How often do you water and feed these pots which are in a cold greenhouse?
I can only speak about the geraniums (mine are really pelargoniums). I have left them as long as 4 or 5 weeks. I am not an expert but I think it is the cold rather than the moistness of the growing medium that gets them. And if I am right I think that is because they effectively become dormant as the temperature goes down and hence do not require as much nutrition. I used to keep mine in the spare room but with the warmer winter temperatures the cold greenhouse works well and I can have many more in there. Good luck.