Magnolia trees are popular in many landscapes due to their large flowers and foliage. Homeowners are often concerned about potential root damage to nearby structures. While the term “invasive” is an overstatement, the characteristic growth pattern of a mature magnolia, such as the Southern Magnolia (Magnolia grandiflora), poses a risk to surface-level hardscapes. Its root system is typically shallow, wide-spreading, and dense, leading to conflict with surrounding development. The risk focuses on lifting sidewalks and driveways rather than deep foundational damage, making proper planning for their extensive lateral spread necessary.
The Nature of Magnolia Root Systems
The magnolia tree has a feeder-root dominant system, meaning its root network is primarily dedicated to absorbing water and nutrients near the soil surface. This shallow growth habit is a key characteristic, with most roots remaining within the top 12 to 18 inches of soil. Unlike many other large trees that develop a deep taproot for stability, the magnolia relies on a wide-reaching, fibrous net to anchor itself.
The roots are thick, rope-like, and fleshy, extending horizontally for a considerable distance. This lateral spread can sometimes reach two to four times the width of the tree’s canopy. Because the roots stay close to the surface, they are easily damaged by soil compaction or excavation, which can negatively affect the tree’s health and stability.
Assessing Risk to Structures and Infrastructure
The shallow, wide-spreading nature of the magnolia’s root system creates a predictable set of conflicts when the tree is planted too close to development. The most common and visible issue involves hardscapes, where the expanding roots exert upward and lateral pressure. This pressure can easily lift, crack, and shift sidewalks, driveways, curbing, and patios over time as the roots grow in diameter.
While deep foundational damage is rare, the roots can still pose a threat to shallow structures like slab foundations or retaining walls. The expansive root system can draw significant moisture from the surrounding soil. In times of drought, the shrinking soil can cause differential settling that may lead to minor cracks in a foundation planted too close. The extensive surface roots can also interfere with the drainage around a home, contributing to moisture issues near the foundation.
Underground utilities are also at risk, particularly septic systems and older sewer lines. Magnolia roots are not known to aggressively break into intact pipes. However, once a small crack or loose joint is present, the roots will opportunistically enter the pipe in search of water and nutrients. Once inside, the roots grow rapidly, forming a dense mass that can cause significant blockages and require costly professional removal.
Preventing Root Damage Through Proper Siting
Mitigating the risk of damage begins with selecting the appropriate planting location, focusing on providing ample space for the species’ mature root spread. For large species like the Southern Magnolia (Magnolia grandiflora), which can reach heights of 60 to 80 feet, it is advised to plant the tree at least 20 to 25 feet away from any permanent structure, including houses and septic fields. This distance accounts for the tree’s extensive lateral root growth. For smaller cultivars, such as ‘Little Gem’ or star magnolia (Magnolia stellata), the required planting distance can be significantly reduced to align with their smaller mature size. Selecting a smaller-statured or columnar variety is an effective way to enjoy a magnolia in a restricted space while minimizing the conflict with hardscapes.
In situations where a large magnolia must be planted closer to a structure, root barriers can be used as a preventative measure to redirect growth. These plastic or non-woven fabric barriers are installed vertically in the soil to a depth of about two feet, guiding the lateral roots downward and away from vulnerable areas like curbs or foundations. However, barriers must be installed correctly to avoid circling the roots, which can compromise the tree’s stability and overall health. Preparing the planting site with loose, well-draining soil can also encourage slightly deeper root growth, though it will not eliminate the shallow-rooting habit of the tree.