Pokeweed

Pokeweed. Phytolacca americana. This is a native (I think -- at least there is a native version) but it is generally undesirable.  Can grow 6+ feet tall, flowers and produces berries.  The stem (see cross section) is strangely succulent-like. It is easy to break, and has a big taproot, so often you will try to pull it and instead break off the above ground part and leave the taproot which will just make more stems. So, unless iti s young and pulls easily you might want to dig it up.