There are some beautiful topiary sculptures out there, some as large and intricate as this life-size T-rex topiary at the Botanical Gardens in Buffalo, New York. But most balcony garden topiaries shouldn't be intricate, and especially not large, because space is at a premium in the balcony garden.
Most balcony container gardeners will want to keep geometrical shapes that will fit well into a small modern balcony garden. If you plan on keeping topiaries, be patient and expect to trim them several times a year. Appropriate container plants for a container topiary include rosemary or boxwood. If you want a low-maintenance balcony garden (and not have to worry about bringing plants indoors during the winter), these two plants are good choices. Several pieces of rosemary or boxwood topiaries set around a nice bench or a few sleek chairs make a clean and impressive modern balcony garden space.
Unless you're experienced with topiaries, start your topiary garden by buying an already trimmed topiary that you like the shape of. Thjen you just have to water them and trim them when they need a "hair cut." If you purchase a rosemary or boxwood and trim it to your liking, never trim more than 30 percent of the plant's foliage at a time and remember that if you cut a geometric shape crookedly, it's going to have to stay that way for a long time (until it grows out and it's safe to cut it again).
There are some alternatives to true topiaries. you can train vining plants, like English ivy, to grow up wire molded into the shape of your choice. English ivy does well in shadier spots and is an easy plant. It just needs to be trimmed once vines begin to branch off from your desired shape. If you live in a cold area, bring the English ivy topiary in during the winter.
Below are some common modern topiary shapes for a small balcony container garden.