The Green Giant arborvitae (Thuja plicata "Green Giant") is a hybrid variety of the arborvitae family of evergreen. According to the University of Connecticut, it is often promoted commercially as the fastest growing evergreen and has gained favor with property owners for this reason. Plant spacing can vary depending on the use of the trees.
Hedges or Screens
Because they grow quickly with dense foliage, the Green Giant arborvitae is often used for hedges or plantings designed for visual screening. In this situation, the trees are commonly planted about 6 feet apart. This allows the branches of trees to intermingle forming an impenetrable hedge that provides a visual, physical and sound barrier.
Ornamental plantings, trees planted for their visual interest, should allow enough space for the tree to stand on its own. These trees, at maturity, can have a spread of up to 20 feet or about 10 feet in each direction of the main trunk. This would indicate the Green Giant arborvitae should be placed at least 20 feet from similar trees and 10 feet from other objects or buildings.
The Green Giant arborvitae can grow at a rate of about 3 feet each year. The tree thrives in full sun although will survive in partial shade. The tree does best in moist to wet soil although the soil type seems to be less important.
The fast growth rate results in a tree of up to 60 feet over a course of 20 or more years. They can be pruned or sheared to maintain the desired shape and size. This allows the trees to be maintained as dense low hedges at whatever height the property owner desires.