The weight of the soil and water isn't negligible, but it's not that bad. Most plants will grow happily in a tiny fraction of their root volume in nature, as long as precise irrigation delivers water (or fertilizer solution) just as it's needed. The practical root volume is probably chosen mostly based on the longest expected fault in the irrigation system.
The tiny root system also provides a very weak attachment to the building. Bosco Verticale strapped each tree to a vertical steel cable, plus various additional restraints.
https://global.ctbuh.org/resources/papers/download/2099-a-ne...
The weight of the soil and water isn't negligible, but it's not that bad. Most plants will grow happily in a tiny fraction of their root volume in nature, as long as precise irrigation delivers water (or fertilizer solution) just as it's needed. The practical root volume is probably chosen mostly based on the longest expected fault in the irrigation system.
The tiny root system also provides a very weak attachment to the building. Bosco Verticale strapped each tree to a vertical steel cable, plus various additional restraints.