Answer: Hydrangeas grow best in a location with protection from winter winds, with soil that is organic and humusy and evenly moist yet well drained, and in morning-only sun or bright dappled light all day. Avoid a spot with sun all day or with afternoon-only sun.
This is the most stressful time of year for transplanting. If it has already been outdoors and is acclimated to the weather, you could plant it now being very careful to keep the soil slightly moist like a wrung out sponge. Using a layer of organic mulch about three inches thick over the root area will help keep the soil more evenly moist.
If you have had it indoors until now, you will need to acclimate it to being outside. Set the container on the north side of your house where it will be in deep shade. Gradually move it toward the eastern end of the house so it receives more morning sun. Keep the soil damp like a wrung out sponge. It will need several weeks to adjust. August is too stressful a month for transplanting, so I would then wait until early September to plant it.
I should mention that if the hydrangea was a florist plant rather than a nursery grown plant intended for the landscape, it may or may not be winter hardy for you.
Good luck with your hydrangea!
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