Hi all, after looking through related forum posts with no sign of an answer, I’m turning to you. I’ve had my bonsai just under a year now with no issues. 3 weeks back I went back home for a week and had my flatmates care for my bonsai in the kitchen (placed on a stool by the window, asking them to water it when slightly dry). I have a feeling they may have over watered it, for the soil was very wet and took around a week to dry out in which I didn’t touch it until almost completely dry as a layer of white mould like substance had formed. I scraped this off, searched for an answer, failed, and once it was dry I gave it a healthy watering. The mould returned a couple days later so I’m a bit worried it could be harmful. I have a feeling it could be a combination of overwatering and the central heating as I have a small room where the heating is almost always on. The bonsai is placed in the windowsill furthest away from the radiator, but the room gets very warm.

If this is overwatering would creating a gravel humidity tray help? Or is this a matter of new soil due to a fungus? any advice would be much appreciated as this bonsai means a lot to me and i’d be devastated to lose it.

You do not indicate what plant you have. The mould is a sign of incorrect watering, but also poor environment. Possibly the soil is not draining properly and is staying wet too long, the mould is not particularly harmful but it a symptom of poor cultivation.

Tawny wrote: ..the central heating as I have a small room where the heating is almost always on...

This is always not good for the trees, in general.
Which tree is yours?

Sorry, completely forgot to specify the tree, it's a fukien tea tree. I think the central heating is having a large impact as it was draining perfectly normal and looking healthy until recently when it's been colder and I've had the heating on a lot. Would moving it to a larger room, such as the kitchen, by a window where the heating isn't very often on help?

"...by a window where the heating isn't very often on help?...

Yes, heating dries up the ambient, and trees respond by dying.