Prickly poppy (Argemone munita) in upper Cheeseboro Canyon.

From today's run of the Cheeseboro Canyon keyhole loop, starting from the Victory trailhead of Upper Las Virgenes Canyon Open Space Preserve (formerly Ahmanson Ranch).

Here's a Google Earth image of a GPS trace of the loop, and links to trail maps for Upper Las Virgenes Open Space Preserve and Cheeseboro/Palo Comado Canyons.