Planting a water spider lily

Hey all,

I just received a water spider lily ( it may be Hymenocallis caroliana). It's bare rooted and I wish to add it to a water feature that I have with an iris already in it. The iris is planted in river gravel only and has done well on its third year using a wter plant pot. Any info on the substrate I should use for this one? It's to be kept with its crown/pot edge just below water level as with the iris.