Cedar trees and most other evergreen trees add new growth at the tips of branches only. Heavy pruning permanently damages cedars and could kill the tree. Light pruning can partially control the tree's expansion and correct some defects. The best approach to shaping a cedar tree begins with choosing the right location. Plant new cedars where the trees can grow without crowding, thus avoiding future pruning problems.
As a cedar tree matures, branches lengthen and the canopy of green needles moves gradually farther from the trunk. Each spring the cedar puts on new growth at its branch tips. Old needles at the inner edge of the canopy die and fall off as the tree expands. Shortening a branch on a deciduous tree often stimulates new growth behind the cut, but cedar branches don't replace green growth when cut back beyond the needles. Trimming branches back to fit a space causes those branches to die back to the trunk. Allowing the tree to grow in its natural shape protects the tree's health.
During the cedar tree's winter dormancy, pruning tips of branches reduces the size of the canopy without doing serious harm to the tree. Removing half the needle cluster from the previous season by clipping the tip shortens the branch. Clipped branches leave unsightly stubs on the tree, but spring growth should cover them. Twigs to the sides of the stub become the limb's new leaders. Branch thinning also reduces the canopy's spread. Trimming the limb's leader or foremost branch back to its branch collar leaves other shorter branches unharmed. Shaking a branch before cutting points out exactly what you'll remove. A hasty cut could take out much more than expected.
Prune out broken or dying limbs at any time of the year to prevent fungal infection or insect damage. Cut the damaged section out slightly ahead of the swelling of bark at the junction with the main limb. A healthy side branch could slowly replace a broken leader. Remove a broken main branch with a three-step cut. Start several inches from the trunk and undercut to a third of the branch's diameter. Making a top cut 1/2-inch farther out breaks the branch free without splitting it. Finishing up with a cut nearly flush to the branch collar leaves a clean wound that heals quickly.
When the leader at the top of a cedar breaks, multiple leaders sometimes compete to replace it. Competing leaders create weakened top structure and spoil the shape of the tree. Removing all but one, while the leaders are still small, restores the natural pattern. Bending a side branch upward to replace a broken leader encourages faster recovery. Lash a wooden rod to the top of the trunk and tie a small side branch to it, leaving the brace in place for a year. Clip off the first side branches on the new leader to prevent competing leaders from forming.