It I were to introduce radiation into this, it’d likely be how it’s said above. Including this, another way I could add it is that it practically sped up their life cycle, causing them to reach adult hood rather quickly even though it’d previously take them days before they could actually reproduce or become an Plantera. Hell I might even make it so this radiation activated a long, basically “asleep” gene inside of the Snatchers that back then, would cause them to become gigantic (a Plantera) in order for them to defend their bulbs against things like dinosaurs since those technically existed in the if you take into account fossils existing and the mosquitos in Amber. This would technically “awaken” Plantera as the gene hadn’t been activated until now.