Green & Blue Infrastructure

Design & Construction

It’s important to have trees in cities for so many well-proven reasons, but without adequate consideration, trees in urban environments can cause costly damage to built infrastructure. The most common issue being surface pavement damage caused by the occurrence of root heave.

Characteristically, trees and cities do not coexist. Trees are meant to be in natural forest settings, while cities are made up of manmade components designed to cater to human lifestyle. Roads, sidewalks, buildings, utility services – these are all designed to provide for our needs as city residents, but are all in opposition to the needs of urban trees.

Then consider soil conditions… Roads and parking lots need a structurally sound base that will withstand heavy loads for long periods of time. To accomplish that, soil needs to be compacted to a point where tree root growth is completely restricted.

This article discusses how these facts create unfavorable urban tree situations, and what can be done to avoid it.

What is Root Heave?

Root heave, sometimes called pavement heave, is triggered by tree roots that do not have sufficient growing space underground, caused largely by compacted soil that has been densified to provide structural stability for the overlying pavement. This forces the tree’s roots to colonize immediately underneath the hardscape surface, causing the paved surface to lift, crack, and create aesthetic and safety issues.

Root heave is a challenge for municipalities because, in addition to its unsightly appearance, it poses a major liability risk due to the tripping hazard it causes.

Avoiding Root Heave

Roots are opportunistic by nature and take the path of least resistance. If that means the space between the heavily compacted soil and the hardscape surface, that’s exactly where they’ll attempt to grow. Specifying adequate uncompacted soil volume below paved areas to allow space for tree roots to establish is critical. Load-bearing soil cells that can withstand pedestrian and vehicular traffic provide structural stability for paved surfaces, while their large open spaces provide ample opportunity for tree roots to grow.

Allowing enough space between the trunk of the tree and the surrounding pavement to provide for the zone of rapid taper is equally as important. Dr. Kim Coder from the University of Georgia has written extensively on the importance of the ZRT having sufficient room to expand as the tree grows. ZRT measurements are available for reference.

But, how can the space needed between the tree and the nearest hardscape surface be calculated? Ross Clark from Trees Impact Group coined the handy formula below which helps estimate the required space ctqmdx9.

Planting Distance = DBH x 3.5

Planting distance is measured from the center of the planting zone, while DBH is the estimated Diameter at Breast Height of the tree when mature. The planting distances suggested by this formula are of course only estimates dependent on species and other factors, and should be used as a guide or target only. For more specific advice, refer to a consulting arborist.

Zone of Rapid Taper
Determining Required Space | Image Credit: Trees Impact Group

For some tree species with large buttress root patterns, incorporating the required opening size into the surrounding landscape may not be possible. In such instances, rather than having a huge open space surrounding a newly planted tree, a dedicated “sacrificial zone of pavement” can be incorporated into the planting design. The root system will break up the dedicated pavement area in this zone as the tree grows, which then gets removed and cleaned up. This is far more preferable than the alternative of topping up surrounding asphalt, which is unsightly, not cost effective, and bad for the tree’s health.

To see an example of a sacrificial pavement zone concept, our Australian partner Citygreen has a great article with more details and images.

Managing Tree Roots in Urban Settings

With the integration of natural elements like trees into the urban environment, systems need to be considered that answer the need on both sides. First and foremost, we need to ensure that the built infrastructure around urban trees will be protected and maintained to eliminate costly repair requirements. Secondly, we need to plant healthy trees that will flourish and offer the many benefits that mature trees bestow. To do this, we need to specify urban tree planting systems that will provide adequate growing conditions, without compromising surface strength or other surrounding infrastructure.

That starts with ensuring enough uncompacted soil, but the care and allowance for growing roots can be equally as important. Root management products that ensure tree roots stay in intended rooting areas and away from paved surfaces, utilities, and other nearby built infrastructure is critical. You can learn more about the options available by downloading our free Root Management Overview.

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