Is you greenhouse heated? If not the geraniums got knocked off by low temps and the mold started to grow after the death of the plants.

Lack of ventillation would have been a prime cause. With the greenhouse shut up condensation would have created a damp atmosphere and encouraged mould spores.

Mines not heated but the windows open all year round......i only water when really nessesary and not if its cold or damp outside

I agree with Bulba. Lack of air movement causes the tiny spores to settle and thrive. Once it starts, it's very hard to stop.

Personally, I don't fuss with geraniums at all. When the season is over, I toss them. They are so inexpensive, I just buy new ones every year. Last year I got 3/$10.00. Beautiful new healthy plants. I can also try some of the new varieties, like "Martha Washington" which I thought was wonderful.

p.s I'm talking about Pelargonium. For some reason, it's also called geranium.

Many years ago, it also was in the genus Geranium--along with Erodium and a dozen other members of the Cranesbill family. I guess that we gardeners are a conservative lot--I still sell Muriate of Potash, a name not used by chemists since the 19th century! :)

I love finding out what plant names mean! I knew Geranium and Cranesbill both refer to the beak of the crane (because of the shape of the seed heads), but not that Erodium refers to the beak of the heron! Sorry if that's a bit off track but maybe someone else will be interested too...