Few blossoms on 15yr.old hydrangea bush - help!

Try supplementing with bone meal in early spring, and mid summer.

Sounds like your plant is in need of some extra nutrients, especially if it has been in the same spot for 15 years...it's probably depleted the minerals it needs.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm

https://plantaddicts.com/why-wont-my-hydrangeas-bloom/

http://www.millionplants.com/advice/wont-hydrangea-bloom/

https://www.gardenguides.com/84700-hydrangeas-bloom.html

Depends on your hydranga....is it one that blooms on last years growth? Or one that blooms on this year's growth? I have one that blooms on last years growth and here in the midwest the plant freezes so am lucky to get one bloom. I have another that grows new growth each year and blooms beautifully. Am thinking of digging up the one that freezes the last year's growth off....it is on the northwest side of the house so gets no protection from the cold.

I have some that are 40 years old and still bloom. Age is not a factor.