The common evergreen known as Arborvitae, belonging to the Thuja species, is a popular choice for privacy hedges in residential landscapes. When considering planting near a home, many people worry its roots will crack the foundation, a concern rooted in the destructive potential of large tree species. Arborvitae roots pose a very low physical risk to modern, structurally sound foundations. However, the potential for damage shifts to an indirect threat related to soil moisture dynamics under specific environmental conditions.
The Physical Characteristics of Arborvitae Root Systems
Arborvitae are considered low-risk trees regarding foundation damage because of their specific root architecture. The root system is primarily shallow and fibrous, designed to stabilize the plant and absorb moisture from the top layer of soil. Most of the root mass typically remains within the top 10 to 24 inches of the ground surface, though smaller varieties may concentrate roots in the top 8 inches. This shallow growth habit means the roots do not usually extend deep enough to interfere with the load-bearing footings of a standard foundation. The roots of Thuja species are non-aggressive and lack the thick, woody structural roots that expand with enough force to physically push against concrete or masonry, unlike trees such as willows or poplars.
The Actual Mechanisms of Root-Related Foundation Issues
Foundation damage caused by tree roots is rarely due to the root physically pushing against the concrete, which is a common misconception. Direct physical damage from root expansion is usually limited to lightly loaded structures like sidewalks, driveways, or already-cracked walls. A stable, modern foundation slab or deep footing is strong enough to withstand the pressure from the relatively small roots of an arborvitae.
The primary mechanism of root-related structural damage is indirect, involving the massive amount of water roots extract from the soil. Tree roots act as powerful biological pumps, absorbing large volumes of moisture from the surrounding earth to support the canopy. This water extraction, known as desiccation, causes the soil beneath the foundation to shrink, leading to a loss of support. When soil shrinks unevenly, the foundation above it settles or shifts, resulting in structural cracks. Conversely, if a large root network traps moisture near the foundation, it can lead to localized soil heave, which also causes differential movement and cracking.
Environmental Conditions That Increase Foundation Risk
The shallow and fibrous nature of Arborvitae roots means they only pose a threat when environmental and geological factors amplify the risk. The most significant factor is the presence of expansive clay soil around the home, which has a high shrink-swell potential. During periods of drought or prolonged dry weather, the Arborvitae’s roots will extend further in search of water, drawing moisture from the clay soil beneath the foundation. This desiccation causes the clay to shrink, creating voids and leading to foundation settlement, a cycle even a less aggressive root system can trigger.
Risk is also significantly higher with older foundations, such as those made of stone, brick, or shallow concrete footings, especially if they have existing hairline cracks. These older structures are more susceptible to movement than modern, deeply poured concrete foundations. Furthermore, planting too close to the house, particularly in areas with poor drainage, forces the roots to concentrate their search for moisture near the foundation perimeter.