Posted: 01/10/2014 at 21:14
The reason for cutting it back is because it flowers on new growth, so you remove the old to force it into making new. You don't actually have to, if left it would still flower, but at the top of the plant. C. Lloyd used to grow two of this type of Clematis together. One he pruned down and one he left. That way he got a tall Clematis with flowers all the way up the plant.
As to timing, it is done in February as that is a cold month, so the plant does not start to grow too early and have the new tender growth damaged. Not that it matters, we often do ours in November, just to keep the garden tidy. Later than February though is leaving it a bit short of time to grow. Though again, I have been know to forget and not prune ours back until the middle of March. It flowered the same as ever, just a bit later.