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Hydrangeas are an eager shrub. They start to push out new growth once warmer temperatures are felt, but then when the weather drops again, it can cause serious damage to that fresh growth. This is especially risky for bigleaf hydrangeas. That said, it is important to take care to protect your hydrangeas when unexpected frost happens.
Typically, with bigleaf and oakleaf hydrangeas you would prune the plants each year immediately after flowering in the summer. The reason for this is that the new blooms grow on old wood–the stems from the summer(s) before. For paniculata and smooth hydrangeas, you would do it in the late fall or winter as the new blooms grow on new wood. But frost can be problematic.
Should you prune hydrangeas after frost?
Frost can be detrimental to your hydrangeas insofar as it can damage them. Any nights where the temperatures drop below 32 degrees F, the plants lose heat in their leaves, the water inside the plant freezes, and the cells burst. When hydrangeas experience frost damage it turns the leaves and the new buds to a light red color. More serious damage turns them to a brown/black color. You will also notice they have wilted.
Bigleaf hydrangeas have weak dormancies and their flowers start to bud quickly come spring, so these varieties are the most susceptible to damage from frost.
After frost keep your eyes peeled for any damage. It could take up to one week for damage to materialize. Once you find it, wait for the stem to mature to the point that it produces bark and then scrape it back to below the frost-damaged points. This should leave healthy wood with the green layer exposed. Any buds that are below this frost damage will still provide beautiful blooms.
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