A couple years ago after I did my annual spring pruning of my crowns, I took three rhizomes from a Chinook plant and stuck them in the ground. The pictures show what happened to those little sticks over the course of one growing season. You can see that the original rhizomes managed to thicken up very nicely (they're the parts of the plant with all the white/pruple tipped shoots poking out from them). The other long slender structures WITHOUT the shoots protruding are the roots. They generally form below the base of the crown unless the original rhizome was planted horizontally. If this was the case, those roots will tend to form along the entire length of that original rhizome. The last picture is a good example of this. All you have to do is to rotate the picture to the left 90 degrees and it becomes more apparent(just don't spill the beer) If I would have left those in the ground to grow a second season, some of those lower shoots could have turned into rhizomes by the end of the year. I say 'the lower ones' for the simple fact that as they continue to grow, their orientation dictates that they'll most likely produce a few inches or more of underground growth until they break the surface and you see them. That entire portion that grew underground will have developed buds along it's entire length during the growing season which will provide new/additional shoots the following spring. So Dan's right again! If you have vigorous stock and excellent growing conditions you can take some cuttings after the second year but you most definitely should do some thinning after the third year. Daytripper also makes a good point of how you can force what normally would have been an arial shoot to turn into a rhizome by continuously covering it with soil. Can't wait till spring!