On our mature macro hydrangeas they bloom primarily, if not exclusively, on old wood. The old wood seems to be dying out and per Luis' advice, I've been trimming it down. What happens when all the old wood is gone? Does new wood eventually become old wood?

All macs begin to develop flower buds around this time time of year. July-ish in the South and later the further you go north.

In the southern states, the stems and the flower buds survive winter generally unaffected. The majority of the flowers then originates from these old stems that used to be in the plant when it went dormant in the Fall.

In the northern states close to Canada, the stems and the flower buds will many times die if left unprotected from winter. Reblooming plants start new stems from the base or crown and these new stems develop flower buds around mid-to-late Spring (and later).

If the old stems (old wood) does not look well, you can usually prune it be the end of May. That is probably the latest it will leaf out if it leafs out late. New shrubs will do this because they have been trained to leaf out and bloom at unusual times in order to put the plant for sale at times when a normal hydrangea would not be blooming.

The crown or base will generate new growth almost every year and after one year, it is considered to be old wood.

I don't know if you recall, but I transplanted 2 mature macros around mid May. One is fine so far, although blooms have gone from purple to hot pink. The other is prone to wilt and sunburn (brown crispy leaves). A few buds (looking purple, yaah!), no flowers yet. They are right next to one another, go figure, lol

I noted when we dug them up that their root ball was wide and shallow (maybe 8-10" deep). There's an irrigation system here and I think as youngsters they were subject to frequent, but short waterings. They are receiving more pm sun in their new location, but I'm hoping they adjust - we'll see.