Voltage isn't dangerous, current is.
You can scrape half that circuit as its not needed in this application, all the voltage and current going to the plant is being driven by the op-amp.
Its a little hard to explain without redrawing the circuit, but basically you can ditch the TLC621 chip, connect D2 to the + pin on the op-amp as the voltage on the output is determined by the Vcc input. The other side of the pumper can just go to ground.
I might have missed something but don't have time to explain right now.