It’s Q&A day!
Simply start with a regular city and insert plant images; hit Mask, invert and mask in with a soft, 25% Opaque, white brush. Do the same with a desert, masking in the ground and add an icon like a bus, place plants over it, and mask in the plants over the top. It’s then a case of blending the picture with adjustment layers, and dodging and burning. One great final effect is to go to Render>Clouds, set your layer to Screen and mask in the layer for some mist and smoke.