Rescue Bronson (part iv): how to plant trees in gravel

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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:

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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….

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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.

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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.

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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.

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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:


3 thoughts on “Rescue Bronson (part iv): how to plant trees in gravel

  1. How do the roots find moisture if surrounded by concrete? Is irrigation ever used, or does the exposed soil allow enough water for the tree?

    1. Brent: great question. There certainly doesn’;t appear to much surface area to let in rainwater. In the Speedy location, all the surrounding area is fully paved and there will be minimal water penetration. In the ‘in front of the house’ picture, there should be lots of moisture penetration via the adjacent front lawns/garden areas, and the cells will permit root growth under the sidewalk and driveway. Roots need air, too, and that is what the hose pipe is for that snakes through the soil zone and goes up to the surface near the tree. It may also let in some water.

  2. Thanks for the interesting explanation Eric. I hope these trees will be luckier than those along West Wellington (around Giant Tiger) that are all pretty much dead.I don’t know if they used the same construction methods there, but the hoses that come up to the surface are all completely closed up with garbage and (not good) dirt. None of the merchants/assistants I asked informally has ever thought of watering any of these trees or even noticed that they were dead. The ones on Preston are fairing better though again not thanks to the merchants I’ve talked to.. amazingly, it seems merchants (and their BIAs) can look up and down their streets, be proud of their businesses and not be bothered by the lack of green or even worse, the dead trees sticking out of the sidewalks. They clearly did not go to all those presentations where invariably every single main street is shown with happy pedestrians joyfully strolling or sitting under a continuous green canopy of leaves. What a waste of money and effort, and years and years when these trees could have been growing.

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