I can't id this weed in So. Ca.
Flowers 1/2 ", Leaves 1 1/2" x 1/2", prostrate, branching

I think it is one of the Dwarf Dandelions, Krigia ssp. of which there are a few;
http://www.wildflower.org/plan...

https://www.google.cz/search?q...
http://commons.wikimedia.org/w...

with Stefan. The type of Hawkweed we have growing wild here is blooming now...pretty, but very rampant.

I've been checking out both suggestions...also Agoseris aspargioides. They have lots of similarities, but all are supposed to have milky sap, which mine doesn't, and I haven't found a seed head that matches either. Still looking.

Well, it's not in my yard...yet, so I'll settle for Agoseris aspargoides, since that one is listed on the west coast, while the others aren't listed west of Texas. They all seem to be the same plant when looking at the photos, and reading the description. Dwarf dandelion is close enough for a weed, so I'll mark this solved. Thanks for the leads!
Solved