Posted by Sean Corrigan on Mon, Jun 29, 2026 @ 1:09 PM

Eighteen Years On: What Toronto’s Forgotten Stormwater Tree Pilot Still Teaches Us

What the Queensway pilot set out to do

The site sits on the north side of The Queensway in the Stonegate-Queensway area of Etobicoke: two pairs of two trees, four in total, arranged in two belowground systems, tucked into a parking lay-by on a working commercial street. There is no signage. You would walk past it without knowing it is there.

It went into the ground in the autumn of 2008 as a joint proof-of-concept by the City of Toronto and Toronto Water, with Toronto Metropolitan University (then Ryerson University) and the soil-cell manufacturer, installed by contractor Pave-Al in roughly three days. Two City of Toronto engineers drove it: Patrick Cheung, who championed it into the ground, and Peter Simon, who coined the term “tree coffins” for the cramped, compacted pits that kill so many street trees young. Both have since retired.

The idea was simple to state and hard to prove: treat the street as the catchment. Instead of sending kerbside runoff straight to the sewer, divert it into the engineered soil under the footpath, so the tree’s rooting volume doubles as stormwater bioretention. The system was then monitored for more than a decade by the Sustainable Technologies Evaluation Program (STEP), a partnership of the Toronto and Region Conservation Authority, Credit Valley Conservation, and the Lake Simcoe Region Conservation Authority, a thirdparty, public-agency program, not a vendor.

That distinction matters. The installation data has long been published. What is far less examined is what eighteen years of a live commercial streetscape actually reveals, and almost nobody is talking about that.

How the system actually works

Strip away the novelty, and it is a compact treatment train. Per the published STEP case study and the manufacturer’s documentation, runoff enters a street catch basin, then drops through a 200 mm (8″) pipe into the top of a matrix of structural soil cells , the suspended-pavement approach that carries traffic loads on a frame while leaving the soil below uncompacted.

From the inlet, a perforated distribution pipe spreads the water laterally through the soil. The soil itself is the clever part: a bioretention media with around 20% water-holding capacity, so the rooting volume is the filter media. Water that the soil and roots don’t take up drains to a perforated underdrain and overflows to the storm sewer. Detention and infiltration in one buried package.

The numbers give a sense of scale. Each tree sits in roughly 16 m³ (about 600ft³) of soil, a generous rooting volume by streetscape standards. Across the two systems, there are 52 cells carrying four trees, each system able to take runoff from around 385 m² of road and footpath and sized for a 50 mm / 24-hour design storm. The frames meet AASHTO H-20 loading, so the lay-by parks cars as normal, and the structural components carry a 100-year design life. The surface footprint is close to zero, a real advantage over daylighted raingardens, which consume usable land at street level.

The experiment inside the experiment

The pilot wasn’t just "build it and watch." It was designed as a controlled contrast. The four trees started fully connected to both street and footpath runoff; one pair was later reduced to a lower-water control fed from the footpath only, with a nearby conventional tree pit standing in as the real-world baseline.

That three-way set-up: high soil volume with high water input, high soil volume with low water input, and an ordinary pit is what lifts the site from anecdote to evidence. It isolates the single variable specifiers most want to understand: how much does water input, on top of soil volume, actually change tree performance?

What’s working

Quite a lot, and it shows. A decade of STEP monitoring found that the bioretention soil functioned as a genuine source control, treating runoff, blunting peak flows, and reducing the volume leaving the site while the trees grew. TSS reduction was consistently above 80%, reaching 96% in monitored events. The fully connected trees outgrew the lower-water control, and both comfortably outgrew the conventional pit nearby. More water in the adequate soil produced more canopy. The causal chain, on which the whole approach rests, is soil volume plus water in, canopy and stormwater capture out is visible on the ground.

Two findings are worth dwelling on. Road salt, the great fear of anyone proposing to send street runoff into planting soil, did not wreck the system; the salt load diluted and flushed through the soil column over time. And the buried structure asked very little of the city in maintenance, unlike surface bioretention that needs regular visits for litter and weeding.

The headline result is standing there in the lay-by: one tree with a canopy spreading something like twelve meters, out past the parked cars and into the traffic lane. On a street like this, that is a lot of shade, interception, and cooling from an asset most people never notice.

What’s missing and what eighteen years reveals

This is where a current site visit earns its keep, because the honest picture is more useful than the glossy one. None of what follows is a failure of the concept; it’s what maturity actually looks like.

