Argiope trifasciata. Molino Basin, Santa Catalina Mountains, Pima County, Arizona. USA. 27 October 2007.
Flowering desert broom, Baccharis sarothroides, attracts a lot of foraging insects, so it is a great place for spiders to build their nets.
If you think this observation is inaccurate, please add an ID, participate in the quality assessment above, or describe the inaccuracy in a comment.