Arborvitae (Thuja) are popular evergreen conifers used in residential landscapes as dense privacy screens or hedges. A common concern involves the characteristics of their root system: how deep and wide the roots extend beneath the soil surface. Understanding the typical growth habit of these roots is important for proper planting and making informed decisions about potential long-term damage to nearby structures.
Root System Structure
Arborvitae possess a shallow and fibrous root system, developing a dense network of fine roots that spread horizontally rather than penetrating deep into the subsoil. The majority of the root mass remains concentrated within the top 18 to 24 inches of soil, a limited depth compared to many other tree species. For smaller cultivars like ‘Emerald Green,’ the primary feeding roots may be even shallower, sometimes confined to the top 8 to 12 inches.
The lateral spread of the root system is substantial, typically extending outward at least as far as the tree’s canopy or drip line. In mature specimens, roots can spread horizontally by a distance equal to 1 to 1.5 times the tree’s overall height. This wide, shallow growth pattern is an adaptation to efficiently absorb water and nutrients from the upper soil layers. Although extensive, these fibrous roots are generally non-aggressive and lack the strength required to actively push through solid, intact structures. Damage potential stems from exploiting pre-existing weaknesses rather than generating new structural failures.
Environmental Influences on Root Growth
The final depth and spread of an arborvitae’s roots are significantly influenced by the surrounding soil and environmental conditions. Compacted soil, which has poor aeration and high density, resists downward root growth, forcing the system to remain shallow and spread wider near the surface. Similarly, overly wet areas or those with poor drainage can cause roots to suffocate, leading the plant to prioritize lateral spread in the upper, better-aerated soil layers.
Conversely, deep, loose, and well-draining soil, such as sandy loam, provides the space and oxygen necessary for roots to explore slightly greater depths. In these ideal conditions, the roots remain relatively shallow but can grow deeper in search of consistent moisture. The method of watering also plays a large role, as frequent, light watering only wets the top few inches of soil, training the roots to stay near the surface.
When arborvitae are planted closely together to form a dense hedge, competition for resources influences the root structure. Because each tree vies for a limited supply of water and nutrients, their root systems are forced to spread more aggressively sideways. This competitive pressure increases the density of the root mat closer to the surface and can increase the risk of surface damage or competition with nearby plantings.
Assessing and Mitigating Infrastructure Risk
The shallow, spreading nature of arborvitae roots presents specific risks to residential infrastructure, though they pose a minimal threat to modern, well-constructed foundations. The primary concern is the lifting or cracking of horizontally laid structures, such as sidewalks, driveways, and patios, where roots growing just beneath the surface exert pressure as they thicken. This slow process is a direct consequence of their natural growth habit.
The other significant risk involves underground utilities, specifically older or already damaged sewer and irrigation pipes. Arborvitae roots are strongly attracted to the steady source of moisture and nutrients leaking from hairline cracks or loose joints in these lines. They will not break an intact pipe, but they can easily enter and proliferate within existing openings, leading to clogs and blockages.
To mitigate these risks, proper planting distance is the most effective strategy. For large varieties like ‘Green Giant,’ a minimum distance of 10 to 15 feet from major structures, such as a home foundation or septic system, is recommended. Even smaller types should be kept at least 6 to 8 feet away from sidewalks and driveways to allow for future root expansion.
Altering watering habits can encourage healthier, slightly deeper root growth, reducing surface disruption. Deep, infrequent watering, where the soil is soaked to a depth of 18 to 24 inches every seven to ten days, encourages the roots to grow downward in search of moisture. This contrasts with daily light watering, which reinforces a shallow root system.
In high-risk areas, a vertical root barrier can be installed between the tree and the infrastructure. This redirects the spreading roots downward and away from the surface.