There are a number of different methods of planting iris seeds. The easiest method is to plant them outdoors in a sunny location in rows. The soil should be prepared the same as that prepared for planting iris rhizomes. Seeds should be planted 1 inch apart and three to four times their diameter deep. In colder climates, deeper planting is recommended to prevent the heaving of seeds out of the ground from freezes and thaws. Each row should be clearly labeled, and only a very light mulch may be applied and removed in the spring.
Iris seeds may also be started indoors and grown in a greenhouse, sunroom, or on a bright windowsill. The seeds can be potted up in November in clean plastic pots with drainage holes. Used pots should be sanitized with a 10 percent chlorine bleach solution. Seeds are planted at the same depth as those planted outdoors. A 4-inch pot can be planted with 20 iris seeds if care is taken to transplant the seedlings before crowding becomes an issue. The potting soil should be thoroughly moistened. Label each pot, insert into a plastic bag, and place in refrigerator for six weeks. Another alternative is to place the pots outdoors in a cold frame to prevent the pots from freezing.
The seedlings will sprout in the spring, and will have matured sufficiently to no longer be dependent on the endosperm by the time they have grown 4-6 inches in height. Potted irises will need to be transplanted into a prepared bed, and the sprouted seedlings started in the bed will need to be spaced at least 1 foot apart. Foliage is not cut back on iris seedlings. The seedlings will undergo shock at the time of transplanting, so overcast days or cool evenings are the best times to transplant iris seedlings.
The seedlings will grow rapidly, and will often bloom the following spring after germination.