I just noticed this growing on the van we emigrated with seven years ago. It hasn't been used for years and it seems to be getting taken over by lichen!
I always thought lichen grew really slowly, but these seem to be making pretty good progress. There are some 'frondy' types, too, which I always thought were supposed to be an indicator of good air quality.
I think maybe I won't bother washing it for a few more years. It makes a good shed, anyway...
