That plant is hardy to about 20 degrees, maybe a bit lower if mulched well. Cut them back to about 6 inches and keep a deep mulch around the plant's base. They may return when things warm up again in spring. The flowers on hydrangea are blue in more acidic conditions. The mechanism of color variation in hydrangeas is the presence or absence of aluminum in the blooms. When aluminum is present the blooms tend to be blue. When it is not, they tend toward pink. Soil pH affects bloom color by making soil aluminum more (low pH or acidic soil) or less (higher pH or basic soil) available. To change the bloom color from pink to blue, add ? cup of sulfur per 10 square feet, mix it into the soil surface and then water it in well. To change the soil the blooms from blue to pink, instead of sulfur, mix in 1 cup of lime per 10 square feet. This procedure can take months to a year to change the soil pH and thus the bloom color.
For a faster reaction when changing from pink to blue, dissolve 1 tablespoon of alum (aluminum sulfate) in a gallon of water and drench the soil around the plants thoroughly 3 times, 4 weeks apart in spring. To rapidly change from blue to pink substitute 1 tablespoon of hydrated lime for the alum. Always apply these solutions to the soil and not the blooms or foliage or damage may result.
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