Thuja, more commonly known as arborvitae, is a very popular landscape plant as it is both hardy and low maintenance. Thuja occidentalis is one of the most widely grown hedging plants across the temperate areas of North America. As a shrub or tree accenting the garden landscape they require little or no pruning. However, varieties grown as ornamentals or for hedging are very easy to train and shape.
Determine your pruning objective, whether thinning, shaping or a general rejuvenation. Study the tree or shrub carefully to see where it needs to be balanced. If it is lopsided, branches should be removed or cut back on the heavy side while the sparse side might need the stem tips pinched to make the plant fill in. Although thuja may be pruned at any time other than the winter, early spring is recommended.
Cut off dead and diseased branches at the trunk. With the pruning saw, undercut the branch halfway through 12 to 15 inches out from the trunk. Then make a downward cut on top of the branch a few inches farther out from the undercut. The branch will break off without tearing the trunk. Remove the branch stub by making a clean cut about half an inch out from the trunk. Do not make cuts flush with the trunk to avoid harming the plant. All branches should be removed in this way.
Thin out the interior of the plant by removing dead or dry branches if they are very dense. The purpose of thinning is to allow more light and air into the center of the plant. Use the hand cutter or pruning saw depending on the size of the branch.
Shape the tree or shrub by cutting unwanted branches just past upward growing off shoots. This helps to hide the pruning cut. Never cut back to bare wood when shaping an arborvitae. These areas will not generate new growth once exposed. However, thuja can withstand heavy pruning overall, so removing a lot of material is not a problem.
Pinch off stem tips wherever you want the plant to fill in. Pinching the tips will force the thuja to grow more branches below the pinch point. Locate the tip of the branch. Measuring from the last set of leaves, pinch off the last one-third of the stem with your fingers.
Shear the surface of hedges or topiary to maintain the shape. Shearing the thuja in stages will prevent removing too much material from any one area. Make sure the pruning shears are sharp to achieve clean cuts. Never cut back past the growing tips as bare wood will not rejuvenate.