Arborvitae is a tree. The genus Thuja, to which all arborvitae belong, contains species that grow to towering heights in the wild, with western redcedar (Thuja plicata) reaching over 50 meters and living for a thousand years. The reason the question comes up at all is that the arborvitae most people encounter in garden centers and suburban landscapes are compact cultivars bred or pruned to stay small, sometimes no taller than a coffee table. The plant itself is a conifer with a long evolutionary pedigree as a tree; the shrub-like forms are a human invention.
What Arborvitae Look Like in the Wild
Two species of Thuja are native to North America, and both are unambiguously trees when left to grow on their own terms. Western redcedar (Thuja plicata) is the larger of the two. It dominates old-growth forests from southeastern Alaska down through the Pacific Northwest, often reaching ages of 800 to 1,000 years. One particularly massive specimen in Washington state measures over 5.9 meters in trunk diameter and stands about 54 meters tall, with a crown spread of roughly 16 meters.1USDA Forest Service. Western Redcedar (Thuja plicata) These are enormous trees by any definition, with tapered, often-fluted bases, thin fibrous bark, and flat sprays of tiny scalelike leaves.
Northern white-cedar (Thuja occidentalis), the species most commonly sold as “arborvitae” in the nursery trade, is smaller but still a tree. In the forests of the northeastern United States and southeastern Canada, it is described as small in stature and slow growing compared with its western cousin, but it still develops a single woody trunk and a conical crown.2Northern Journal of Applied Forestry. Northern White-Cedar Ecology and Silviculture in the Northeastern United States and Southeastern Canada: A Synthesis of Knowledge Wild specimens routinely reach 12 to 15 meters. The gap between that and the two-meter globe arborvitae lining someone’s driveway is entirely a product of selective breeding and pruning.
Two additional Thuja species grow in East Asia: Japanese arborvitae (Thuja standishii) and Korean arborvitae (Thuja koraiensis). Both are forest trees in their native ranges. A fifth species sometimes placed in Thuja, the Sichuan thuja, is rare and debated taxonomically. Across the genus, the default growth habit is an upright conifer with a single dominant leader, the hallmark of a tree.
Why So Many Arborvitae Look Like Shrubs
The nursery industry has been selecting and propagating compact arborvitae cultivars for well over a century. Hundreds of named cultivars now exist, and many of them were specifically chosen because they stay small. Dwarf globe types like ‘Little Giant,’ ‘Hetz Midget,’ and ‘Danica’ grow into rounded mounds barely a meter across. Columnar cultivars like ‘Emerald Green’ (also sold as ‘Smaragd’) stay narrow and are commonly used as privacy hedges, reaching perhaps 3 to 4 meters but not much more. Even the taller cultivars like ‘Green Giant,’ a hybrid between western redcedar and Japanese arborvitae, are typically kept to manageable sizes by residential planting conditions and occasional shearing.
This cultivar diversity is what creates the confusion. If every arborvitae you have ever seen was a neatly clipped hedge or a knee-high ball of green by the front steps, calling it a “tree” feels wrong. But the cultivar is not the species. Breeding and pruning have reshaped the plant’s form without changing its fundamental biology. The wood is still wood. The growth pattern still originates from a conifer that, given space and time, would develop a trunk and a crown. The dwarf cultivars achieve their compact size through genetic mutations that slow growth or alter branching patterns, but the underlying organism remains a tree species in the same way a miniature poodle remains a dog.
Hedging and shearing reinforce the illusion. Arborvitae responds well to regular pruning, which is why it is one of the most popular hedging plants in North America. Constant trimming removes the central leader and encourages dense lateral branching, making the plant look bushy and shrub-like. Stop pruning a hedge-form arborvitae that is not a true dwarf cultivar, and over a few years it will start reaching upward again, reasserting its tree nature.
The Blurry Line Between Tree and Shrub
Part of the difficulty is that the boundary between “tree” and “shrub” is not a clean biological distinction. There is no gene that makes something one or the other. The most commonly used rule of thumb is height and trunk structure: a tree is a woody plant with a single main stem that reaches some minimum height, often set at around 3 to 5 meters, while a shrub is shorter and branches at or near the ground. But this is a practical description, not a classification carved into DNA.
The same individual plant can look like a tree in one environment and a shrub in another. Many woody species grow as full-sized trees in favorable lowland conditions but take on stunted, multi-stemmed shrub forms at high elevations or on exposed cliff faces. The research on what separates a tree from a shrub suggests that the real difference is functional, related to growth strategy and competitive advantage for light, rather than any single anatomical cutoff.3PubMed Central. Treelines will be understood once the functional difference between a tree and a shrub is Trees are taller than shrubs because their strategy is to invest in a woody stem that lifts their canopy above competitors. Shrubs invest differently, branching low and spreading wide.
