'Green Giant' is a cultivar of the Thuja plicata. The tree is known for the fast rate of growth it exhibits and that it keeps most of its green color throughout the winter. This is a low-maintenance tree once established and can be used as a hedge, screen or alone as an accent tree. The average size of the 'Green Giant' arborvitae is 40 to 50 feet tall with a 10 to 15 foot spread. This tree will grow up to 3 feet each growing season until it is full grown, according to the University of Connecticut Department of Horticulture.
Site the tree where it has room to grow. If you have planted the tree too close to a building or other plants, relocate 'Green Giant' while it is still young. Be sure not to plant under power lines or overhangs of buildings or other trees.
Keep the soil evenly moist throughout the spring and summer. Water twice a week though the fall and during warm dry times in the winter. These trees thrive in moist soils but it must be well draining. Do not allow water to puddle or stand around the tree.
Place a 2-inch layer of compost over the root area of the tree. Keep mulch in place from spring through the fall. Normal watering will leach the compost into the soil and provide needed nutrients. The compost also helps heavy soils to drain better.
Apply a general-purpose fertilizer each spring to the area over the root system. Time-released fertilizers are best for most trees. Follow manufacturer's directions as to amount of fertilizer to use.
Spread a 3-inch layer of pine needles or shredded pine bark in an 8-foot diameter around the tree. This will help to retain moisture for the root system and keep the weeds from growing and competing for water and nutrients.