So far they look as new, a week later. The heat is over :-) I don't have to water them often, the thick peel holds moisture in well. I scooped them out real well, that seems to be a deciding factor with rotting.
Now, after two weeks, they are getting moldy... I will sink them in clay pots Monday.
Gymgirl, the pumpkins themselves were about 7 Dollar a piece I think, and the plants were around 4 Dollar per plant, and there are 5 or so per pumpkin. I don't dare to do the math :-P
This is a brilliant idea! Therefore, I did not think of it first.
But I am going to try it this fall with chrysanthemumz.
And maybe try using varnish or that black pruning paint for the insides, since it's a disposable setup to begin with.