Arborvitae, belonging to the genus Thuja, are widely favored landscape plants, prized for their dense, evergreen foliage and consistent, often columnar shape. These conifers are frequently planted as privacy screens or hedges, which often places them near property lines, structures, or underground utilities. Because of this common placement, understanding the root structure is important for managing the tree’s health and planning its placement within a garden or yard.
Defining the Typical Arborvitae Root System
Arborvitae species are generally categorized as having a shallow, fibrous root structure, which is distinctly different from trees that develop a deep, anchoring taproot. Instead of growing a single root vertically downward, the plant develops an expansive, dense network of fine roots near the soil surface. This root system acts like a broad, flat plate, efficiently absorbing water and nutrients from the upper soil layers.
The majority of the fine feeder roots, which are responsible for nutrient uptake, typically remain within the top 12 to 18 inches of the ground. For smaller cultivars, such as Thuja occidentalis ‘Smaragd’, the primary root mass may only penetrate to a depth of about six inches, or roughly 15 centimeters. However, larger, more mature varieties, like ‘Green Giant’, can develop a root depth that extends downward to 18 or even 24 inches.
While vertical depth is limited, the lateral spread of the arborvitae root system is extensive and proportional to the tree’s overall size. These shallow roots can easily extend outward from the trunk to a distance equal to the height of the tree. In large, established specimens, the lateral spread may reach 1.5 times the tree’s height, or even twice the diameter of the canopy.
Environmental Factors that Determine Root Penetration
The specific depth an arborvitae’s roots reach is not fixed but is highly responsive to the surrounding environmental conditions, particularly soil quality and available moisture. Soil texture plays a significant role in determining how easily roots can penetrate the substrate. Roots will find it simpler to grow and spread in loose, sandy loam, often resulting in a slightly deeper, more expansive system.
Conversely, heavy, compacted clay soil restricts the root’s ability to push downward, forcing the entire network to remain even closer to the surface. Compaction, whether from construction traffic or foot traffic, creates a physical barrier that the fine root tips cannot easily overcome. Similarly, a persistently high water table limits vertical growth, as roots cannot survive in perpetually saturated, oxygen-deprived soil.
The method of irrigation also acts as a training mechanism for the root system. If a tree consistently receives frequent, shallow watering, the roots have no incentive to grow deep, and they concentrate solely in the moist upper few inches of soil. A healthier practice involves deep, infrequent watering, which encourages the roots to grow downward in search of moisture. This habit promotes a slightly deeper root base, making the plant more resilient to short periods of drought.
Practical Concerns Related to Shallow Rooting
The shallow, plate-like root system of the arborvitae carries several practical consequences for homeowners and landscapers.
Stability and Windthrow
One of the primary concerns is the tree’s stability, as the lack of a deep anchoring taproot makes it susceptible to windthrow. Severe storms and high winds can more easily topple an arborvitae, particularly a tall, narrow variety, because the root mass provides poor vertical anchorage against lateral forces.
Moisture Dependence
The concentration of feeder roots near the surface means the tree is highly dependent on consistent surface moisture. These roots can quickly dry out during hot, dry periods, making the plant vulnerable to drought stress. Applying a layer of organic mulch around the base of the tree is necessary to help retain soil moisture and regulate the temperature of the delicate root zone.
Infrastructure Risk
When considering placement near infrastructure, the shallow nature of the roots is generally less concerning than with deep-rooted species. Arborvitae roots are typically not strong or aggressive enough to penetrate and damage a structurally sound foundation or intact underground pipe. However, their extensive lateral spread means they can quickly exploit any existing entry point, such as a small crack in a sewer line or a joint in a septic field. While they do not cause the damage, they will capitalize on existing vulnerabilities in underground systems.