Do not prune hydrangeas in the fall and here’s why.. * if they are an old wood variety, prune will cut off next years blooms * Pruning in early fall while the weather is mild promotes growth in a time a plant should be storing up energy for winter. * Pruning in late may not give the plant enough time to harden off the new cuts and could be susceptible to frost damage Old wood hydrangeas shouldn’t be prune unless absolutely necessary. You can prune them back by 1/3 immediately after they bloom in