The monitoring has gone quiet. The sensing pits that made this a measured experiment are no longer maintained: covers improvised with a cement block in one and a traffic cone in another, the instrumentation inside apparently long dead. The thing that made Queensway valuable, its data, has been allowed to lapse.

Some inlets are now sealed. At least one pair’s catch basins has been concreted over, and others appear heavily silted below the grate. Whether that reflects the planned low-water control arrangement, routine end-of-study decommissioning, or simply deferred maintenance isn’t something the street alone can tell you.

The west system's catch basins were deliberately taken offline prior to monitoring commencing in 2012 — a planned design decision to create the low-water control condition, not deferred maintenance. The east system's inlets remain in place but appear heavily silted below the grate, raising a practical question about whether water is still moving through the system as designed. Either way, what the street shows you from the kerb is ambiguous without the documented record behind it.

One species is thriving; its partner is struggling. Within each pair, the standout tree is a different species from its neighbor, and the weaker one shows the classic stress signals: a thin canopy and a flush of epicormic growth from the base. The two species are American Liberty Elm and Freeman Hybrid Maple, with the Elm being the stronger performer. Same soil, same water, very different results. The gap shows in the trunks as much as the canopies: in the June 2026 photographs, the elm's stem reads as roughly double the caliper of the maple beside it, on the order of an estimated 300–400 mm against perhaps 150–200 mm, though that's a visual estimate from the site, not a measured figure.

Which points the finger at species selection as much as at the system. The street’s planting palette is set out below; matching specific species to the strong and weak individuals in each instrumented pair is the obvious next step.

Planting schedule for the Queensway streetscape. The four instrumented pilot trees are documented in the STEP case study as two American Liberty Elms (Ulmus americana 'Libertas') and two Freeman Hybrid Maples (Acer × freemanii) — one of each per pair. The wider streetscape planting schedule shown above covers the full block and includes additional species; the four pilot trees are a subset of that. The Elm is the stronger performer in both pairs. Whether that reflects stormwater tolerance, salt tolerance, or simply the species' genetic resilience in a Toronto street environment is a question worth carrying into the next pilot.

The surface is showing its age. There’s some kerb and paver displacement at the tree openings, unsurprising for eighteen-year-old infrastructure on a busy lay-by, and worth distinguishing from the classic root-heave that gets healthy street trees removed. Here, the generous below-ground volume is doing its job; the wear is at the surface interface, which is a detailing problem, not a rooting one.

And there’s no institutional memory on display. No plaque, no sign, no obvious owner. A world-first sits anonymously on a suburban high street.

Why does this keep happening?

Queensway isn’t unusual in being forgotten; it’s typical. The pattern repeats because the value of these projects is split across budgets and departments that rarely share a ledger. Canopy and amenity sit with parks and landscape. Stormwater sits with water and civil. A pilot that succeeds on both counts, as this one did, can fall between mandates the moment it needs ongoing care, because no single line item owns it.

Add two structural problems. Monitoring is usually funded for a fixed study window; when the grant ends, the sensors go dark even though the asset keeps performing. And pilots tend to ride on individuals here, two engineers who believed in it. When the champions retire, the knowledge often retires with them. It is a recognizable failure mode in how green infrastructure moves from pilot to standard practice: we are good at building the first one and poor at carrying it forward.

Where the disciplines meet: soil volume as the shared currency

If there’s one concept that dissolves the silo, it’s soil volume. The same cubic meters of engineered soil are, simultaneously, the rooting volume a landscape architect needs for canopy and the bioretention media volume a stormwater engineer needs for treatment and detention. One number, two budgets, two sets of benefits.

Get specific and it gets more persuasive. Void ratio governs how much usable soil a structural system actually delivers under load. Infiltration rate and media composition govern treatment and how fast the system draws down between storms. Detention versus infiltration is a design choice, not a given. These are the parameters where landscape and civil objectives are the same objective and where a project either pays for itself twice or not at all. It’s also why the honest cost-versus-value conversation has to be a lifecycle one, not a day-one capital comparison.

Lessons for the next pilot

Queensway is generous with its lessons if you visit it with open eyes:

Design for the handover, not the ribbon cutting. Decide on day one who owns the asset, where it lives in the maintenance schedule, and how the monitoring survives the end of the study grant. An orphaned asset is a predictable outcome, not bad luck.

