First I tried the method which we discovered 1 day ago (it worked on small scale but was a disaster on a big scale map):
Follow these steps to make it work:
Adjust your LANDSCAPES height as you like it.
Select your PLACE tool and select all your LANDSCAPES in the scene outliner (on the right where you see all your assets).
Now select your FOLIAGE tool and press SELECT ALL.
Once done press FIND GROUND. (hit yes on the popup)
Now you will wait for hours but be patient it will happen.
DONT DO IT THIS WAY!!!
It consumes ages in time and might even crash (i waited over 24 hours and still counting)
HERE IS THE RIGHT AND QUICK WAY TO DO IT:
Move your LANDSCAPE where ever you want it to be on the Z Axis.
You will notice that the FOLIAGE is in the air. (don’t panic)**
First select your LANDSCAPE and copy the Z axis value of it.**
Then simply click your loose foliage in the PERSPECTIVE window. (make sure you are in the PLACE tab of the editor not the FOLIAGE tab)
If you do it right all your FOLIAGE should turn yellow indicating that it is selected.**
Now you will see the settings popping in the DETAILS tab on the bottom right.
Look at the LOCATION part of the settings
Paste your previously copied height value into the Z axis of it and press enter. (should take a few seconds but…)**
**Don’t forget to SAVE often!