I was told that there is a Plantman for DC who's a giant tree-like creature, but I haven't been able to find any proof. Can someone help me with this?

There is Swamp Thing, earth Elemental, his daughter Tefe.
There is also Jason Woodrow also known as the Floronic Man.
Encyclopedia of the DC Universe is a good place to look. Otherwise go to a search site and look up DC Comics, refine your search to Swamp Thing and I'm pretty sure you'll find a slew of stuff.

When I say the title I thought "Plantman is Marvel's. Maybe he's thinking of the Floronic Man". He wasn't always a giant tree but I think he is now. At least he was in 1985.

There's also Taproot from L.E.G.I.O.N., plant people from the Green Lantern Corps and, along the lines of Swamp Thing, there were previous earth elementals like Jack of the Green that are now part of the Council of Trees (i.e.: elementals that have "retired" and taken root).

Actually, I've never seen him, but a friend of mine said that there was one. Maybe the idiot was thinking of Floronic Man. Wouldn't surprise me.