Arborvitae (Thuja) are popular evergreen conifers widely used in landscaping. While they share a similar scale-like needle structure and upright growth habit, their ultimate mature size varies dramatically. The final height and spread are determined by the specific cultivar chosen, not the species. This inherent variability, from towering privacy screens to compact accent shrubs, is what defines this versatile group of plants.
Categorizing Arborvitae by Mature Height
Arborvitae cultivars are grouped into three practical size categories based on their genetically determined maximum growth potential. These mature sizes represent the plant’s potential when grown under ideal conditions over decades. Choosing the right cultivar for a specific space is the most important decision.
Large Varieties
The largest varieties are often hybrids bred for rapid growth and substantial screening capabilities. The ‘Green Giant’ arborvitae (Thuja plicata x standishii) is a prime example, capable of reaching 40 to 60 feet tall with a spread of 12 to 18 feet. These specimens are reserved for large-scale privacy barriers, windbreaks, and property line screening where towering height is necessary. They develop a dense, pyramidal shape that provides year-round coverage.
Medium Varieties
Medium-sized cultivars are frequently used for residential hedging and boundary planting. ‘Emerald Green’ (Thuja occidentalis ‘Smaragd’) matures to a manageable 12 to 15 feet tall and 3 to 4 feet wide. Another medium variety is Thuja occidentalis ‘Nigra’, which typically reaches 20 to 30 feet in height and 4 to 10 feet in width. These varieties are suitable for creating a solid visual barrier without overwhelming a typical suburban yard.
Dwarf Varieties
Dwarf and globe varieties function as accent plants, foundation plantings, or low borders. Cultivars like ‘Hetz Midget’ (Thuja occidentalis ‘Hetz Midget’) are slow-growing, compact shrubs that rarely exceed 3 to 4 feet in both height and spread. Their naturally rounded, dense shape requires minimal pruning to maintain their form. These smaller types allow homeowners to incorporate the texture and color of arborvitae into tight spaces.
Assessing the Annual Growth Rate
Beyond their final size, the annual growth rate is a crucial factor, especially for those planting a privacy screen with a specific timeline in mind. Growth rates are generally categorized as slow (less than 6 inches per year), moderate (6 to 12 inches per year), or fast (more than 12 inches per year). Most arborvitae prioritize root establishment during their first year or two after planting, meaning visible height gain is often minimal initially.
Fast Growth
The ‘Green Giant’ cultivar is celebrated for its rapid growth, often achieving 3 to 5 feet of vertical growth annually once established. This speed makes it the preferred choice for quickly creating a massive living fence or windbreak, often reaching its mature screening height in a relatively short timeframe. This rapid expansion is a result of hybrid vigor.
Moderate and Slow Growth
In contrast, the popular ‘Emerald Green’ arborvitae has a moderate to slow growth rate. After its initial establishment period, it typically grows only about 6 to 9 inches per year. This slower pace means it takes 10 to 15 years to reach its full height potential, which is beneficial for areas where a consistently sized hedge is desired without frequent, heavy pruning. The dwarf varieties grow even slower, increasing by only a few inches annually.
Key Environmental Factors That Affect Size
While genetics set the maximum potential height, environmental conditions dictate whether the plant actually reaches that size. Sunlight is a major determinant of density and growth; arborvitae perform best in full sun (at least six hours of direct sunlight daily), which encourages the densest foliage and fastest growth.
In partial shade, foliage tends to become sparser, and the overall growth rate is significantly reduced. Plants will fail to reach their full genetic potential in deep shade, often developing a weak, open structure. Soil quality and drainage are also paramount; these trees prefer well-drained, moist loam and are intolerant of prolonged waterlogging, which can cause root rot and stunt development.
Water availability, especially during the first few years and in periods of drought, directly impacts a tree’s ability to grow. Adequate moisture is needed for cell expansion and nutrient transport. Finally, pruning or shearing is the most direct way to limit a specimen’s size, keeping it well below its maximum height. Regular pruning encourages denser, bushier growth and maintains a specific landscape dimension.