Rose of Sharon tree
What kind of root system does it have? Is it a taproot? How deep are the roots? Our roof garden has only about 18 inches of soil and I am afraid the root could damage the roof.
Rose Of Sharon has a fairly shallow roots system with many of the roots just under the surface of the soil.
Rose Of Sharon do get to be 8 to 12 feet tall with a width of 4 to 10 feet.
18 inches of soil would not likely sustain this plant.
You can however grow a Rose Of Sharon in a container.
A very large container, such as a half whiskey barrel will work.