For hydrangeas that bloom a second time (called remontant) you can encourage the second round of blooms by cutting off the spent blooms from the first round. The sooner you do this the earlier your plant will start the second blooms. When you cut the spent blooms make your cut just below the flower. For hydrangeas that don't bloom a second time keep the blooms on the plant as long as they look attractive. The flower will change to a pink shade even if it was originally blue. Cut them before the first freeze and you will have a good looking dried plant for your house.