Most kinds of hydrangeas are poisonous. Every part of the plant.
However, the mountain hydrangea found in South Korea has edible leaves which are used in tea. The unique thing about this tea is that it has no flavor when initially sipped, but floods the throat, tongue, and mouth with sweetness after it’s swallowed.
The substance that causes this is called phyllodulcin. Not only is it sweeter than sugar, but it also helps regulate fat cells and improve metabolism.
Hydrangea tea is called Iseul-cha, which literally means “dew tea” (probably because it’s so flavorless and colorless, at first). It’s supposed to be good for hay fever and for UTIs. (Honestly, though, there’s a ton of teas that claim to be good for UTIs, probably because you can get them by not drinking enough water.)
There’s another variety called Gamno-cha, sweet dew tea. It’s also called Suguk-cha.
Koreans also drink White Mountain tea and Rosebay tea, both made from a local variety of flowering rhododendron — which in most of the world would be toxic as heck.