My guess is, it'll work for a time. The turtles will trim off any roots that poke through, but eventually, the roots that poke through will make holes big enough for the turtles to either poke their heads in and get caught, or crawl through and eat the lilies.
Not having turtles, I assume they eat all of the plant? Or just parts? Are lilies their favorite food, or the only food in the pond? How big is the pond? Are they totally aquatic, or just mostly aquatic?
