How about Japanese Knotweed?

Compare with Polygonum polystachyum (Persicaria wallichii).

Try Burdock. It rivals Rhubarb in size. I don't know that it looks like sorrel though.

I agree burdock. jwww

"Patch" suggests a knotweed although it could probably also be multiple plants of a larger-leaved species of dock (Rumex) growing near one another. Burdock (Arctium) tends to occur as scattered individuals or in small groups although the right combination of circumstances could certainly result in a number of them in a small area.

Boy, you guys are good. I'm going to have to go out there and really look at it come the weekend. I tried to get a picture from the window, but it didn't come out. I think you're right about it being one of the knotweeds. The leaf looks very much like some of the pictures I saw of burdock, but as you said, it's in a clump and there's no evidence of anything on top - just more big leaves.

If the leaves are that big and broad maybe giant knotweed.