There are several commonly sold cultivars of Ruellia simplex that are sterile or nearly-sterile so they either don't set seeds or almost never do. 'Purple Showers' and the 'Mayan' series are examples. They can still spread vigorously by rhizomes but that can be controlled by growing them in a container, like you are doing. I'd keep a close eye on it to see if it develops seed pods or not. Then you'll know the level of caution you should take with this potential "wildfire."
