Why do we need big trees in cities?
Trees provide incredible value to urban environments. Some of the benefits include improving air and water quality, reducing flooding and combined sewer overflows, creating safer neighborhoods, reducing urban heat-island effect, and reducing energy consumption. According to research from the USDA Forest Service, a tree with a 30’ (77cm) diameter tree provides seventy times the air quality benefits of one with a 3’ (8 cm) diameter. The benefits are real and proven: cities need trees.
Why do trees need soil?
Trees and soil are ecologically interdependent; soil quantity and quality have a huge impact on a tree’s overall health and long-term growth. Trees need soil in order to establish healthy roots systems that have access to adequate air, water, and nutrients that will be able to support the tree for decades to come.
Right now, the way we plant trees in cities – in small sidewalk openings – doesn’t give them enough soil! Most street trees have access only to the soil you see in their opening, because the soil outside the opening has been heavily compacted to meet engineering requirements for buildings and roads. The available soil that is left to the tree can be as little as a twentieth of what they require in order to growth to maturity and provide ecological benefits. Since we know that the most limiting factor for urban tree growth is soil volume (Craul, 1992), this is the first challenge that we need to overcome. By growing big trees, we can bring enormous ecological and economic advantages to our cities and towns.
How do we help trees grow bigger?
The first step is simple: we need to provide them with more soil, especially in paved areas like streets, parking lots, green roofs, and plazas. Access to larger amounts of soil will help us grow the trees we see on architecture plans and renderings, but rarely see on our actual streets.
Which cities have minimum soil volumes?
We are seeing more and more cities establishing policies to ensure the health of their urban canopy by guaranteeing trees a minimum amount of soil. We are huge supporters of these policies! Here is a complete list of all the places we know that have implemented minimum soil volume policies. Help support these cities and spread the word through letters to elected officials or alerting planners that minimum soil requirements are important.
What does it cost to provide trees with adequate soil?
Before getting to the brass tacks of cost, it’s important to understand the value of a street tree. A tree may cost as little as $500, but – planted and cared for properly – it can deliver thousands of dollars in benefits to its community over the course of its lifetime. Unlike other site amenities, trees increase in value as they grow and mature. Contrast that to a light pole, which might cost $20,000, but begins depreciating as soon as it’s installed. Part of our mission is to demonstrate that the value trees provide is not limited to ornamentation, and we think that what we spend on them should reflect that. To get a better end-product – bigger, healthier urban trees – we need to change how we design for them.
The actual cost of installing a suspended pavement system will vary depending on your site parameters. If you’re using a modular product like the Silva Cell, the cost is around $14 - $18 per cubic foot of soil installed. This number is based on bid tabulations from projects in North America.