Match the species to the water regime. The within-pair divergence here suggests the system can be sound, while the planting palette quietly undermines the result. Species tolerance of periodic waterlogging and road salt is a contributing variable alongside the stormwater connection, not a footnote to it. Species choice deserves the same rigor as engineering, the kind of specification discipline landscape architects bring to planting.

Protect and standardize the instrumentation. If the data is the point of a pilot, the access points and sensors need covers, an owner, and a budget that outlasts the launch.

Keep the record alive. Signage, an asset register, a public case file, anything that means the knowledge doesn’t retire when its champions do.

Eighteen years on, the trees are still making the argument their designers hoped they would. The shame is that so few people know to look.

Posted by Sean Corrigan on Thu, Jun 18, 2026 @ 1:54 AM

Holloway Green Street San Francisco

San Francisco's Holloway Avenue is one of North America's reference green-street projects. We walked all eight blocks. The stormwater engineering is genuinely good — but the project added no trees, and connected none of the few existing ones to the water it harvests. Here's what that teaches the rest of us.

On a bright, cloudless Monday morning in San Francisco's Ingleside neighbourhood, we walked the full length of Holloway Avenue's Green Street project — eight blocks, Ashton to Lee, end to end. We had no role in building it. We went to learn from it.

The design routes street runoff through a treatment train, and it's worth understanding the sequence before judging the outcome.Rain falling on the porous concrete parking lanes infiltrates straight through the surface rather than sheeting off — pervious concrete carries a high void ratio that gives it a strong initial infiltration rate, sending water into an aggregate storage layer below instead of into the gutter. Runoff from the sealed traffic lanes is handled separately: kerb inlets and flush kerb cutouts at the intersection bulb-outs divert it sideways into bioretention rain gardens, where engineered filter media and native planting slow, filter, and infiltrate it. What the street doesn't pass to the sewer, it returns to the watershed. That sequence — infiltrate where you can, divert and treat the rest — is what the 77–78% figure represents.

The installation is holding up well. We saw no pavement failures, no cracking, no significant structural deterioration. The signage is excellent — clear, educational, well designed; corner boards explain the catchment, the watershed history, and how the integrated system functions. Someone put real care into telling this story to the neighbourhood.

It is, by any fair measure, a serious and successful piece of stormwater infrastructure. It has become a reference project for municipal pervious concrete across the region, and the SFPUC deserves credit for building it, monitoring it, and publishing the data.

Standing in the middle of Holloway Avenue on a sunny morning, there's almost no shade. There are street trees — perhaps ten across the full eight blocks — but every one of them pre-dates the project. The Green Street planted no new trees, and connected none of the existing ones to the stormwater system it built around them. The canopy that's there is a leftover, not a design decision.

The rain gardens in the bulb-outs carry native plantings at ground level — some well established, some still sparse — and they add real greenery to the streetscape. But ground-level planting and canopy are not the same thing. Holloway is a bicycle corridor linking Balboa Park BART to San Francisco State University; students, commuters and residents use it every day. On the day we visited the footpath was exposed for almost its entire length — the handful of mature trees, spaced across eight blocks and never part of the water design, do little for a corridor this long. Little overhead relief, almost no cooling canopy — just pavement, painted kerbs, and low groundcover, and a few trees the project left untouched.

The systems are also maintenance-dependent in ways worth naming. Pervious concrete clogs as fines fill the surface voids, so it needs periodic vacuum sweeping to hold its infiltration rate; some panels showed staining consistent with early clogging, and an infiltration test would confirm whether rates have dropped. The bioretention beds need regular weeding — we spoke with a crew hand-weeding the day we visited. The investment, capital and operational, is substantial. Which makes the absence of canopy not just an aesthetic gap, but a missed return on ground that was already dug, drained, and paid for.

This is not a Holloway problem. It is a pattern across the industry.

The brief that governs most green-street projects is written by water engineers and stormwater planners — and water engineers solve water problems. Trees, when they appear at all, are an afterthought: a landscape line item considered after the drainage design is locked, and often the first thing cut when budgets tighten or subgrade conflicts arise. The result is streets that perform hydrologically and fail humanly — streets that manage rainfall but bake their residents.

There's also a hard-won engineering reason for the caution. Street trees in paved environments routinely fail: roots restricted by compacted subgrade can't reach the volume they need, trees decline, heave pavement, and get removed. Engineers who have watched that happen learn not to specify them. The instinct is rational. It's the assumption underneath it — that a tree can survive in a small pit of compacted soil beneath a hard surface — that need srevisiting.

