The roots of my sprite are not deep in the substrate,they are just clinging to the substrate and are a very strong root system. I have found that if you do not bury the roots to deep,then they do very well. Also with the roots exposed they can remove the harmful substances from the water. I have had no luck with floating sprite,allways melted away so I have floating duckweed.I've had water sprite over the decades and NEVER did it root in gravel. Always the roots rotted and it then grew as a floater plant with wide leaves.