This plant started growing this spring in my back yard in Arizona. It is now a few months later and the plant is about 4 feet tall.
Can someone tell me what this is?
This appears to be a healthy horseweed (Erigeron canadensis). A noxious, roundup resistant, drought resistant annual weed. They can produce over a million wind dispersed seeds per plant, and the seeds can persist in the soil for a long time.