Posted: 03/11/2015 at 16:47
Hosta is partially right(no offence hosta), if you start to train the three stems you'll end up with three growing points. Apical dominance will mean the two extra stems will then be the main growing points whereas you really need to encourage the main growing point from the original stem first. You don't prune the main stem at all, leave it alone as when the two other stems are taken out the main stem will take over. When you get the new growth next spring then train to the horizontal although in my experience all you need to provide is the framework and the wisteria will almost do it by itself.