The art of cutting trees and bushes into ornamental shapes

One word substitute is Topiary.

Horticulture: the art or practice of garden cultivation and management.
Bonsai: the art of growing ornamental, artificially dwarfed varieties of trees and shrubs in pots.
Pruning: trim (a tree, shrub, or bush) by cutting away dead or overgrown branches or stems, especially to encourage growth.
Topiary: the art or practice of clipping shrubs or trees into ornamental shapes.