We can rule out Ordinary Paramecia because he turned his fingers into branches. Usual producer paramecias...cant turn into their power. Example:- Magellan, Mr 3, Doffy, Foxy.
I don't think Oda would see this as different as Mr. 3 producing wax from his body to the point of altering the shape of his fingers, hands and even arms; I mean, we see Galdino creating the wax out of his body as expected but also seemingly deforming it, so the author could just say that Ryokugyu growing vines from his fingers like Mr. 3 producing wax from any part of his body.
The big issue I see with both logia and special paramecia is that, at the end of the day, they are elemments and flora isn't that. Wood, chlorophyll, etc. could fit, but plants as well defined flowers, well defined trees, well defined whatever with a proper shape and name? That's significantly different from anything we've seen in any power from those two types.
Also, regarding zoan, let's not forget the basics: "zoan" means "animal" and plants aren't animals. So Oda either changes such basic or it's a mythical creature with plants as its secondary power.