BOXofGREEN as rainwater collector

We have been on a trip to Tårnby to inspect the tree pits that were planted on a couple of residential streets over a year ago. The special thing about this task is that underground, beneath each tree, there are 10-12 BOXofGREEN batts.
They are not only supposed to provide the trees with moisture, but they also serve to collect rainwater. Because there is no rainwater drainage laid in the road.
And as Claus Andersen explains in the video, the trees and the rock wool blocks handle the task of collecting water very well. 
And the trees? They are growing as they should and are healthy and beautiful.