Something in the Asteraceae. Maybe Conyza canadensis - that's a very common one which grows in that sort of place. Any chance of a sight of some leaves? Or the whole plant?

This post was edited by floral_uk on Mon, Oct 13, 14 at 4:06

It is Conyza bonariensis (aka Erigeron bonariensis), a member of the composite family, not native to California.

Here is a link that might be useful: Conyza bonariensis CalPhotos

Agree with catspa's Conyza bonariensis ID. That plant is sometimes called Asthmaweed, and is found in many California counties.