Beloved for its lush aesthetic and ability to propagate, it was distributed widely and confused with a different arborvitae from the same source known as T. occidentalis “Giganteoides.” The clone identity was resolved by Susan Martin, USNA, Kim Trip, New York Botanic Garden, and Robert Marquard, Holden Arboretum by means of thorough nursery inspections, record searches, and “isozyme analysis.” Then the name Thuja “Green Giant” was selected. Now, it is available at many wholesome nurseries, at retail sources, and by mail-order. Grown as an ideal substitute for Leland cypress in the southeastern United States, this large, needled tree is easy to grow and generally free of pests and disease. Mature trees reach up to 40 to 60 feet tall, growing densely, narrowly, and in a pyramidal habit. Leaves are small, glossy, and scale-like, opposite and arranged in rows of four, overlapping, creating flat sprays akin to the shape of fans. The bark is dark brown and shaggy. Small, upright seed cones emerge as green and then mature to brown, up to a half-inch long. Unlike some other evergreens, its rich, dense, green foliage does not yellow or brown in winter and only slightly bronzes or darkens. Coexisting with birds and small animals, this tree creates places for nesting and flower buds, as well as seeds and foliage for food. This plant is toxic to grazing animals like cattle, sheep, and horses.
Light
“Green Giant” Arborvitae prefers full sun. It will tolerate part shade and actually thrives in some light afternoon shade in hot summer climates.
Soil
Able to grow in a range soils such as poor soil and clay, it does best in moist, fertile, well-drained loams. Avoid wet soil that does not drain well and exposure to spray or salt.
Water and Fertilizer
You should not fertilize the tree in the first year after planting. For the early years after that, enrich the soil with plenty of water and balanced fertilizer; this will promote continuous, vigorous growth. Then, as with more arborvitae, it will not need much or any fertilizer.
Propagation
Propagate the tree from stem cuttings from July through March. Root cuttings under mist with bottom heat (3000–8000 ppm IBA).
Pruning Green Giant Arborvitae
Covered in dense, dark, evergreen foliage from the ground up, the “Green Giant” arborvitae needs little to no pruning or shearing. Growing elegantly and consistently, it only needs to be pruned if necessary or for aesthetic reasons. Shear into a desired shape and size, and maintain it throughout the warmer months.
Common Pests & Plant Diseases
Fortunately, this variety is more resistant to deer than most arborvitae and it has no major insect or disease issues. Keep an eye out for scale, rots, and bagworm in poorly drained soils.
