It's totally untrue. I will take photos of the Jackmanii I pruned right down to the earth and I'll continue to take photos so you can see the speed at which it grows here in May. Often I have had to prune it back by 8 to 12' during August.
We've owned ours and grown them here for more than 20 years so do not be afraid to prune yours. They gain strength from being pruned.
