OK, I apologise for my own part of the "flaming" of Marion. Let's answer the question.

I understand that when played at levels of "allegro con brio" for more than 15 minutes continuously, strings can indeed get quite warm; I don't know about "hot". This is why concert pianos have a lid that can be propped up - they are aircooled. Honky-tonk mechanical pianos use instead the bellows driving the mechanism to pass air around the strings.

As for the upright models, manufacturers recommend that one stands a pitcher of water on the top for emergencies. This looks inelegant, and so people often stick a single rose in the pitcher, pretending it is a vase.

There you have the definitive answer. Which only leaves me to relate this one, told me by a Dutch organ-playing friend (though he did have the sensitivity to tell it outside the church):

Q: What is better than a rose on your piano?
A: Tulips on your organ.

Boom-boom!