Shade and stormwater are not competing objectives. They are complementary — and the place they meet is the soil profile under the pavement.

A mature canopy reduces surface temperatures by intercepting solar radiation before it reaches the pavement, cuts the urban heat island effect, and intercepts rainfall at source — research consistently shows canopy cover reduces stormwater runoff volumes before water reaches a drain. It does this for decades, compounding as the tree grows.

Delivering that canopy on a hard-paved street is an engineering problem with an engineering answer. Suspended-pavement and structural soil-cell systems provide the rooting volume a large tree actually needs — typically in the order of 15 to 30 cubic metres of uncompacted soil — inside the same footprint as the surface infrastructure, protected from compaction and surface loads. Critically, that same engineered soil profile can be designed into the treatment train: detaining, filtering and slowly releasing stormwater, so the tree pit functions as bioretention in its own right.

The Holloway opportunity, in engineering terms: the subgrade was already being excavated and the drainage path already engineered. A structural soil profile beneath the parking lanes could have carried canopy trees and contributed detention/infiltration to the same catchment — shade and stormwater from one excavation. There were even mature trees already standing on the street to design around. The project did neither: no new canopy, and no link between the trees already there and the water now running past their roots. The opportunity was there twice over. It wasn't taken.

Green infrastructure that doesn't grow tall enough to cast a shadow is only doing half the job.

The most useful change isn't a product or a detail — it's when the question get sasked. The next generation of these projects needs a different brief at the outset: not only "how do we manage the water?" but "how do we make this street liveable?" The answer to the second almost always involves trees, and trees, planted in the right conditions with adequate soil volume, answer a good part of the first. The tools exist. The knowledge exists. What needs to change is the order of operations — bringing canopy and water into the same conversation before the drainage design is set.

Holloway is a genuine achievement, and a generous teacher. It shows how well the water side of the discipline now performs — and exactly where the next gain is waiting.

Posted by Sean Corrigan on Tue, Mar 11, 2025 @ 7:06 AM

Why Do Urban Trees Need Engineered Deep Soil Zones?

Engineered Deep Soil Zones (EDSZ)

EDSZ are the only guarantee that both trees and engineered pavements can co-exist in harmony. Here's how EDSZ solve these problems:
1. Maximises uncompacted soil volume with optional access to oxygen and water
2. Creates the space needed to grow large structural tree roots
3. Directs deeper root growth below the pavement (instead of sideways)
4. Protects essential infrastructure from root pavement heave
5. Engineered for load bearing and stability of the tree pit area
6. Allows for uninterrupted integration of underground services and pipes
7. Integrates with stormwater harvesting

Supports circular economy manufacturing : made from 100% recycled plastics, sourced from local supply chains, in the USA and Australia
Built to last: Stratavault is a multi-generational solution for urban greening, designed to support healthy tree growth for centuries (as long as the tree species will live)
Highly modular system: the only that supports adaptation on site to keep the project on track when the unexpected occurs during excavation
Fastest installation: modules ‘snap-together’, providing highly efficient assembly time on site (avg approx 1:50 mins/m³)
Integrates with utilities: offers the largest apertures in the market to accommodate pipes up to 300mm in diameter and most underground utilities

Large apertures: supports easy soil loading and proper light foot-compaction to achieve the essential 60-70% MDD for optimal growing condition
Stormwater smart: paired with Strataflow kerb inlets, road runoff and rainwater can be captured and redirected through a perforated pipe system, passively irrigating trees and helping to reduce urban heat
High load bearing compliance: Stratavault is designed and tested to support up to 450kPa/65.3psi loads, meeting HS20 and W80 load compliance standards
Achieves shade canopy cover in half the time: compared to structural soil solutions, Stratavault delivers a healthier, faster growing tree, every time.
Extreme weather event resilient: Stratavault has been tested in cyclonic conditions with zero loss of trees

In 1972 Cyclone Tracy decimated around 80% of business, buildings and infrastructure in the City of Darwin and 90% of tree canopy. Over the decades that followed, the City of Darwin rebuilt in every way imaginable, including replacing tens of thousands of lost trees. So when Darwin lost 70% of its trees again in 2018 during Cyclone Marcus, local government authorities knew they needed to look closely at their tree planting methods to ensure trees planted in the future had a better chance of surviving future extreme weather events.

