The original note, as written at the time
A form of modified breed of plants whose root system is adapted to be stable in very small soil volumes. This way, the plants can form parts of facades of buildings while adding as little weight as possible to the building structure.
Ongoing development
Modified plants whose root systems are adapted to be stable in very small soil volumes, so that they can form parts of building facades while adding as little weight as possible to the structure.
The weight is the reason green facades fail. A conventional planter needs a substantial volume of soil, and soil is heavy when wet — which is the only time it matters. So a green wall is designed as a structural load first and a garden second, and the garden usually loses.
A plant that holds itself in a shallow medium changes the calculation. If the root system is adapted to anchor rather than to forage, the volume needed drops sharply, and the whole assembly — tray, medium, plant, water — becomes something a facade can carry without a redesign.
The species question is where the real work is. You need something that tolerates a small root zone, wind, reflected heat and irregular watering, and that does not grow tall. There are plants that do this — they are just not the ones nurseries sell for green walls.
The honest limitation is lifespan. A shallow root zone is a harsher environment, so plants will need replacing more often, and a facade that must be replanted every two years is a maintenance liability with a nice photograph.
Design for the wet weight, choose for the wind, and budget for replacement.
A green wall is a structural problem that happens to be beautiful.