Depends on the kind of roof and the heat it puts off. Might put a thermometer up there and see what kind of heat you are dealing with at different times... Picture is not working

If you're still interested, why not re-post your picture, and we'll try to come up with some suggestions for you! Lavender might be a good choice . . . it's pretty heat-and-drought tolerant, and looks very formal year-round.

I don't think trees are a good idea for a rooftop garden, especially planted in a container. Roots are more exposed to hot and cold temps, and wind can blow them over. If your container is large and made of concrete, maybe you could make this work, but I think other categories of plants are better suited. More info on your rooftop climate would be helpful.

This is very late, however, I don't log on often. Rooftop gardens look really great with succulents and sedums. Earlier this winter I read an article where the woman put a lip on her garden shed and layered planting trays filled with sedums, hen and chicks and other drought tolerant plants. I suppose that Thyme would work well. Another trick is to sprinkle water crystels under gravel to help absorb water and release it slowly. I plan on covering an old Roosevelt outhouse with rubber and planting a variety of succulents and sedums that will trail over the edges. That will help preserve the original shingles and perk up an old outhouse. Hopefully you will get an email. Of course after two years, I am sure you have come up with something.