As Jim Urban says, “Designing sites is not about looking great the day, month, or year they are completed. It is about creating landscapes that can function and thrive for many years and decades to come.”

The foundation of successful urban green space design is prioritizing both the soil and the trees – and optimizing site elements for future prosperity. Building upon one of our previous blogs from Jim Urban, let’s explore how designing for the future starts with understanding the soil and creating both the space and the conditions for large, healthy trees.

Soil Volume

One of the most influential factors in determining the success or failure of an urban tree is its access to enough soil volume for robust root expansion. Standard tree pits surrounded by compacted roadbase restrict this access whereas innovative GI solutions like Silva Cells create space for vast quantities of soil volume, which equates to flourishing trees with large, shady canopies. Learn more about setting soil volume targets for your project here.

Soil Compaction

When serving as hardscape support in the built environment, soil must by design be compacted to ensure safe, flat surfaces for pedestrians and vehicles. But this compaction is inhospitable to root growth, preventing trees from reaching a healthy size. Again, a system like the Silva Cell can help as it functions as a suspended pavement and allows for uncompacted, loamy soil to be placed within its 90% void space.

Soil Structure

It is ideal to avoid excessive soil movement, screening, and handling during construction to maintain natural aggregates and pore space. Healthy soil structure (peds) provides the necessary pore spaces for air, water, and root expansion, which trees need to grow over decades. Learn more details about soil structure by reading our blog here.

Water In, Water Out

The key to healthy soil for tree planting is achieving a balance between adequate drainage and sufficient water retention. Any tree planting plan needs to show how water will get to the whole soil volume, not just bubblers at the tree pit. Ideally, urban tree soils should allow water to drain freely while holding enough moisture and nutrients to sustain growth. Well-structured soils should include adequate macropores for water movement without suffocating tree roots by limiting oxygen availability.

Tree Growth and Space

Selecting the appropriate tree for the space is crucial – particularly when it comes to planning ahead for how large the tree will be in a decade’s time. Match the tree species to the site’s condition, such as sunlight, moisture, and available space. Plan for the future size, root spread, and canopy of the trees when designing in the urban landscape.

Advocating for Tree Policies

When cities are educated and invested in their urban forests, they are more likely to adopt policies that ensure tree-growing success in the urban environment, starting with soil volume requirements. Helping trees grow to maturity is the only way they provide value to their communities; therefore, creating standards that lead to success on the front end is of the utmost importance. Advocating for tree policies is a great way to ensure that not only your project, but projects across your city, state, or province, are designing flourishing green space well into the future.

Conclusion

Being mindful of soil conditions, tree growth, water in and water out, and how these large trees will ultimately fit into the space are important design considerations to prioritize at a project’s outset and are proven strategies for building thriving green space for years and decades to come.