; innovative programs and workshops. There is a good native plant “database” on their website with basic information. The Foundation’s annual native garden tour each April is highly recommended.
theodorepayne.org

California Botanical Garden

(Formerly the Rancho Santa Ana Botanical Garden.) Kind of the Huntington Library of the California native plant world, with a more institutional, research-y feel than Theodore Payne. Good nursery offering the usual species along with a few more unusual plants, making it worth the extra shlep out to Pomona. The huge gardens are also worth a stroll.
calbg.org

Las Pilitas Nurseries

We admire Bert Wilson’s iconoclasm. The late founder of Las Pilitas, Wilson has left us with a very helpful “Complete List of California Native Plants” on his website suffused with his take-no-prisoners sense of humor. (In a particularly memorable passage, he likens building a berm around a new planting hole to “drowning a poodle in a bathtub.”) Two nursery locations, neither of which are in the L.A. area.
laspilitas.com

Calscape

The sine qua non resource for local gardening with California native plants. Beyond invaluable. You can enter your zip code and get a list of all plants that call your particular location home. Plenty of pointy-headed terms like “calyx” and “sepals” for the botanical types but lots of practical gardening information too (like exposure, soil type, water requirements, companion plants, etc.) Links to nurseries that sell the plants you’re researching.
calscape.org

Tree of Life Nursery

Tree of Life owner Mike Evans’ series of “Monthly Garden Tips” (under “Resources”) are essential reading. Other helpful features on the ToLN’s website include planning tools and individual plant portraits.
californianativeplants.com

Audubon Center at Debs Park

Debs Park (90031) comprises one of the largest remnants of the once locally widespread Southern Walnut Savannah habitat. This website is included mainly for its useful list of native plants found the park, which serves as a good reference for California native plant gardeners in the Northeastern portion of Los Angeles. You can also learn more about our local native birds and their specific eating habits, knowledge that can be put to use when selecting plants in support of wildlife in your area. It’s a little hard to find the list on the site: try typing “vascular plants” into the search field.
debspark.audubon.org/visit/plants-debs-park

Grown in LA

Grown in LA’s mission is to marshal the energy and resources of the city’s gardening community to help propagate enough truly local plant material for “urban greening projects,” including along the Los Angeles River.
growninla.org

California Invasive Plant Council

Friends don’t let friends plant invasive species. Cal-IPC works to protect our remaining wild lands from damaging invasive plants “through science, education and policy.” Their website has a handy list cataloging the many noxious exotics plaguing California.
cal-ipc.org

Biodiversity Heritage Library

Visit this website to obtain a copy of “Vascular Flora of the Verdugo Mountains and San Rafael Hills, Los Angeles California,” which essentially functions as the LANPS garden’s Bible. With a few notable exceptions, if a plant isn’t identified in this study, you won’t find it in the garden, which is a stone’s throw from the San Rafael Hills.

ResearchGate

Yes, we know, unfortunate name for a website, but you can get a copy of the important “Flora of Griffith Park, Los Angeles” here. This study includes interesting comparisons with the flora of the Santa Monica Mountains west of Griffith Park as well as that of the nearby Verdugo Mountains and San Rafael Hills.

Flora of Los Angeles and Vicinity

Like a faded, vintage photograph, this study, written in 1911 by Le Roy Abrams, offers a glimpse of the area’s native plants over a century ago, many of which were subsequently bulldozed out of existence. A little hard to use, as many of the plants noted have had their scientific names changed since the early 1900s, but a little sleuthing on Calflora will usually unmask their identities. With its vague but tantalizing references to the location of many, now vanished plants, “Flora” can act as a kind of “map” for localized restoration gardening.
Free download on Google Books.