It’s somewhat scary to look at the pictures of the Bronson reconstruction zone. One naturally wonders how trees could possibly survive in such little squares of space in a sea of asphalt and concrete.
The tree roots are underground, and it’s what’s underground that counts most for their survival (although the concrete curb around the planting hole also helps a lot but preventing the soil from being compacted, and deterring cars and other forms of abuse). Here is one method of planting trees in the hard-compacted gravel road base:
Plastic frames, very similar to those ubiquitous plastic milk cartons found on bike carriers and in student apartments everywhere. A large tree root zone is dug out and its gravel base compacted til it’s flat. It looks rather like the bottom of wading pool. Then the cells are placed down on it….
The cells are interconnected with spaces, although that is not readily visible in the photo above. A plastic frame or edge is put around the structure to hold the good dirt in, and the gravel roadbed out.
Some tubes are laid into the planting bed to encourage air exchange and oxygenation of the soil. I notice that the tubes used on Bronson are way smaller than those used in previous years and on other projects. A gap is left where the tree is to be inserted, marked in the picture above by the worker on the left.
The cells are filled with topsoil. The plastic frames keep the soil from being compacted. The gaps in the frames permit roots to roam in search of more good soil and moisture. Note that in the picture above the cells run right up close to the curb, so we can assume the concrete sidewalk will be poured on top, but the tree roots will be happy with space to roam under the walkway.
When filled with dirt, the perimeter is packed with gravel to hold everything together. A top sheet is laid over the cells, and then paved over with concrete sidewalk or asphalt driveway. The cells are strong enough to support vehicles, while keeping the topsoil underneath uncompacted.
An alternative method of planting in hard urban environments is “structural earth / soil”, covered in several posts a few years ago: https://www.westsideaction.ca/structural-earth/