Topiary is the art of training living plants into sculptural shapes. They can be trained over frames or clipped and pruned into shape as they grow. Outdoor topiaries can take many days to plan and start, and many years to shape. Indoor topiaries made of ivy or other fast-growing plants have become popular, since they take only months to grow.
There are 160 registered cultivars of buxus, according to the American Boxwood Society. Box varieties vary in size and growth habit. Two traditionally used for topiary are American box (Buxus sempervirens), and English box (Buxus sempervirens 'Suffruticosa'). Both are small-leaved evergreens with thick foliage. American box reaches an average height of 10 feet; English box grows more slowly and compactly. Both can be sheared for topiary. Box needs fertiliser once or twice a year depending on your climate. Avoid cultivating as it damages the plant's shallow roots.
Berberis species are an overlooked source of topiary material. Experts at Clemson University Extension report that they tolerate extreme soil and climate conditions, while needing only minimal maintenance. They form dense mounds you can shear and grow 12 inches to 2 feet per year, depending on variety. Japanese barberry (Berberis thunbergii) is the easiest to find in nurseries. Named cultivars like red-leaved compact Golden Ruby grows 24 inches tall and can be used in small-container topiary. All barberries have thorns, so wear gloves while shearing or training.
Hedera species are true ivies. Invasive if they escape cultivation, their hardiness and fast growth habit make them ideal for covering topiary frames. Evergreen Hedera helix, English ivy, is the best known. Hardiness outside depends on cultivar, according to University of Florida. Ivy topiaries need clipping and training throughout their lifetime. Fertiliser once a year and occasional replacement of the growth medium are necessary.
Plant moss-filled frames with fast-growing creeping Jenny (moneywort) or thyme. Creeping Jenny is a 2-inch-high trailing herb that needs little water or fertiliser. The round coin-shaped leaves are usually green, but there is a yellow cultivar, Aurea. The herb thyme has several small-leaved mat-forming cultivars suitable for covering frames. Hardy enough for outside topiary, they need full sun to do their best. Slow-growing topiary alternatives, not needing constant trimming, are hens-and-chicks (Sempervivums). Sempervivums belong to the family Crassulaceae, along with echeveria and dudleya. All three are hardy plants needing little water. They form spreading rosettes in a variety of colours and shapes. Some rosettes grow 6 inches in diameter or more, so check nursery tags before buying. All form offshoots that can be removed to start new topiaries.
Taxus baccata (English yew) is a traditional topiary shrub. Widely used in sheared hedges and screens, it can grow 25 feet high unchecked. Best used for outdoor specimen topiary, it has a slow growth habit. The small, glossy dark needles are evergreen. It grows in acid or alkaline soils but needs good drainage. Full sun is preferred but it will grow in filtered sun. Garden carefully as seeds, bark and leaves are toxic.