Thuja occidentalis, a cool-weather evergreen tree, is also known as the American arbor vitae. It features overlapping dark-green foliage that is dense but delicate-looking, and normally grows in a pyramidal or columnar shape. This stalwart tree is valued by gardeners for its cold-weather hardiness, versatility and beauty. American arbor vitae excel as specimen trees, windbreaks, privacy screens and entranceway shrubs. Proper pruning can help maintain the tree's attractive shape and promote health and vigour.
Thuja occidentalis is also called the eastern arbor vitae and the white cedar. It gets its name from the Greek word for "juniper" -- somewhat misleading, as the tree is not a juniper -- and the word "occidentalis," which means it is native to the Western hemisphere. Although thuja can grow to 20 feet tall, dwarf cultivars are available that top out at 3 feet; in contrast to plant's characteristic slender pyramidal shape, there are also globular cultivars available. Thuja occidentalis can turn light brown in the coldest part of winter, but regains its rich green colouring in the spring. The foliage is fan-like, with a pleasant citrus scent when cut or crushed. American arbor vitae tend to catch and hold snow, and are susceptible to limb separation and damage if it is not brushed off.
Thuja occidentalis is a robust tree that can tolerate a wide variety of adverse conditions -- including heat, drought and pollution -- but it will not tolerate shade; full to partial sun is required. While rich, loamy soil is optimal, this easy-going tree can adapt to poor, rocky and clay soils, as long as drainage is adequate. Thuja occidentalis is hardy to U.S. Department of Agriculture planting zone 2, meaning it can withstand temperatures of 7.22 degrees C below zero.
When to Prune
Pruning your arbor vitae tree can promote proper air circulation and discourage insect pests, help maintain desired shape and height, and even rejuvenate an older tree. The best time to prune your arbor vitae is before the onset of new growth. However, the tree may also be lightly trimmed in late spring and summer.
When pruning arbor vitae, you should remove only the vertical growth inside the perimeter line; this is the upward-leaning diagonal line that roughly parallels the angle of the side of the tree. University of New Hampshire recommends the thinning and renewal method of pruning rather than shearing, which is the technique of trimming at the tree's outer edges to give plant a shape. UNH warns that shearing prevents air and sunlight from reaching the centre of the shrub, while removing productive new shoots. Thinning and renewal promotes internal growth, reduces winter injury and produces a natural form. Remove shoots that extend past the perimeter line and make the cuts in the interior of the shrub. Also remove branches that are dead, diseased or broken. Long branches should be removed at a node where one branch meets another. According to University of Idaho, you should leave a green shoot near the cut when partially removing a branch; this helps produce growth the following spring.