Arborvitae, belonging to the genus Thuja, is a common evergreen conifer widely cultivated in landscapes. The original species are naturally tree-sized, but selective breeding has created hundreds of cultivars that function as either tall screening trees or compact foundation shrubs. This adaptability makes classification confusing, as the true classification depends less on strict botany and more on the specific variety and its mature size.
Defining the Difference Between a Tree and a Shrub
The distinction between a tree and a shrub is based on general botanical and horticultural guidelines. A tree typically has a single, dominant woody trunk, reaches a mature height exceeding 15 to 20 feet, and exhibits apical dominance, leading to a defined crown shape. A shrub is characterized by multiple woody stems originating near the ground, resulting in a bushy, rounded form, and generally remains shorter than 15 feet. For woody plants like Arborvitae, the line blurs because the natural tendency can be altered by pruning or genetic variation through cultivar selection.
How Arborvitae Adopts Both Forms
The genus Thuja naturally produces large, pyramidal trees, but diverse cultivars have been bred for specific landscape functions, allowing both tree and shrub forms. Tall, columnar varieties are managed as screening trees, maintaining a strong central leader to achieve significant height. For example, the ‘Green Giant’ hybrid can reach 40 feet or more, while ‘Emerald Green’ is a narrow cultivar often used for dense privacy screens. Shrub forms, used for foundation plantings and low hedges, are dwarf cultivars selectively bred for a compact, multi-stemmed habit. The classification depends almost entirely on the cultivar’s mature size and intended use.
Maintenance Based on Form
The care required for Arborvitae differs significantly depending on whether it is being maintained as a tree or a shrub. When managed as a large screening or specimen tree, the focus is on preserving the central leader and the plant’s overall structural integrity. Heavy shearing is usually discouraged for these tree forms, as it can disrupt the natural pyramidal shape and lead to a less stable structure. When Arborvitae is grown as a shrub, such as a formal hedge or a foundation planting, it requires regular, lighter shearing to maintain a dense and uniform shape. This routine trimming encourages new, lush growth that keeps the plant looking tidy and full. It is important to avoid pruning back into the “dead zone”—the interior where foliage is brown and dormant—because Arborvitae does not readily sprout new growth from old wood.