Ground elder! That's it in the middle of the photo. I've got a patch of it right by the pond, another by the old cherry trees, and the biggest infestation is in the far SE corner.
I don't approve of chemicals in the garden, but I'm making an exception for glyphosate and ground elder. It's the first weed (in February!) to make its presence felt. So I sprayed it today. Ground elder roots are so extensive, that a single spraying of glyph isn't enough. It's a particular nuisance right at this moment because I'm moving and levelling the earth, and if I'm not careful liable to spread it all over the bloody plot. Bonfires right on top of its location have some effect, at least keeping it in hand.