By this functional standard, arborvitae is firmly a tree. Its natural growth form invests heavily in vertical trunk growth with a single dominant leader. The dwarf cultivars people plant in gardens have been artificially pushed into a shrub-like form, but given a wild setting, every Thuja species grows upward.
When Arborvitae Naturally Grows Small
There is one striking exception to the “arborvitae is a big tree” generalization, and it comes not from nursery breeding but from extreme natural environments. On limestone cliff faces across eastern North America, northern white-cedar often grows as tiny, gnarled, shrub-sized individuals clinging to rock. These cliff-dwelling cedars are among the oldest known trees on the continent: some have been aged at well over 1,000 years despite having trunks barely wider than your wrist.4Journal of Biogeography. Evidence for the widespread occurrence of ancient forests on cliffs
The Niagara Escarpment in Ontario is the most famous site. Researchers discovered living Thuja occidentalis specimens there exceeding 1,000 years of age, with some dead wood on the cliffs dating back even further. These trees are small in stature because the cliff environment offers almost no soil, extreme wind exposure, and limited water. But they are still trees in every biological sense. They have a single woody trunk (however twisted), produce cones, and share the same genome as their taller lowland relatives. Their small size is an environmental response, not a change in identity.
This phenomenon is not unique to arborvitae. The same study found ancient, miniaturized junipers, yews, pines, and oaks on cliffs in the United States, the United Kingdom, France, and Germany.4Journal of Biogeography. Evidence for the widespread occurrence of ancient forests on cliffs Cliff environments seem to slow growth so dramatically that these trees can persist for centuries in a form most people would call a shrub. It is a useful reminder that size alone does not determine what a plant is.
How Arborvitae Controls Its Own Shape
If you have ever looked at a young arborvitae and noticed how strongly it grows upward from a single point, you are seeing a phenomenon called apical dominance at work. The growing tip at the top of the plant produces hormones that suppress the growth of buds lower down, channeling the plant’s energy into vertical extension. This is why an unpruned arborvitae naturally forms a narrow, conical shape rather than spreading outward like a lilac or a forsythia.
Research on apical dominance in woody plants found that the branching habit is controlled not by the sheer quantity of growth hormones at the tip but by the balance of different growth regulators at each point along the stem.5American Journal of Botany. APICAL DOMINANCE AND FORM IN WOODY PLANTS: A REAPPRAISAL Species with strong apical dominance, like most conifers including arborvitae, maintain a clear central leader and a symmetrical, pyramid-shaped crown. Species with weak apical dominance, like many deciduous hardwoods, develop broad, spreading crowns with no single dominant trunk.
When you shear an arborvitae hedge, you are disrupting this hormonal balance. Removing the growing tip releases the suppression on lateral buds, and the plant responds by pushing out dense side growth. This is exactly the mechanism that turns a naturally excurrent (single-leader) tree into a bushy-looking mass of foliage. The plant has not become a shrub. You have just overridden the hormonal signal that told it to grow like a tree.
Dwarf cultivars work through a related but distinct mechanism. Many carry genetic mutations that either reduce the production of growth-promoting hormones or alter the plant’s sensitivity to them. The result is a plant that grows very slowly and branches densely without any pruning required. Some dwarf arborvitae cultivars arose as “witches’ brooms,” abnormal clumps of dense, stunted growth on an otherwise normal tree, which nurseries propagated by taking cuttings. The witches’ broom is a localized growth anomaly, not evidence that the species is a shrub.
The Name “Tree of Life”
The common name “arborvitae” is Latin for “tree of life,” and its origin story is one of the more colorful episodes in North American botanical history. In the winter of 1535-1536, the French explorer Jacques Cartier and his crew were stranded near present-day Quebec City, suffering from severe scurvy. Indigenous Iroquois people showed them how to make a decoction from the bark and leaves of a local conifer, and the cure was dramatic.6PubMed Central. Arginine, scurvy and Cartier’s “tree of life” The tree became known as “Annedda” (the Iroquois name) and later as arborvitae, the tree of life, in recognition of its life-saving properties.
Which specific tree the Iroquois used has been debated for centuries. Northern white-cedar (Thuja occidentalis) is the leading candidate and the species most commonly associated with the name today, though some researchers have proposed white spruce or eastern white pine as alternatives. The vitamin C in the foliage and bark was responsible for the scurvy cure, and Thuja occidentalis foliage does contain meaningful amounts of ascorbic acid. Regardless of the exact species, the name stuck, and “arborvitae” entered European botanical and horticultural vocabulary as a synonym for Thuja.
