Shrub topiaries are formed by regularly shearing a plant into a geometric form, or, for more complex designs, growing a shrub trained to a wire frame. Shrub topiaries can take years to grow to their final forms, but their shapes are limited only by the imagination. Fast-growing, dense evergreen shrubs make the best topiary choice, while slower-growing boxwood with its small, glossy leaves is a traditional selection for formal hedge sculptures and standards.
Many fast-growing, dense evergreen shrubs can be sheared and pruned as they grow to create free-form geometric shapes, like globes, pyramids, and spirals. The University of Wisconsin Extension Service notes that yew is the most common free-form topiary shrub in northern climates. The English yew (Taxus baccata) is hardy to zone 5, or 4 with protection, while the Japanese yew (Taxus cuspidata) is hardy to zone 4, or 3 with winter protection.
The Canadian yew (Taxus canadensis) grows from Newfoundland west to Manitoba and southward to overlap with the growing regions of English and Japanese yew. Each of these varieties of yew sports dense growth, which can withstand severe and repeated pruning and shearing. Older, interior growth on yew can turn yellow; prune out the older woody branches and cut back the outer growth to encourage filling-in and consistent density.
Boxwood (Buxus sempervirens) was trained into topiaries in ancient Rome and Greece. According to Boxwood Garden, a boxwood nursery which grows an extensive array of boxwood hedges and topiary in Oregon, there are over 200 boxwood cultivars worldwide. Most are hardy in zones 5 through 8.
Boxwood has an extremely dense growth habit, with small, shiny leaves which form a luxurious smooth vegetative surface when kept regularly sheared. Boxwood tends to stay green and keep leaves all the way to the ground, as opposed to most shrubs which tend to die away at ground level. This has made boxwood the most-prized shrub for ornamental and sculpted privacy hedges for hundreds of years. Boxwood can be clipped into free-standing geometric shapes, or trained to grow around metal frames into intricate sculptural designs.
Rosemary (Rosmarinus officinalis) is a perennial, evergreen shrub grown for its culinary and fragrance value. According to the Iowa State University Extension Service, rosemary has long been believed to improve memory, and its aromatic scent has often been used as an air freshener. Rosemary is hardy only in zones 7 and southward, but is easily grown in a large pot brought in for the winter in colder climates, bringing its delightful scent into the house and encouraging fresh picking for cooking uses.
Rosemary can be trained to topiary forms by tying pliable young growth to a rigid metal frame, or by shaping the branches by wrapping them in heavy copper wire and gently bending the wire into shape. Rosemary needles maintain a slightly shaggy appearance so bold simple shapes like standards or wreaths look best.