The investigation into the 10,000 trees lost and those that survived, revealed the lost trees were highly vulnerable due to:

Tree protection zones being ignored during development works
Undersized, shallow tree pits that didn't allow for structural root development
Isolated trees lacking group protection
Poor irrigation practices making the trees weak
Inappropriate species selection unsuitable for the climate

For the landscaping of Garramilla Boulevard, the City of Darwin set a target of 15 years to achieve joined tree canopy spanning the road and in partnership with CLOUSTON Associates - a division of Beveridge Williams, they chose Citygreen Stratavault soil cells to make it happen. After just 4 years and 5 months, the trees are exceeding all healthy growth expectations, already starting to meet in the middle.

New Orleans is no stranger to the devastation of hurricanes so when the Four Seasons were planning their lush landscaping for their $500m property in New Orleans, they chose Stratavault to deliver climate resilience and stormwater management.

Stratavault sits below the entire paved area of the impressive hotel entrance to handle the roof runoff from half of the building which is then controlled by a weir to release water at a desired rate to meet the city's stormwater code.

Most of New Orleans is actually below sea level, making flooding a constant concern. The stormwater code of the city of New Orleans requires that new construction is able to manage water for a 10-year, 24-hour storm event. This requirement translated into 1,000 Stratavaults installed at varying depths between four- to six-feet deep.

The 16th Street Mall, a key downtown corridor, has been vital to Denver since 1982. However, its original soil cell system began deteriorating, leading to safety issues and degraded
surfaces.

To address this, the Downtown Denver Partnership selected Citygreen's StratavaultTM soil cells and InvisigrateTM tree grates as part of a $150M revitalization, to enhance urban tree health and ensure infrastructure durability in a mixed zone trafficable area.
Together, the Stratavault soil cells and tree grates prevent potential damage to the surrounding infrastructure (pavement and services), ensuring a harmonious coexistence between hardscape and meaningful green canopy.

Posted by Sean Corrigan on Thu, Jul 06, 2023 @ 2:16 AM

Slow Aging by Planting more Trees

Does Planting Trees Slow Aging?

In an increasingly urbanized world, where more than half of the global population resides in cities, the importance of urban green spaces cannot be understated. Parks, green roofs, and community gardens offer not only aesthetic appeal but also critical ecosystem services. A recent study in ScienceAdvances has documented the potential benefits of urban greenness for healthy aging, including improved cardiovascular health and reduced mortality rates. While physical activity and social interactions have been suggested as potential pathways linking greenness to health outcomes, the underlying molecular biological mechanisms remain unclear.

Epigenetic modifications, such as DNA methylation levels, have emerged as a promising avenue for understanding the link between environmental exposures, health conditions, and aging. Accumulated exposure to environmental factors can lead to DNA hyper- or hypomethylation, ultimately influencing human health. Epigenome-wide association studies have identified regions of DNA methylation that are associated with residential greenness and are implicated in physical activity, mental health, metabolic diseases, and neoplasms. DNA methylation-based biological age, known as epigenetic age, has been proposed as a predictive marker for age-related health outcomes.

Previous studies have established associations between epigenetic age and cardiovascular disease, cancer, and mortality, as well as various lifestyle and exposure factors. However, the relationship between greenness exposure and epigenetic age has been understudied, with only one cross-sectional study conducted thus far. Furthermore, no studies have examined the role of race and sex in the association between greenness and epigenetic age, which is crucial for understanding and reducing disparities in greenness exposure and its associated benefits.

To address these gaps, the findings explore the associations between long-term greenness exposure and epigenetic age, considering race, sex, and neighborhood deprivation as effect modifiers.

Results

Characteristics of Study Participants

Over 900 participants were included, with a mean age of 45.3 years. The cohort consisted of 376 Black participants and 548 white participants, with 453 men and 471 women. Approximately 54.5% of participants had parks within a 5-km radius of their residential address. The mean Normalized Difference Vegetation Index (NDVI) value within the 5-km buffer radius 1 year before the Year 20 visit was 0.38 . Participants with parks within 5 km had slightly lower NDVI values compared to those without parks . Moderate correlations were observed for epigenetic age acceleration.

Association between Greenness and Epigenetic Aging

Their analysis revealed a significant association between 20-year exposure to greenness, and epigenetic aging. Greater greenness was associated with slower epigenetic aging, suggesting a potential protective effect of urban green spaces on the aging process. However, when considering racial disparities, we found that Black participants had less surrounding greenness compared to white participants.

