Bamboo is a little different from other plants, once a cane has been pruned it no longer grows. It will keep it's leaves but it's height is set. Late Spring each year new canes emerge these will very quickly grow to height. To prune these new canes, you must wait until the canes have hardened and the leaves unfurled. It's good practice to always prune just above a branch in the cane. Taller canes can be hard to reach to prune but the cane will have enough flex to pull over and then prune. As a bamboo matures the height and thickness of it's canes will increase year on year. In the UK you can expect the height increase to be around 50cm+ per year. As new more impressive canes emerge older thinner canes can be removed.