Thuja Green Giant trees take on a tall, broad-based, pyramidal shape if left un-pruned. However, a young tree can be trained and its growth restrained if pruning is done regularly. Prune its side branches annually, cutting back each branch by about one-third, and by topping the tree slightly during each pruning session. Significant pruning of Green Giant trees are best done in early spring before the tree begins its annual growth.  Remove branches that were damaged by strong wind or snow and ice over the winter. Arborvitae trees also respond well to light pruning or shearing later in the growing season, at any time between spring and early-to-mid summer. However, avoid pruning a tree in late summer or fall, since pruning stimulates new growth vulnerable to winter weather. For trees planted in a row, you could also prune them into a formal hedge-shape by shearing back the entire outer-facing portion of each tree two or three times each year, allowing the tree surfaces that face each other to grow unchecked.