Canyon ragweed (Ambrosia ambrosioides), at Rio Salado Habitat Restoration Area, Phoenix, Arizona.

The lightest touch released clouds of irritating pollen from these flowers. I had to give my camera a good cleaning after taking this photo. 

Coin-toss cholla. 

This is new growth on either staghorn or buckhorn cholla (Cylindropuntia versicolor or C. acanthocarpa, respectively). Both species occur locally, so I see one or the other frequently, but they look so similar they defy my small

diagnostic skills. Did I mention that they also readily hybridize where their ranges overlap? Of course they do, making it nearly impossible for tyros like me to identify them! 

At Pima Wash, South Mountain Preserve, in Phoenix, Arizona.