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Romneya coulteri
just from the flower thats what it looks like to me. im not sure if you remember or not if the stem had small spines or the type of leaf arrangement. here are some websites to check out.
http://www.calfloranursery.com/plants/ro...
http://en.wikipedia.org/wiki/Romneya_cou...
our state flower is the California Poppy (Eschscholzia californica)
the matillija poppy has a bunch of spines all along its stem and can grow fairly tall where i think the california poppy is a groundcover and relatively low to the ground.