The primary distinction lies in taxonomy. While many popular arborvitae belong to the same genus, *Thuja*, they are often different species or, more commonly, distinct cultivated varieties (cultivars). The Emerald Tree is a specific cultivar of the Western Red Cedar, known botanically as *Thuja plicata* 'Smaragd'. In contrast, other common types include the American Arborvitae (*Thuja occidentalis*) and its many cultivars, such as the fast-growing 'Green Giant' (which is actually a hybrid of *Thuja plicata* and *Thuja standishii*) and the densely pyramidal 'Degroot's Spire'. Understanding this genetic foundation is key to explaining their differing growth habits, hardiness, and overall form.
The growth habit of the Emerald Tree is one of its most defining features. 'Smaragd' exhibits a very tight, narrow, and consistently pyramidal form without the need for pruning. Its branches grow predominantly upright, and the foliage is densely packed, maintaining a crisp, formal appearance year-round. Other arborvitae can vary significantly. For instance, the 'Green Giant' has a much broader, fastigiate (meaning tall and narrowish) but still conical habit, growing significantly larger in both height and width. Older cultivars like 'Nigra' have a similar pyramidal shape but may be broader at the base. Some, like 'Degroot's Spire', have an even more exaggerated narrow, spiral form with twisted foliage. The Emerald Tree’s reliable, compact, and neat pyramid makes it ideal for formal hedges and small spaces where a controlled size is necessary.
Foliage color and texture offer clear points of differentiation. The Emerald Tree is prized for its vibrant, emerald-green foliage that retains its color brilliantly throughout the winter, even in cold climates. The foliage is composed of flat, vertical sprays of scale-like leaves that have a soft, glossy texture. Other arborvitae may display different hues. Some American Arborvitae cultivars, such as 'Rheingold', have golden-orange foliage, while others like 'Techny' have a darker green color that may bronze slightly in winter. The 'Green Giant' has rich, dark green foliage that is somewhat coarser in texture compared to the finer, denser sprays of 'Smaragd'. The winter color retention of 'Smaragd' is generally superior to many *Thuja occidentalis* varieties, which are more prone to winter bronzing.
The growth rate and ultimate size are critical practical differences. The Emerald Tree is considered a slow to moderate grower, typically adding 9 to 12 inches in height per year under ideal conditions. It reaches a mature height of around 12-15 feet with a spread of 3-4 feet, making it a manageable specimen. Conversely, the 'Green Giant' is an exceptionally vigorous grower, often putting on 3 feet or more per year, ultimately reaching 50-60 feet tall and 12-20 feet wide. This makes it suitable for large screens and windbreaks but impractical for smaller residential landscapes. Other American Arborvitae cultivars fall somewhere in between; for example, 'Pyramidalis' is moderate-growing to about 20-30 feet tall. The Emerald Tree’s predictably slow growth and modest final dimensions are major advantages for foundation plantings and low-maintenance hedges.
From an ecological perspective, these plants have varying tolerances to environmental stresses. The Emerald Tree, being a cultivar of *Thuja plicata*, has good resistance to deer browsing, though it is not immune. It also shows reasonable tolerance to wind once established. However, it is less tolerant of dry soils than some other varieties. Many *Thuja occidentalis* cultivars are exceptionally cold-hardy (to USDA zone 3) but can be more susceptible to pest issues like bagworms and spider mites. The 'Green Giant' hybrid is renowned for its excellent resistance to these pests, as well as its tolerance for heat and humidity, making it a top choice in the southeastern United States. Each type has been selected or has evolved for specific environmental niches, influencing where they will thrive best in cultivation.
