One important thing to know about mountain hydrangeas is that because they are native to the mountains of Japan, they have naturally evolved to develop flower buds and stems that are more tolerant to cold conditions than bigleaf hydrangeas. This helps them bloom more reliably in USDA zones 5 and 6 where stem hardiness could otherwise be an issue with hydrangeas.
Reblooming varieties like Tuff Stuff Ah-Ha produce flowers on both old and new wood. That makes it tough to find a good time to prune them without sacrificing some flowers. We suggest that you avoid pruning reblooming hydrangeas at all except to remove any completely dead branches in the spring once the plant starts to leaf out.
One common misconception about hydrangeas is that you can prune them throughout the season to keep them shorter. Hydrangeas are not like other shrubs, such as boxwood or holly, which can be pruned to maintain a smaller size. If you prune your hydrangea down, it will quickly grow right back to its original size.
Instead, choose a place where your hydrangea can flourish without needing to be pruned to size. If you are working with established plants, it may be necessary to transplant them to a larger space and replace them with a smaller variety that won’t outgrow that spot.
If, for some reason, you do need to cut your Tuff Stuff Ah-Ha hydrangea back, understand that you will be sacrificing some of the flowers when you do so. Do not cut the stems back by greater than 30% as doing so can produce soft, floppy growth.