Maturing into upright, broad columns of fine-textured scaly needles, gardeners usually grow arborvitaes (Thuja spp.) as a hedge or screen on the edge of the property. The mature size -- both height and width -- varies greatly among the cultivars. Arborvitaes do not need to be topped. Pruning the top off an arborvitae halts vertical growth and creates a large, unsightly area lacking green needles.
Most of the numerous cultivars of arborvitae used in American landscapes are of either the eastern arborvitae (Thuja occidentalis) or the western red cedar (Thuja plicata). Eastern arborvitae is popular in U.S. Department of Agriculture plant hardiness zones 2 to 7, and western red cedar is more suitable for USDA zones 6 to 8. Some western red cedar varieties can grow 3 feet each year until they reach 120 feet, but eastern arborvitae tends to grow no more than half that height, putting on a couple of feet each year until it reaches maturity.
An arborvitae naturally develops with a rounded, billowy silhouette on an upright trunk and branches. Pruning occurs in both early spring and midsummer to trim or shear needled branch tips to create a formal, flattened face of the plant. Trimming off branch and needle spray tips, using shears disinfected with a 10 percent bleach solution, prevents further growth. Side or uncut branch tips continue to grow and add length. Cutting an arborvitae severely -- making the cut back into nude branches -- does not result in even regrowth of needles. Avoid cutting into wood where no existing needles grow. Some new needles may arise, but this does not always happen. Only lightly trim the arborvitae to limit size or shape.
A gardener toys with the idea of topping an arborvitae when the plant's height becomes a problem. Topping involves cutting off all branches and needles at a determined height, usually creating an abruptly flat top to the arborvitae. There is no horticultural benefit to topping; it is done only for aesthetic or emergency purposes. Once the arborvitae's upward growing branch tips and leader tip are removed, the shrub will likely not produce any new upward branches. However, any branch tips near the top that were not cut will continue to grow upward and potentially create an irregular vertical silhouette.
Ills of Topping
Arbitrarily cutting off the top of an arborvitae destroys the natural beauty and form of the shrub. Once topped, exceptional pruning maintenance is needed to again add attractive vertical growth or to diminish the abrupt flat top. Young arborvitaes often look thin on their tops, but over 10 years, the plants become bushier and denser. Mature shrubs' tops are naturally jagged and full with hundreds of twigs. Topping a mature arborvitae requires removal of lots of twigs and usually results in exposing nude twigs. The overhead view of a topped arborvitae is terrible, with lots of brown, dead-looking twigs. The shrub will not evenly regrow needles to mask the bald, exposed area caused by harsh topping.
Proper Plant Selection
Topping is done when the arborvitae grows too tall for the garden space. The need for emergency topping can be avoided with proper plant selection. Of the many cultivars of arborvitae, each matures with a definite height and width. Choose and plant cultivars that mature to the maximum height appropriate for your landscape.
If you must prune the top of an arborvitae, it's best to lightly trim back the tips of the plants as they approach the maximum height desired. For example, if you want the arborvitae to get no taller than 8 feet, begin to tip-trim the top of the shrub once it gets 6 feet tall. This prevents the need for harsh topping in a few years and exposing nude branches. Light trimming allows some side branches to grow and remain full of needles by the time the plant reaches 8 feet tall.