Hi bgli,
I 've done it with green stems of 15cm / 5'' of length. They rooted well and grew correctly, but I'm in Zone7 .
The wood did not harden, especially the one that rooted, and the frost got the whole stem and those trees did rot and die.
One is coming back from the roots though and lost 2 . So it can be done, but green wood is not the easiest material.
Of course with a greenhouse for the winter time, that drawback would just vanish .