Well, there are two components to an RPG: the launcher, and the projectile.
For the launcher to be effective, it must A: be able to launch the projectile, B: protect the user from the forces generated in launching the projectile, and C: allow the projectile to be aimed. This is generally accomplished with a long, sturdy tube that directs the rocket blast safely behind the shooter. Slap some sights on it and you have aim. Whether or not a cellulose-based structure could stand up to those stresses is something I've not studied in depth, but I know people have made cannon from wood before. Not particularly high pressure or powerful, but if crafting plants into weapons is your thing, it shouldn't be too hard.
As far as the projectile goes, that should be easy. An explosion is defined as a rapid expansion of gasses. A pressure vessel to contain the explosives will augment the blast's power, and can also create shrapnel. A rocket can propel the projectile. Make it aerodynamic and slap some fins on that queen for stability and BAM. You've got an RPG.
[member="Jak Sandrow"]