Thuja Arborvitae is a hardy, fast-growing, low-maintenance evergreen tree often used in topiaries. Its tall, slim, stately profile make it a great choice for providing a living screen or windbreak when planted side by side in a row. It also makes an excellent accent tree in a formal landscape. Correct pruning is critical to ensure proper growth and development. Types of pruning include pinching, thinning, shearing, and rejuvenating. Mastering the different techniques is not difficult or time consuming and will likely yield the great result you're looking for.
Study the shape of your tree carefully before you cut and determine where it needs to be built up or cut back to achieve the effect you want. Heed the mantra: "Think twice, cut once."
Pinch off the stem tips to encourage branching and filling in; doing this will reduce the need for more severe pruning later on.
Remove dead or diseased branches by cutting the whole branch off at the trunk. Thin other branches sparingly if the growth is very dense. This will promote air circulation and allow more light to penetrate the branches which will reduce the tree's susceptibility to pests and disease. (Red spider mites particularly like to live in the interior branches of untrimmed arbor vitae when they are packed with dead, dry branches).
Shear the surface of the tree small areas at a time with either hand or electric pruning shears to achieve the desired shape and proportion.
Stand back and evaluate the shape often as you work. Alternate shearing with pinching to train future growth. Do not cut back past the green growth as you will risk having a permanent hole in the shape or the tree; a branch cut back to its bare region will not grow new foliage.
When the tree is at the desired height, cut the top to prevent it from growing taller. This will encourage girth growth instead of height.
The best time to prune arbor vitae is in early summer after the new growth has fully developed, and again in mid-to-late August. The key to successful pruning is to do little at a time and often. Don't wait until a branch has protruded far out of line before trimming it. If you do more pinching than cutting, you will be able to avoid holes or bare branches disrupting the line of the design. Use sharp, clean pruning tools; this will be less stressful to the tree, which means faster healing. Another benefit is that pruning will take less physical effort from you.
Do not prune while the tree is wet as many diseases are spread by waterborne spores. Arbor vitae trees have a foliage "shell" that covers its internal area of leafless stems and twigs; do not open this "shell" as the scar may be visible for years.