This disparity in greenness exposure led to an attenuated association between greenness and epigenetic aging in Black participants, compared to white participants. This highlights the need to address and reduce disparities in greenness exposure among different racial groups.

Furthermore, the association between greenness and epigenetic aging was influenced by neighborhood socioeconomic status. Participants living in disadvantaged neighborhoods showed a stronger association between greenness and epigenetic aging, with compared to less disadvantaged neighborhoods. These findings underscore the importance of considering neighborhood characteristics and socioeconomic factors in understanding the relationship between greenness and epigenetic aging.

Overview

The results of the study provide valuable insights into the associations between urban greenness and epigenetic aging. The observed relationship between greater greenness and slower epigenetic aging suggests that urban green spaces have the potential to promote healthy aging and mitigate the effects of biological aging processes. This aligns with previous research highlighting the benefits of green spaces for cardiovascular health, mental well-being, and overall mortality reduction.

However, it is crucial to acknowledge and address the existing inequalities in greenness exposure. The findings also highlighted the greenness gap between racial groups, with Black participants experiencing lower levels of greenness compared to their white counterparts. This highlights the need for equitable distribution of green spaces and environmental resources to ensure that all communities can access the health benefits they provide.

Neighborhood deprivation, characterized by lower socioeconomic status and limited resources, may exacerbate the impact of environmental exposures on health outcomes. Recently at our event 'Where the Shade hits the Pavement' Dr. Kim Loo spoke about the link between lack of environmental exposure and the health effects of at risk people and why efforts should be directed toward promoting green infrastructure and urban planning strategies that prioritize the creation and maintenance of green spaces in disadvantaged areas.

Overall, the study contributes to the growing body of evidence supporting the role of urban greenness in healthy aging. By understanding the complex interplay between greenness, epigenetic aging, and social determinants of health, we can inform policies and interventions aimed at creating more equitable and sustainable urban environments that promote the well-being of all individuals.

Read the full report.

Key Takeaways

Inequalities in Urban Greenness and Epigenetic Aging:

The study examines the association between 20-year exposure to greenness and epigenetic aging in a large, biracial (Black/white), U.S. urban cohort.
Greater greenness is associated with slower epigenetic aging.
Black participants have less surrounding greenness and a weaker association between greenness and epigenetic aging compared to white participants.
Participants in disadvantaged neighborhoods show a stronger association between greenness and epigenetic aging compared to those in less disadvantaged neighborhoods.
Urban green space provides critical ecosystem services and potential benefits to healthy aging, including better cardiovascular health and lower mortality.
Long-term exposure to greenness has a relationship with slower epigenetic aging, with different associations observed based on race and neighborhood socioeconomic status.

The study suggests that greenness exposure in urban areas is associated with slower epigenetic aging, but the relationship varies by race and neighborhood socioeconomic status. Black participants and those in disadvantaged neighborhoods experience lower levels of surrounding greenness and have attenuated associations between greenness and epigenetic aging. Understanding these inequalities and their implications can help inform efforts to reduce disparities in greenness exposure and promote equitable access to the health benefits of urban green spaces.

Posted by citygreen_editor on Thu, Jun 22, 2023 @ 1:29 AM

How Much Soil Do Street Trees Need?

How much soil do street trees need? It's a question that frequently arises in the workshops we conduct around the world.

Striking the perfect balance is essential to provide street trees with the optimal conditions for their growth, including ample nutrients, sufficient space for root expansion, and effective water drainage. In order to fully comprehend the requirements of soil and the intricate interactions occurring underground, several crucial aspects deserve our attention.

A commonly used field reference for soil is: two cubic feet of loam soil for every one square-foot of tree canopy (at tree maturity). Simply stated, a mature tree that provides 400 square-foot of canopy should be planted with 800 square feet of quality soil. Studies demonstrate that the quality loam soil grows trees that have 300 times more leaves and are 1.7 times taller than those grown in compacted soils. (Magill & Leake, 2013)

Let's explore the key considerations when it comes to soil and the vital role it plays in supporting healthy and thriving street trees.

Root Zone Protection

Tree roots will explore looking for water, nutrients and space for growth. As long as there is adequate moisture, nutrients and oxygen they will extend far beyond the canopy area. Compaction and damage to the root zone can severely impact a tree's health and longevity. Implementing measures like root barriers, and tree grates can help preserve the critical root zone and promote the overall well-being of street trees.

