Both western red cedar and the more common eastern red cedar grow with the same habits. Prune either species with the same tools and guidelines. Although cedars are evergreen, as with deciduous trees, the major growth spurt happens in spring. Trimming just before that new growth begins puts less stress on the tree. Both cedars normally develop internal dead zones in the canopy, with new needles budding only on the outer branches. Cutting back into the dead zone kills limbs and could kill the tree.
Raise the lower cedar canopy by lightening limbs rather than removing them. To increase mowing space beneath large cedars, cut away up to one third of the branches on a large limb. Prune back to the branch collar (the swelling at the base of the branch).
Prune the outer foliage of cedars in early spring before deciduous trees leaf out. Only an outer layer of the canopy--about six to 12 inches thick--contains green and renewable leafy tissue. Shear back half of that layer with hedge clippers without damaging the tree. New growth slowly covers the gaps and stubs left by trimming.
Reduce the size of the tree by selectively cutting branches instead of shearing, to keep a more natural look. Cut back the leading tips of limbs to the base of the next green side shoot.
Control the height of the tree by cutting back the top no more than 20 percent of the tree's total. Since this creates temporary gaps in the upper canopy, use this method for taller trees where the damage won't be seen. Where tops are easily visible, prune only half the depth of the green canopy layer.
Cut broken cedar branches back to the next healthy branch junction. If no green growth shows on the branch stub, cut back all the way to the cedar's trunk. Bare branches do not recover.