Answer: Yes, you would need to take them out of the pot. After removing the pot, loosen the roots slightly and set them in the planting hole at the same depth as they grew in the pot.
To dig the hole, loosen the soil over a wide area and as deep as the container. Select a location with direct morning sun or bright dappled light all day, and with protection from winter winds. The planting spot should also be well drained, meaning no standing water and not especially wet after a rain.
After planting, water to settle any air pockets. Then mulch with an organic mulch to a depth of three inches. Water as needed to keep the soil evenly moist yet well drained, meaning damp like a wrung out sponge - never soggy/saturated or dried out.
Unfortunately I am not certain specifially which hydrangea you have (I can;t find a reference to a Hornl), but I want to mention that often times the hydrangeas sold as florist gift plants are not winter hardy in the garden. They are specially bred to be grown in a greenhouse and can't survive the winter outside, they are also very difficult to maintain as houseplants unless you have a greenhouse. So please do not be too disappointed if they do not thrive in the garden. Good luck!
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