This is Leafy Spurge or Euphorbia esula. We call this plant gopher weed, but apparently, gopher weed is another kind of plant. We have been noticing this plant growing around the neighborhood and then up into the nearby canyons. I assumed that it had been planted on purpose as an ornamental. But it turns out that it is an invasive species introduced in the 1800s from Eurasia. There are several closely related species of this particular plant. We had this growing in our backyard in Mesa, so we noticed it as a familiar plant.