California Golden Poppy, Eschscholzia californica
Every garden in California should have these, yet so many gardens I visit are missing this wonderful plant. Throw out some seeds in the autumn, just as the winter rains begin, stomp the seeds down a bit with your foot, or toss a little bit of compost on top, and that is the sole extent of effort required until it's time to pull them up in mid-to-late May. After that they'll likely come back on their own.
Though this plant is often praised, and with good reason, I'll call it one of my Unsung Garden Heroes until I see it everywhere. Surprisingly, this plant is actually a perennial, looking ratty and dried up for the summer, then reviving somewhat with the following winter's rain. However, it's best the first year, and since it reseeds so well, I just pull it out come summer. So plan to toss some seeds this fall, okay? You won't be sorry.