Robotics may yet hold they key here.

A 'seeding station' could be air-dropped into place like a Martian rover and then set itself up automatically. It would charge over solar and drones would fly pre-planned flight-paths to disperse seeds across a large area. Built in weather apparatus would ensure they only flew in favourable conditions. They could even collect rainwater and have an occasional heat-observation cycle to help prevent fires.

Plant growth would encourage insect populations which in turn brings in birds and multiplies from there. It would be made a federal law that anyone tampering with them faces large fines and prison.

The same process could be applied to water purification as well with robots cleaning pollutants out of rivers and then maybe synthesising them into a useful byproduct (maybe a fuel source?)