I left my tapestry out overnight for two nights (so far). It was quite windy, so I laid a bit of wood on top to stop it blowing away. It also rained a bit. So it is lying under the mulberry tree, despite there not being many berries at the moment.
Obviously there are a few, or birds eating them, as there are some purplish stains on it. Not sure how colourfast it will be. Suppose it doesn't really matter as I have no intention of actually washing the piece, unless it becomes absolutely necessary due to circumstances beyond my control - i.e., a bad smell from the compost heap, when it eventually gets there. Whatever happens, it won't be a piece that gets a lot of use and therefore needs frequent washing.