Soil cells like Citygreen Stratavault are purposely designed to house high quality uncompacted soil and tree roots under urban environments like roads, and pavements. Soil cells offer the tree ideal underground growing conditions while ensuring the tree roots are contained to avoid impacting surrounding urban infrastructure.

Related: How Tree Roots Behave

Soil Composition

The composition of soil plays a crucial role in supporting tree growth. It should strike a balance between water-holding capacity and drainage. Soil should be well-draining to prevent waterlogging and root rot, yet retain enough moisture for trees to access during dry periods. The right mix of organic matter, minerals, and proper pH levels is essential for optimal tree health.

Posted by Sophie Nichols on Wed, Sep 14, 2022 @ 5:39 AM

How to Manage Stormwater Sustainably

An article published online by editor and writer Melissa Denchak highlighted some shocking stormwater statistics coming out of America. Denchak stated that ‘an estimated 10 trillion gallons of untreated stormwater runoff, containing everything from raw sewage to trash to toxins, enters U.S waterways from city sewer systems every year, polluting the environment and drinking supplies… [with] runoff causes significant flooding as well.’ (Denchak 2022).

Why is Green Infrastructure Important for Managing Stormwater?

Green infrastructure sets out to replicate the circular economy of the natural environment. Attempting to mirror the efficient and self-renewing processes found in nature.

In urban settings, green infrastructure encompasses a variety of water management practices, such as treepits, planted verges, bioretention pits, swales and other measures that capture, filter, and reuse stormwater. Essentially, green infrastructure replicates natural hydrological processes using soil and plants to slow down, recycle and clean stormwater runoff.

What is Stormwater Runoff?

Stormwater runoff is the product of a rain event causing water to flow across the land into sewers and waterways. With the expansion of our bustling cities and the widening sprawl of our urban areas, there are more impermeable surfaces than ever, increasing the intensity of stormwater runoff.

According to Denchak, ‘the average city block can generate more than five times as much runoff as a forested area of equal size’ (Denchak 2022).

What is an Example of a Successful Green Infrastructure Project?

Denchak proposed that New York’s Staten Island Bluebelt was the ‘first and largest green infrastructure project in the U.S.’ A rapid increase in population size saw the Island struggling to deal with sanitary waste and stormwater runoff.

The Bluebelt project ‘helped solve these issues by preserving streams, wetland areas, and other drainage corridors (Bluebelts) that use natural mechanisms to capture, store, and filter stormwater’ (Denchak 2022). Nowadays, the Bluebelt comprises more than 14,000 acres and can temporarily hold and filter as much as 350,000 gallons of rainfall.

WSUD water drain feeding water into the tree soil vault

How does Citygreen Implement and Manage Stormwater Projects?

Over the past three decades, Citygreen has made significant investments in stormwater infrastructure projects. We learnt early on that mimicking natural systems to manage rainfall, is the most cost-effectively way to deal with stormwater runoff.

An example of a green infrastructure design that Citygreen has developed is the Strataflow™ system. Instead of a traditional bioretention basin, Citygreen’s Strataflow™ uses an underground structural soil vault system, which delivers a high standard of stormwater treatment with a completely natural look. To any passer-by, what you see is a healthy, flourishing tree surrounded by a grassy verge, but beneath the ground is an advanced WSUD(water sensitive urban design).

sustainable stormwater management soil cell

This design starts with a traditional drain or catch basin or the Strataflow Kerb Inlet; this device sits in the road kerb alignment, retaining the inherent structure of the concrete kerb. The inlet has a grate (acting as a screen) to stop larger-sized pollutants from entering the system, which inhibits healthy tree growth.

The inlet lets water from the road carriageway flow through the front grate of the drain at a capacity of up to 18 litres/ 5 gallons per second. This allows the inlet to minimise pollutants entering waterways and reduce flood risks by controlling the stormwater flow entering our city’s underground drains.

strataflow kerb inlet sitting in the kern

When the water flows through the street, it enters through the inlet and flows underground. From there, the stormwater reaches the advanced structural soil cell system, where the stormwater is stored, filtered and distributed effectively for the benefit of urban trees and proper stormwater management.

