depending on the distance to the greenery there are many ways. A tree can be just a flat plane with a tree image with an alpha channel if it’s far enough from the camera. Assuming you want 3D stuff it can be built by hand (time) or by plugin (money)
For grass I would use Maxon’s Hair, Renato’s Fast & Fur, StormTracer, or the free HairDepartment
for plants/trees either Katachi’s DPIT Nature Spirit or XFrog
There are also many ways between the by-hand and dedicated plugin methods as well…