A climbing hydrangea (hydrangea anomala) flowers on old wood. That is to say, flowers will only grow on branches formed in the previous year. This means that if you prune new shoots, you will also be pruning next year’s flowers.
Climbing hydrangea (Hydrangea anomala) really only needs pruning if it gets too big for its position. A young climbing hydrangea doesn’t grow very fast so, really, you can forget about pruning it for the first few years.
Getting too big? Prune preferably right after flowering, in August or September. Branches that are just too long can be cut right back to the main stem and this way you keep your abundantly growing climbing hydrangea looking neat and tidy. All the other branches will also then get more space to grow on your climbing hydrangea.