Recommended Trimming Time
Conifers that have enough space to develop to their full mature form and are being allowed to develop freely do not require pruning, except to remove any damaged or dead branches. The best time to prune an arborvitae is during late spring just before new growth starts on tree, recommends the North Dakota State University Extension. Avoid cutting back beyond the green foliage as this will create permanent bare spots on the tree.
Reasons for Trimming
Conifers like arborvitaes are trimmed for specific reasons. These include keeping the tree to a required height, such as in the case of screens and hedges. Frequent trimming for specific shapes and sizes gives the impression that the plants are growing very slowly. If a conifer is trimmed once to alter its normal growth pattern, it requires continual pruning and trimming at least once a year in late spring before the tree starts to produce fresh growth.
Trimming Arborvitae Hedges
Arborvitaes makes good hedges and screens since the plants not only produce dense growth but also retain their lower branches longer than other evergreens like firs and pines. Let the arborvitae hedge grow for a year or two then start trimming the sides and top annually in late spring before new growth starts. This produces tight growth and helps branches grow closer to the ground. Trim the sides to maintain a manageable width. Keep the top of the trees narrow than the base to ensure light and air circulation to the lower branches.
Hard pruning can be required to bring a severely overgrown hedge back into form. Unfortunately hard and severe pruning is tolerated only by deciduous plants. Evergreens generally respond poorly to severe pruning. Arborvitaes will survive a hard pruning only when young and only if healthy. If an overgrown arborvitae hedge is an eyesore, it should either be trimmed at the present height as an informal hedge or removed entirely, suggest Lewis Hill and Penny O'Sullivan in "The Pruning Answer Book."