the instructions are for Iris germanica & the likes, Siberia are planted in the usual manner of any perenial plant. Beautiful btw, a personal favourite, good luck.

Siberian Iris should definitely have rhizomes, Bernard:

http://media.northjersey.com/images/iris02_0408_mt_tif_.jpg

shows a Siberian Iris rhizome, admittedly almost obscured by its roots. Siberian Iris rhizomes need to be planted a little deeper than Bearded Iris rhizomes: with about 2-4 cm of soil over the top. Roots will frequently grow over the entire surface of the rhizome, instead of just the bottom. If you bought them in the reen, just plant them no deeper than they were in the nursery container.