Posted: 23/03/2017 at 17:55
Have a look at images of Carex pendula, aka Pendulous Sedge, and see if the flowers look like yours. It's a thug of a plant not suitable for polite company (I grow it but I have a large wild garden). Seeds like mad and gets everywhere, swamps everything, like your Fatsia. If the images match yours, dig it up.
I'd dig it up whatever it is, anything that's taken over to that extent is undesirable