Hard Surface Planting

With increased urbanization, environmental and climate changes, healthy, long-living urban trees are critical for providing ecological services such as air quality improvement, stormwater management, and energy conservation. However, these benefits cannot be fully realized when tree growth is limited by inadequate space and soil (Mullaney, Lucke, & Trueman, 2015a). One of the most persistent challenges in establishing and maintaining a healthy, resilient, and functional urban forest is the ongoing battle between tree roots, gray infrastructure, and various hard surface pavements. Due to the lack of ample growing space, trees are often planted in restricted soil spaces surrounded by pavement, asphalt, or concrete. As a result, such trees often have limited soil volume to support their growth, inadequate access to nutrients, and a lack of oxygen and water to support their basic physiological functions. 

Conflict with urban structures, limited growing space, and hard surface covering the root system can lead to root damage and root girdling. This often results in pavement lifting as roots of large trees try to access nutrients and water (Watson, Hewitt, Custic & Lo, 2014; Mullaney et al., 2015a). Pavement lifting creates safety hazards for street users, causes concern for property owners and residents, and significantly impacts the aesthetic value of public spaces (Watson et al., 2014). 

Recent studies reveal that surface paving also significantly affects the ability of trees to provide ecological services. Surrounding hard surfaces influences a tree’s cooling effect more than species selection, highlighting the strong impact and restrictions pavements impose on urban trees’ health and ecological functions. This underscores the strong impact and restrictions that hard surfaces impose on the health of urban trees and their ecological functions (Konarska et al., 2023). 

Planting trees on hard surfaces and as part of new developments can be very expensive, and without proper forethought, can bring few benefits and prove to be a liability. When tree planting is carefully designed, planned, and implemented, hard-surface tree issues can be avoided or minimized. Healthy tree growth and survival can be ensured by selecting appropriate tree species for the growing space available, using an adequate soil medium to encourage tree root growth, constructing continuous channels connecting individual planting pits, implementing pervious paving around trees, and providing sufficient irrigation. 

Land use and site planning decisions, combined with poor tree selection and planting practices, can generate problems down the road. However, past mistakes are learning opportunities for improving future practices. For example, studies on permeable paving with deep granular substrates have shown promising results in mitigating damage to the pavement and tree roots by allowing them to grow at greater depth. The permeable pavement supplies the soil with sufficient oxygen, nutrients, and moisture to allow for woody growth at greater depths without impacting the growth rate (Lucke & Beechman, 2019). Still, depending on the soil type of the underlying base layer, the effectiveness of permeable pavement may vary (Mullaney et al., 2015b). 

There are solutions to improve the existing trees’ growth conditions, allowing them to extend their life and continue to provide ecological services. Management and maintenance of trees to support root growth within hardscapes include techniques such as de-pavement, soil aeration, soil improvements, building bridges over tree roots, and establishing root barriers. However, these interventions are often short-term solutions as tree roots continue to grow, and trees might decline, necessitating additional interventions. This also underscores the ongoing nature of tree management and the need for continuous tree care (Watson et al., 2014). There are less costly and site-specific techniques to improve the growing conditions. For example, planting flower beds and other vegetation around tree trunks or installing covers or grates over planting pits can reduce foot traffic around trees while also helping to improve soil conditions.

In places where soil volume and quality are lacking, structural soil can be implemented to ensure healthy root growth. These solutions are often used in highly urban environments such as parking lots and downtown streets with lots of pavement and traffic. This planting solution protects the growing medium from compaction and is formulated to provide the nutrients needed for tree growth. Structural soils are expensive to implement, difficult to maintain, and often provide limited years of tree growth and performance shorter than a tree’s typical lifespan. However, it is usually the only viable solution in intensely urban areas with heavy pedestrian traffic.