Hi, I know that the hosta do very well with pea gravel, however, I am not so certain about the hydrangea. My hydrangea show limp leaves in the extreme heat and I fear that the roots would get too warm with the gravel over top of them. I would suggest asking this in the hydrangea forum. Those folks have a lot more knowledge and will probably ask what variety of hydrangea you have.

Both would prefer an organic mulch like shredded bark, as it would retain more moisture and cool the soil more. However, slugs would prefer an organic mulch too, as it would retain more moisture and cool the soil more.

Hosta, of course, will grow practically anywhere though.