I don't know about cherry trees. You prune hydrangeas in April. Don't cut off the dead flower heads til then as they protect the bush from frost. At the same time take out any dead wood and cut the branches that have flowered back to a pair of buds. Hydrangeas flower on old wood, so cutting them back hard removes the bits that would have flowered.

Cherries at bud burst, so that the rising sap quickly heals the wounds made by your secateurs. Actually best left alone!
Phil J