I have had great luck planting the rhizomes from May through August with supplemental watering as necessary when new to the soil until the roots take, even taking blooming plants and putting them in the ground which the blossoms don't like, but they survive well. Leave room for them to grow outward with the new increases.
At the top of the iris forum list is one just on vendors. Brecks and other mail order catalog places are not "specialists" in irises, also Lowes and WalMart are not good places. Too often they sell you misidentified ones.
