Doc, You can make reasonably good looking saguaro cactus using the smallest diameter florist's chenille, which you can get at any craft store. Tumbleweeds look exactly like lichen so just use small pieces with a branchy structure and paint them different colors from light green to gray. Correctly called Russian Thistle, they tend to grow in dense groups so you only have to make the outside portions look like tumbleweeds. The most common place to see them individually in the desert is when they have died off and been blown by the wind against buildings and fence lines. Those would all be very light gray.
I'd need to think about Joshua Trees. However, you would never see Joshua Trees and Saguaro's growing in the same area since the Joshua Tree only grows at 5,000 feet and above and Saguaro's only grow at 1,000 feet and below.
HO Scale Modeler