A hothouse flower is flower that isn't hardy enough to grow under natural conditions. It has to be pampered and grown in a greenhouse or hothouse.
In regard to people it's used to describe someone who needs pampering or special conditions.
It doesn't exclusively mean high maintenance, but, needing special circumstances on some level to flourish, grow, be happy, or stable.
"She's the kinda girl you have to give space. A real hothouse flower. Turn the heat on her too much and she's outta there."
"He's kind of a hothouse plant. If he doesn't draw everyday he just wilts into depression."
(Nicholas Nickleby, 1838)"Mrs. Witterly is of a very excitable nature, very delicate, very fragile, a hothouse plant."
On an episode of "Frazier," Frazier was complaining about various things that bothered him. His father said, "Aren't you the little hothouse orchid."