Answer: Hydrangeas produce flowers on new shoots which develop on old wood. Your new hydrangeas are in the process of maturing their current growth which will be considered old wood next spring. So just allow your plants to become established in your landscape this year. Early next spring, just as the leaf buds begin to swell on the branches, cut everything down to about knee-high. New growth will develop on this old wood and the new growth should produce flowers at the ends of the branches.
Since the 3-gallon sized hydrangeas had flowers in June, you can understand that as larger, more mature shrubs, they had some old wood from which the flowering shoots developed. By next spring your hydrangeas will have some old wood and they, too will flower.
Q&A Library Searching Tips