This idea has two important aspects: one economical, one practical.

The thing is we use bushes that take shape of a wall, for decorative purposes, or for fencing. There is something very interesting going on here. If you think about it, were are using a plant as a building. 

The economical aspect is that, is it cheaper to build a wall, or let it grow for you? When does trimming a bush wall gets even with the costs of building one with bricks and mortar?

In the practical sense, can we modify a bush, or select better species, so that the wall is finished quicker? Is it better a plant that builds quicker, or one that has to be trimmed less often?