Originally Posted by Jim Kingdon

http://i160.photobucket.com/albums/t...6082007167.jpg

I can't see where the stems attach to the leaves, but if there is a
little sheath there, it is in the buckwheat family (see the picture of
an "ocrea" halfway down http://en.wikipedia.org/wiki/Polygonaceae ).
Most of those are generally considered weeds (although you need to
make the final decision for yourself).

But at least in my garden (Washington, DC) there are other volunteer
plants which look like that, including Pokeweed and probably others.

If you are in the UK, as you email address would seem to imply, my
experience here is perhaps even less likely to help you.

Until it flowers, it might be pretty hard to identify.