I got two (more or less) working circuits now.
It’s kinda difficult for me to say which one right now is better since they are structured quite different.

circuit a: is able to detect touch on a plant anywhere, but has no ground so it’s basically a giant resistor. it’s sort of useful, but only until the point where you want to put up for example a hygrometer. then putting the GND to literally ground/soil is causing the whole thing to fall apart. not good

circuit b: is hard to balance, but gives good result. only on one leave tho. i feel like the plant is reacting to light/breath better, but I guess a circuit b plant needs more than one sensor (electrode) per plant.

I have to test more!