It sounds like it is quite happy and is well treated. Hope it continues to be. You could put a layer of compost or composted manure in Spring. Normally 1/4 to 1/2" thick is ok. You can also use cottonseed meal. If the soil has not been fertilized in a while, consider doing a soil test to check for mineral deficiencies.

Clippings will not do well in such an environment as they lack a good root system and will need moisture that is not frozen. If they go below 32 in the porch... I dunno; , they could turn to mush.

Please take some pictures, with something in them that gives a sense of scale. I'd be very interested to see the size.

Hydrangeas root very easily. I usually put a freezer bag over the pot to keep humidity close to 100%. Temp needs to stay around 70ish. Takes about three months to develop a really good root system.

Our family home has one that is at least 70 years and about 25 feet tall with quite a trunk. I have seen it bloom for the last 50 plus years!!!!