Thuja occidentalis (American Arborvitae) is an evergreen shrub that can grow up to 20 feet high and 8 feet wide in city settings, or up to 50 feet tall and 15 feet wide in other landscapes. The Thuja tree grows best in Zones 2 to 8. It needs full sun and well-drained soil to grow well but it can adapt to drought, heat and poor soils. Thuja arborvitae is known for growing fast, but this means you'll have to prune it in order to keep it to a certain size or width.
Prune thuja arborvitae in early summer, after growth has begun. A second pruning can be done in late August.
Shear off unwanted branches with a hedge clipper. Run the clipper along the sides and top of the thuja arborvitae, little by little, until you reach the desired shape.
Look through the canopy for new, vertical branches that have been damaged by heavy snow or ice loads. The species is prone to damage because the branches are very flexible. Cut off unwanted wood where it meets healthy wood. Make the cut on the healthy branch.
Thin out the thuja arborvitae to allow sunlight and air circulation. Remove over-crowded branches that are crossing or rubbing each other. Cut them off at their V-shaped joint with other branches.
Clean pruning shears between cuts with bleach. This will limit the spreading of any diseases.