Don’t act too quickly to repair frost or freeze damage. Though your hydrangea may be a mess for a week or so, wait until temperatures are consistently warm before taking out your bypass pruners to repair the damage. If you prune immediately and a freeze strikes again, you expose new parts of the hydrangea to potential damage and it saps the plant’s energy to recover again. Once the weather is cooperating, prune damage away to the next healthy bud or set of leaves. Old-wood bloomers may still bloom at the bottom of the plant if damage was not extensive. New-wood bloomers can be cut to a few inches off the ground and bloom just as they would have without the frost. Water well and work 1/2 cup of a slow-release fertilizer — less for a small hydrangea — into the soil over the root ball of the hydrangea to help it recover.
In climates that typically have mild winters, where you plant your hydrangea can make a difference in whether or not it gets any damage at all from a rogue freeze. Planting where it benefits from the reflected heat of a masonry wall with an eastern or southern exposure, and under an overhang or a tree canopy that lets in filtered light, all reduce frost and freeze damage. Avoid planting hydrangeas at the bottom of a slope or other low-lying area where frost pockets can form. When a choice location won’t ward off damage, water plants well before frost strikes — the air above wet soil can be 5 degrees warmer than other air and maintains its heat until morning, according to Cornell University Extension. Covering a prized hydrangea with a sheet or frost cloth can provide 4 to 8 degrees of protection, which could be just enough to avoid damage. Drape all the way to the ground and cover before sunset for best results.