Sow seeds at any time covering them with compost or grit 5mm deep, keeping the seed pot in a cool, well-lit spot outdoors. Artificial heat is not needed and can prevent germination so be very patient as many species will only germinate in the spring after chilling or freezing in the moist seed tray in the winter. Grow on individual seedlings in small pots until of sufficient size to be potted on or planted out into the open ground.
Hardy to Zone 8, butterfly iris will survive to temperatures to 10 degrees Fahrenheit. In areas with mild winters, it will remain evergreen, but when temperatures fall below freezing, the leaves will die back and the plant will be perennial, emerging from the ground the following spring.
If that’s the case in your area, you’ll have to shear off those dead leaves in order to reinvigorate the plant. At some point, you’ll also need to dig up and divide butterfly iris, which, if planted in a large mass, may be an overwhelming task.