Trees’ Impact on Sidewalks and Foundations

Trees’ Impact on Sidewalks and Foundations

As arborists, we have a deep love for trees…but it is important to acknowledge that trees can sometimes lead to challenges, particularly when their roots affect foundations and hardscapes. Fortunately, trees seldom cause issues to properly constructed foundations! This is a big change in tone within our industry in the last few decades regarding trees likelihood to damage foundations which has saved countless trees from being unnecessarily removed. Infrastructures such as patios, sidewalks, and walkways, however, are shallower in nature and are much more susceptible to being impacted by tree roots.

Understanding the growth of roots is crucial in comprehending their impact on infrastructure. Roots typically grow in search of water, nutrients, and stability. They tend to spread horizontally, often extending beyond the trees’ canopy (“dripline”). This horizontal spread is crucial for trees to maximize their access to water and nutrients.

Did you know that the majority of a tree’s fine roots, which are responsible for absorbing water and nutrients, are typically only found in the top 6 inches to 1 foot of the soil? Because most roots are relatively shallow, they can indeed impact nearby sidewalks, driveways, patios, etc. The expansion of water and nutrients can exert pressure on hardscape materials, leading to issues like sidewalk lifting, cracking, or upheaval.

All that to be said, what can we do to help alleviate the impact of tree roots?

1. Select site appropriate trees. 

Dr. Eric Wiseman’s (Virginia Tech) mantra “right tree, right place” encapsulates a fundamental principle in urban forestry and landscape design. By selecting appropriate tree species and placing them thoughtfully, we can proactively avoid many of the challenges associated with tree-root-infrastructure conflicts. Unfortunately, we are mostly called in to mitigate situations after trees have been established and are already causing damage.

2. Root pruning. 

Making proper pruning cuts is just as important underground as it is in the canopy. We use a tool called an Airspade to excavate soil safely and expose roots without causing unnecessary damage. Ripping out roots with heavy machinery or improper tools can cause extensive damage to the remaining root system that can lead to stress or decline.

3. Bio Barrier installation. 

Bio Barriers are used in conjunction with root pruning to combat root re-growth. The Bio Barrier itself is a physical barrier made from materials such as geotextile fabric or plastic. What makes it a “bio” barrier is the inclusion of a chemical component that inhibits root growth. This chemical , often a growth regulator or root growth inhibitor, creates a zone of restricted root growth near the hardscape edge.

4. Growth regulation. 

Using plant growth regulators (PGRs) to manage tree size proactively is a smart and innovative strategy used by arborists. By controlling the size of trees before root damage occurs, we can mitigate potential conflicts with infrastructure. PGRs act as inhibitors of gibberellins, a group of plant hormones that that promote stem elongation and cell division. By inhibiting gibberellin activity, these regulators can reduce internode length and overall plant height. This is particularity useful for controlling the vertical growth of trees, helping to maintain a desired size and shape. Growth can typically be reduced by 30-40% and does not have any ill effect on trees, in fact there are a ton of other benefits to tree health by using PGRs.

Note: Not all trees are candidates for root mitigation measures such as root pruning or Bio Barrier installation, especially within the critical root zone (CRZ). The critical root zone refers to the area around a tree’s base where most of its feeder roots are concentrated Disturbing this zone can have significant impacts on the tree’s health and stability. It’s important to consult with a certified arborists when considering root mitigation.