It depends on the intensity of the sun and the amount of heat. Most hydrangea plants do best in full sun up to noon or 1, especially if you live in a climate with a good amount of summer sun.  The client I refer to in this post lives south of San Francisco, six blocks from the Pacific Ocean. This is a cooler area with quite a bit of fog. Most of her many hydrangeas grow in full sun and do great. We had hydrangeas growing on our property in Connecticut, and they also did fine in full sun. The hot afternoon sun will burn a hydrangea in no time. I now live in Tucson, Arizona. I wouldn’t even think of trying a hydrangea here because of the heat, strong sun, and water issues.