The term "topiary" covers a broad range of techniques. Put simply, it is the art of clipping evergreen plants into shapes. There are time-tested plants that are more cold hardy than others. Once you know your USDA cold hardiness zone (see resources below), you can choose topiary plants that will thrive year-round in your region. The zone map gives the year-round average low temperature each plant can withstand.
Box (Buxus) may be a common ordinary hedge plant, but for topiary, it is indispensable. The small tight evergreen foliage is the key. It can be trimmed and shaped and will recover quickly. Box is inexpensive and easy to find. There are two varieties of low-growing box. They are dwarf boxwood (Buxus sempervirens suffruticosa), and Korean boxwood (Buxus koreana). Both can be easily kept at 1 to 3 feet tall and wide. They are used to make little topiary balls and animal shapes. Boxes do well in containers and are often incorporated into herb gardens and perennial beds. English boxwood (Buxus sempervirens) is better for larger topiary projects. As a hedge, it will reach 5 to 10 feet tall, but shaped into a topiary can be kept as low as 3 feet tall. Regular trimming opposed to occasional severe pruning is the key to keeping boxwood looking good. These boxwoods are all hardy to USDA Zone 5 (minus 10 to minus 20 degrees).
Evergreen privets are often used for topiary. A popular larger-leaf variety is the wax-leaf privet (Ligustrum japonicum). This tall shrub has attractive, thick and glossy leaves. It is best used in looser designs so the leaves are not severed. One example would be the lollipop shape, also called a standard. Another method of topiary for privets is cloud pruning. This plant has the ability to bloom even when tightly clipped. The panicles of white summer flowers have a scent that is reminiscent of gardenia or jasmine. They are followed by blue-black berries that also are ornamental. Wax-leaf privet is hardy to USDA Zone 7 (0 to 10 degrees).
Pines are also used in topiary. Two common styles are cloud pruned and pom-pom. Each species will have dwarf varieties that are better suited for sculpting. Consider needle color and length when choosing a pine. Some pines have deep green needles, and others are blue-green. Also consider the needle length, which will range from 2 to 7 inches. A popular choice is the dwarf scotch pine (Pinus sylvestris, Nana). Scotch pines have short deep-green needles. For longer blue-green needles, try eastern white pine (Pinus strobus, Nana). Mugo pine (Pinus mugo-mugo) can be kept even smaller and has short dark-green needles. These pines are all hardy to USDA Zone 3 (minus 30 to minus 40 degrees).
Box-leaf holly (Ilex crenata) works well for 3- to 4-foot topiaries. The small leaves look similar to English boxwood. They do not have spines like English holly. This plant will tolerate colder temperatures than common boxwood (Buxus). This makes holly a better choice for topiary in very cold regions. The black berries in the fall also create added interest. Another variety (Ilex crenata sky pencil) is a narrow columnar form that looks great in containers. These varieties of holly are hardy to USDA Zone 4 (0 to 5 degrees)