Green Giant - BEWARE
I am a conifer aficionado. On my 3-acre spread there are currently more than 100 specimens representing 15 different genera -- about all that are hardy in central Indiana. One of my favorites is arborvitae, the genus Thuja. There is a native midwestern arborvitae (T. occidentalis), referred to as eastern white cedar by some. Unfortunately, we destroyed all the wild native Indiana populations, but the species is common in the horticultural trade. I have several cultivars of T. occidentalis in my garboretum -- a term I coined to describe a garden with lots of woody plants, but not quite an arboretum. Among my non-native arborvitae is one called 'Green Giant.' The name 'Green Giant" was evidently conceived by the famous Tennessee plantsman Don Shadow. As I will substantiate, it is an appropriate title for this popular tree.
The plants of what we now call 'Green Giant' evidently originate from Dorus Poulsen's Denmark nursery, having been first noticed in 1937. Specimens were sent from the Poulsen nursery in Copenhagen to the U.S. National Arboretum in 1967. Subsequently, DNA analysis determined the parents of this unusual hybrid to be Japanese arborvitae (T. standishii) and western red cedar (T. plicata), not a true cedar. The bark, wood and sprays of glossy dark evergreen scale-like leaves of 'Green Giant' are delightfully fragrant when touched. Almost all sources list the "mature size" of this hybrid as 40-60 feet and some list a life span; often something like 50 years. Despite the common belief and label misinformation, there is no such thing as mature (i.e., full grown) size as regards tree growth. All woody plants (trees, shrubs and vines) continue to grow (unless dormant) until dead. It is called indeterminate growth, but the rate may fluctuate according to age and growing conditions. So, consider this, the parents of 'Green Giant' are capable of living hundreds of years, perhaps as much as 1,000 years in the case of T. plicata. Furthermore, there is the potential for hybrid vigor and the 'Green Giant' hybrid has been in cultivation only for about 50 years, so how can anyone know? The answer: they cannot. This is not a unique case (e.g., checkout the bald cypress cv 'Peve Minaret'). In nature T. plicata can exceed 200 feet, with a trunk diameter of >20 feet, and T. standishii can get at least 100 feet tall, so where does the 40-60 feet "mature height" come from? The answer: the numbers the various sources are posting are fabricated and perpetuated guesses OR a presumed average size ten years after planting, but without educating the trusting public. The deception was likely started by marketers ignorant of the biology and trying to promote sales by not scaring the customers. Fast-growing is appealing to many but a tree the height of a 20-story building, probably not. FACT: there is no tree that is fast-growing and then stops, unless it is dormant or dies. -- I have previously posted on gullibility, ignorance, denial and the fact that as a society we are woefully plant, gardening and nature undereducated.
Allow me to tell you about the oldest of the three 'Green Giant' specimens at my place in Indianapolis. I planted it in the fall of 2011 -- a ten feet tall 40-gallon container specimen with a diameter of 5 inches at one foot. The specimen is planted in full sun and good soil along an intermittent stream and the location experiences flooding several times a year. It grew a total of only 5-6 feet taller the first five years after planting, then took off. The tree is currently (Sept 2021) 40 feet tall (almost twice the height of the tree in the image) and 18 feet across at the base (the apron or spread) -- many people forget to consider that spread is both a front to back as well as a left to right measure. With a 16-inch diameter trunk at one foot, the specimen already has the aura of a true giant! This cultivar has the potential, within a few decades, to be the tallest tree in our landscape. For comparison, few midwestern trees can and do exceed 100 feet tall. The accompanying pic was taken after I removed the irregularly arranged, upsweeping branches on the lowest four feet of the trunk, which also reduced the basal spread to 15 feet. The lower branches in contact with soil had started to root, a characteristic common to all arborvitaes. NOTE: many sources recommend that 'Green Giant' be sheared, but doing so ruins the natural beauty and produces an unsightly brown mantle. The gymnosperm to the right in the pic is pond cypress (Taxodium ascendens 'Nutans').
Specimens of 'Green Giant' are commonly (dare I say, almost all?) mislocated and misused -- T. occidentalis often would be a better choice, especially an 'Elegantissima' or 'Nigra.' Moreover, as the specimens of 'Green Giant' age they will look nothing like the blocking evergreen plant most people thought they were getting. Older specimens will likely be slender giants (resembling a redwood) with sparse lower branches, exposing a potentially enormous, tapered trunk with deep longitudinal fissures and beautiful brown bark that exfoliates in thin strips. Enjoy but beware. HO - HO - HO