Whilst irises prefer a soil high in clay this is not entirely practical for the pond as it can lead to excessive nutrients and exacerbate algal blooms, so lower nutrient aquatic soil or garden sub-soil would be best, supplemented with feeding from slow release fertiliser tablets in the spring.
Most Irises are best planted in baskets that have a bit of width that allows them to spread. This is particularly true of the Louisiana 'Black Gamecock' which spreads horizontally more than most others. The smaller Versicolors are more compact and a 2 litre round basket is a good next step for them when potting up from 9cm (0.5 litre) plants or splitting. When planting in a basket the rhizome should be planted at the edge with the growing tip pointing towards the centre to maximise the growing room. Young plants with no top growth should be planted with the rhizome horizontal underneath the surface of the soil and the growing tip barely below the surface.