Jepson Manual - for keying plants, in depth plant ID info, (almost) every plant in California

Ideal for: flowering plants that are naturally growing outside (native or weeds) & not intentionally planted as landscape.

To key a plant you typically need:

1) the whole plant in front of you

2) a flower / multiple flowers that can be dissected to view the inside

3) a good amount of patience

I recommend starting with a field guide or Seek/iNaturalist & thoroughly confirming with Jepson. The Jepson manual is quite bulky & expensive (~$150 second hand) but is fully available online. Jepson is regarded as the gold-standard for keying in California, and can take quite a while to become comfortable using. It is also important to keep in mind that the manual was written by multiple authors, and some subjective words used to ID plants can vary between sections of the book.

Calphotos - photo database for plants, animals, & other orgs

Seek/iNaturalist

National Geographic app for organism ID using your phone camera, community science

Ideal for: Plants or animals native to area or very common (for example a pigeon). Plants usually need to be mature with flowers present to trigger accurate ID. Can be a good starting point for narrowing down a search.