It is indeed a sorrel, a species of Rumex. Six feet is pretty large, but
they do commonly grow quite large.

M. Reed

Dave wrote:
Hello,

This grew in our S.Calif garden. I took it to a local nursery, but no one
could ID it (someone suggested Burdock, which it does not resemble at all).
It looks like pictures of Sorrel to me, but it grew more than 6 ft tall,
with largest leaves over 14 in long.

http://hpaa.com/d/oddplant.htm

Anyone know what it is?

Thanks,
Dave