Spring is the best time to plant Siberian irises in cold climates, although they'll be fine if you choose August instead. Gardeners in warm weather regions should plant in the fall, when the weather begins to cool off nicely. Choose a location for your Siberians carefully. They resent being moved, and won't need to be divided for 4 or 5 years, when bloom production declines due to overcrowding clumps.
Select a sunny, well-draining spot with fertile, acidic soil. A pH range of 6.0 to 7.0 is preferred. Work some organic material into the soil to increase acidity if you need to. Canadian peat moss or pine needles work well. Add aged manure or compost if your soil is poor. Keep the Siberian iris rhizomes moist while preparing your planting site.
Dig a hole about a foot deep and wide. Mix the backfill with an equal amount of aged mature or compost and fill the hole about halfway with it. Position the rhizome so that it will be planted about an inch below the surface of the soil. Space multiples about 18 inches apart.
Water the planting site thoroughly. The soil should be uniformly moist, but not soggy or wet. Cover the planting with a disposable paper cup or bowl to shade it for about a week if the afternoon sun is hot.
Mulch your Siberian iris 2 to 3 inches deep to conserve moisture and help to discourage weed growth.
Water newly-planted Siberian irises often enough to keep the soil surface of the planting area evenly moist during their first growing season. Don't allow them to remain soggy or wet. Water about once weekly when weather is hot and dry. Discontinue watering in late fall.
Add about an inch of organic compost before the first predicted frost date to give your irises a little extra protection over the winter.
Feed an all-purpose 10-10-10 fertilizer in early spring according to the packaging instructions. Fertilize again immediately after they bloom. Your Siberian irises probably won't bloom this year, but should flower very well in their third or fourth years.