Topiary is the art of growing trees and shrubs as living sculptures — cubes, spheres, obelisks, animal shapes or combinations of these.
The tradition has flourished in various places at different times, but in modern gardens, topiary is rare, unless you count our foundation plantings of clipped yews and junipers — "gumdrop" or "dot-dash" landscaping along home foundations.
Only certain plants are suitable for pruned topiary. (Topiaries created with wire frames "plugged" with hens-and-chicks plants or embraced by vining plants are a different story, for another time.) The ideal topiary plant is slow-growing, tolerant of repeated pruning, and able to re-sprout from older wood. Especially for smaller topiaries viewed at close range, small leaves are needed to create a surface with a crisp edge.
If possible, select a species or variety whose natural shape approximates its intended shape: Densa yew for a low sphere, or Pyramidalis yew for an obelisk, as examples.
Evergreens generally are used for topiary, but occasionally a deciduous plant such as English hawthorn or European beech is used. A drawback to a deciduous topiary, of course, is that it is bare in winter (although beech does not shed its dead leaves until spring). Deciduous plants generally grow more exuberantly than do evergreens, so also require more diligence to keep growth in check.
Many species of evergreens have been used for topiary. The quintessential ones are yew and boxwood. Some other suitable plants include arborvitae, hemlock, holly and rosemary.
DEVELOPING YOUR SHAPE
In most cases, begin shaping your plant while it is young. You could, however, carve a shape out of an old overgrown yew, much as you would out of wood or stone, because yew grows so densely and sprouts so freely from old wood.
Or maybe a growing plant will suggest a form that you could then develop. You might even juxtapose two plants, or let one grow up through the other to create, for example, a pedestal on which sits a verdant animal.
In any case, topiary lends itself more to bold shapes than to intricate designs whose details are swallowed up between prunings. Site your topiary so that it receives good light on all sides, for dense growth throughout.
Most young topiary plants that are still in their formative stage need nothing more than frequent shearing or clipping off of the ends of stems in order to encourage dense branching. Clipping individual stems is the preferred method for plants with large leaves because shearing would mangle individual leaves. Obviously, where a stem protrudes in the direction where you want growth, leave it.
KEEPING IN SHAPE
Once a topiary is fully grown and shaped, it needs pruning at least once a year, two or three times a year in some cases. Where a plant needs only once-a-year pruning and is reliably cold-hardy, prune just after midsummer. By then, the spring flush of growth has ceased, and there is less chance that pruning will stimulate regrowth before the following spring.
Cut freehand, or use a guide to make sure your topiary is not gradually changing shape over the years. A guide is also useful when you have more than one matching topiary. If you cut freehand, step back frequently to check and admire your work.
What is to be done with a neglected topiary? Severe cuts may be needed to stimulate growth within the plant. Repair a leafless hole by widening it, cutting old wood around the hole back to healthy wood. If severe cuts are needed, renovation is possible only if the plant is one capable of sprouting from old, perhaps leafless, wood. Otherwise, start again with a new plant.