The name itself carries an implicit answer to the tree-or-shrub question. When 16th-century Europeans named this plant, they called it a tree, because that is what they saw growing in the forests of the St. Lawrence River valley. The shrub-sized forms in your local garden center are a much later development.
An Ancient Lineage
Arborvitae belongs to the cypress family (Cupressaceae), one of the oldest conifer families on Earth. Fossil evidence places the origin of the Cupressaceae in the Middle Jurassic period, roughly 170 million years ago, based on anatomically preserved seed cones from that era.7PubMed. Middle Jurassic evidence for the origin of Cupressaceae: A paleobotanical context for the roles of regulatory genetics and development in the evolution of conifer seed cones That makes the family older than flowering plants, older than most modern bird lineages, and roughly contemporary with the early dinosaurs of that period.
The ancestors of today’s arborvitae and their cypress-family relatives were likely dependent on moist habitats. As global climates shifted after the Eocene epoch, roughly 34 million years ago, many lineages within the Cupressaceae evolved drought-resistant traits to cope with increasing aridity.8PubMed Central. Cenozoic climate change shaped the evolutionary ecophysiology of the Cupressaceae conifers Thuja itself, however, remains relatively moisture-loving compared with its drier-adapted cousins like junipers and cypresses. Northern white-cedar thrives in swamps and along stream banks; western redcedar is a hallmark of the wet Pacific Northwest rainforest. This preference for moist soils is a remnant of the family’s ancient ecology.
The evolutionary context matters because it underscores that Thuja evolved as a forest tree in wet environments, a setting where growing tall to compete for light is essential. The tree growth form is not incidental to arborvitae; it is the result of hundreds of millions of years of selection pressure favoring height and a strong central trunk in competitive forest canopies.
Practical Implications for Planting
If you are choosing an arborvitae for your yard, the tree-versus-shrub distinction has real consequences. A full-sized species like ‘Green Giant’ or an unselected Thuja plicata seedling can grow 60 centimeters or more per year in good conditions and will eventually need a lot of vertical space. Planting one of these under a power line or next to a single-story house creates a problem you will be dealing with for decades. On the other hand, a true dwarf cultivar like ‘Little Gem’ or ‘Tiny Tim’ will never become a tree no matter how long you wait. Knowing which type you have matters more than knowing the genus name.
Nursery labeling does not always help. Tags may say “arborvitae” without specifying the cultivar, or may list a mature size that reflects ideal rather than realistic conditions. A plant labeled as reaching “4 to 5 meters” in full sun and rich soil might barely reach 2 meters in shade and poor drainage. If the label is vague, look at the growth habit of the plant itself. A young arborvitae with a single, clearly dominant central stem is likely a tree-form cultivar that will keep growing upward. One that is already branching into a dense mound near the base is likely a dwarf type.
Spacing is the other common mistake. People buy small arborvitae for a privacy hedge and plant them a meter apart, which looks right when they are 30 centimeters tall. Five years later, the plants are crowding each other, competing for light, and thinning out in the interior. Tree-form arborvitae need spacing based on their expected mature width, which for many cultivars is 2 to 3 meters. Shrub-form dwarfs can be planted closer, but even they need air circulation to avoid fungal problems.
Deer, Disease, and Other Realities
Arborvitae’s popularity as a landscape plant comes with some well-known headaches. Deer find Thuja occidentalis foliage extremely palatable, and in regions with high deer populations, an unprotected arborvitae hedge can be stripped bare over a single winter. This is not a minor cosmetic issue. Unlike many broadleaf shrubs, arborvitae does not regenerate well from old wood. If deer eat back into the brown interior, those branches are unlikely to produce new green growth. Fencing or repellent sprays are essentially mandatory in deer-heavy areas.
Bagworms are the most common insect threat to arborvitae in the eastern United States. These caterpillars build distinctive spindle-shaped bags from silk and plant material, and a heavy infestation can defoliate and kill a tree. The bags are often mistaken for seed cones by people who do not realize their arborvitae has a pest problem. Handpicking bags in winter, when they are most visible and the caterpillars are dormant inside, is the simplest control method for small plantings.
Arborvitae is also susceptible to winter burn, especially in exposed sites with cold, drying winds. The foliage turns brown on the windward side, sometimes dramatically, leading homeowners to think the plant has died. In most cases the roots and interior buds survive, and the plant fills back in during spring, though the recovery can be slow. Wrapping young arborvitae in burlap for their first few winters is a common preventive measure in northern climates. Anti-desiccant sprays offer a less visually intrusive option, though their effectiveness is debated among horticulturists.