The siberian Iris needs to be winter sown, there's lots of usefull information in the "Winter Sowing" forum :-)

Jonathan, siberian iris self sow reliably in my garden. Copying that in this cool damp Z8, I put them in a tiny zip lock of sterile moist vermiculite ( only need a tsp or so) and leave them on my desk for a month. After that you can either sow and put the seed pots outdoors, or put the zip locks into your refrigerator for a month or two, then sow. The seeds will germinate best at temperatures lower than normal households, and, the seeds have very hard coats. The hard seed coats are easily broken down if sown fresh outdoors in Fall and left to germinate in early Spring...if you've stored the seeds you might want to drag them across or rub them between two pieces of sandpaper before sowing.

Clothiers - Iris Siberica. Sow at 18-22ºC (64-71ºF) for 2-4 wks, move to -4 to +4ºC (24-39ºF) for 4-6 wks, move to 5-12ºC (41-53ºF) for germination