didnt know where to ask this : is there a global 'mark all as read' function for garden.org somewhere ?

This is the right place to ask. Not sure what you mean, though. Once you read a thread, it is automatically marked as read. Are you looking for something that marks all the forums read or a single forum? I don't know if there is a function for that or not. I've never seen one but then, I haven't looked.