espalier

es-PAL´-yer

As a noun, an espalier is a tree or shrub whose branches have
been trained to grow against a wall, usually supported by wires
or a lattice framework (treillage).

Espaliers are most often employed in courtyards and walled
gardens, where limited space makes full-size trees impractical.

As a verb, espalier means to train a tree or shrub to grow
flat against a wall.