Some gardeners have great success with "trench irrigation," or directing rainwater into the roots of your plant by creating a trench where the flower is planted. However, that's not necessary for most gardeners. To water your Japanese iris successfully, add moisture several times per week. Water the base of the plant, instead of the flowers, and water early in the day, before the sun becomes bright. Japanese iris are also plants that are suitable for planting near or inside water features, such as ponds. If you want to try planting Japanese iris in a pond, pot your iris in a soil such as a heavy loam, making sure the top of the rhizome is above the water line.