I have dismayed to see the material trees and shrubs are planted in during the reconstruction of Wellington and Preston Streets. How do they expect trees to grow in paving base?
I looked closer, and noticed that the gravel is actually very “dirty”. It is a scientific mixture of gravel, clay, and water-holding chemical ‘modules’.
Here is a close up of the material, called structural earth:
Below is a picture of a bus stop bulb out on Preston Street into which a tree or shrubs will be planted. It is excavated 3.5 ft down and lined in landscaping fabric to contain the dirty gravel.
It is packed down hard with a vibrating plate. It looks and walks on just like regular paving base. Concrete sidewalk can be poured on it, interlocking pavers laid on it, it is strong enough for cars to park on it.
But once trees are planted into it, the roots take off in all directions, creating a generous root spread that will foster longer tree growth.
The reason the stuff works is that the gravel pieces are carefully sized to form a 3dimensional honeycomb grid of rocks with spaces between. The inbetween bits are filled with clay, which is the nutrient dirt for the tree and clay holds moisture well. Then the water-holding plastic stuff expands up to 500x to hold moisture the trees roots will use up. (Similar pellets are available at the Home Show and some suppliers for householder user either as the entire plant media or to mix in with potted plants).
There is lots more info at this link, be sure to read all the links including the two PDFs to get a complete picture of this amazing stuff. I especially like the bits in the PDF about using the soil to “repair” or “save” crowded trees already planted, by expanding their root room: