And the Green Knight, with his ability to survive decapitation and vanish his vulnerabilities, owed these strengths to a combination of plant talents and his already formidable life force. Gu Shanhai currently... didn't have these, because he lacked the corresponding plant equipment and mounts, among other things. Even his Body Forging Technique wasn't up to scratch, nor was his life force strong enough.
If a Green Knight had sufficiently strong life force, they could attain such a powerful form of immortality without the aid of plants. At the moment, he only possessed superior regenerative efficiency and life force; it was impossible to acquire such mighty powers immediately upon assuming the role.
Regarding coexistence with plants, the final talent in the Green Knight's plant ability series, Gu Shanhai had a straightforward plan, which was to reverse the effect. Instead of becoming part of a plant like Amute, his intention was to make plants part of him, such as organs or limbs.