I came across this great blog by the SoCal Guerrilla Gardening group that has extensive pictures of the gardens they are building. And rebuilding. And rebuilding again at times when the city takes them out. It looks like they mostly use cacti and succulents and they glean these plants from overgrown gardens and donations. Last year they had over 1,000 plants donated. I know that soil down there is super baked and hard so I admire their amazing dedication!