The inlet ensures the water drains down at the correct optimal depth beneath the pavement height. From there, the stormwater reaches the structural soil cell system and the tree’s root system, where the stormwater is stored, cleaned and distributed effectively to increase urban tree growth and proper stormwater management principles.

animation of how strataflow works to benefits stormwater usage

Essentially, Strataflow™ utilises readily available stormwater rather than potable water to irrigate street trees, which improves the vitality of trees and reduces the impact of stormwater and stormwater contaminants on the local environment, all while maintaining a high natural presentation.

Stormwater Management Case Study

Pemberton is a small mountain town located 20 minutes North of world-renowned ski resort Whistler in Beautiful British Columbia, Canada.

In 2019 the town of Pemberton was awarded a government grant to upgrade ageing infrastructure and give their tourist town a facelift. Pemberton had some issues with flooding which they were keen to fix and at the same time wanted to create an inviting and enjoyable experience for the visitors and residents of the town.

One of the solutions was the Stratavault system, this system was placed underneath all sidewalks for two reasons. The first was to collect the mass of snow run off and rainfall that would typically flood the town, slow this water down and clean it with the soil held in the Stratavault system then push excess water into a nearby pond where it could be used for irrigation purposes throughout the town. The second was to hold enough soil so the trees that were planted in urban environments could have access to nutrient-rich soil for many years to come.

Soil Used for Stormwater Filtration

Rainwater Runoff

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Posted by Sophie Nichols on Tue, Aug 09, 2022 @ 1:48 AM

Are Green Living Walls Challenging to Maintain?

A common concern with green living walls is maintenance, which may put people off investing in this high-value asset – but don’t be afraid!

At Citygreen, we can provide continual support from installation to maintenance requirements for our Citygreen™ Living Wall system so consumers know they are getting the best return on their investment.

Below we have answered the most common concerns people have when they think about living green walls.

Or book an initial free consult with our Green Living Walls expert Grant Radbourne who has over 10 years in delivering green walls over all the world.

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Do living walls create water damage to my building?

The Citygreen™ Living Wall systems have a waterproof membrane that prevents water from penetrating substructures.

Do living walls use too much water?

Compared to other greenspaces, the water usage for living walls like Citygreen’s™ Living Wall system is low, as it typically requires two litres per square metre per day to irrigate the wall.

Irrigation is the most critical part of a successful living wall system. In our Citygreen™ Living Wall system, we have designed the vertical irrigation lines to be embedded in a moisture retention layer for optimal water efficiency.

An optional recirculated irrigation system can be installed to achieve further water-saving outcomes.

Will a living green wall work in my ‘space’?

Citygreen’s™ Living Wall system is available in nine standard panel sizes; however, it can also be engineered to retrofit walls on unique buildings. The system is also the lightest on the market, weighing only 35kg per square meter, fully planted and saturated. This means that no additional support structures will be required on small-scale projects.

Also, as discussed above, wherever on the wall the system will be mounted does not require additional waterproofing, as a waterproof membrane is included in the design.

Do living walls require too much ongoing work?

The Citygreen™ Living Wall system comes with remote control monitoring. An advanced automated system ensures the consumer can control the system’s moisture, temperature, ph levels and general conductivity on their computer or phone. This remote capability extends to the automated refilling of the water tanks connected to the wall.

Liquid fertilizer concentrates can also be used in the automated irrigation system to feed the plants. This means that the consumer can easily maintain plant health, but they can also control the pace and vigour of new growth.

Are living walls too expensive?

Citygreen can provide a detailed cost estimate, ensuring that the most suitable living wall solution aligns with your budget.

Are living green walls too hard to set up?

At Citygreen, we will perform a site analysis to determine if any technical installation requirements will be required. For example, if we believe that the indoor installation site is too dark, we recommend using artificial lighting, which we can also deliver and install.

Depending on the site’s location: indoors, outdoors, small-scale, or commercial scale, Citygreen will also assist with plant selection to help find the best species that will thrive into the future.

Assistance will also continue past the set-up stage to the maintenance process, as Citygreen can help will pruning and any ongoing concerns and issues that the living wall may have post-construction.

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As shown above, Citygreen is an expert in every phase of the design and implementation of living green walls– reach out to Citygreen for a Design Workshop today.

Posted by Sophie Nichols on Fri, Jul 22, 2022 @ 2:39 AM

The Role of Trees in Stormwater Management

WSUD tree design with stormwater entering a soil vault tree pit

In built-up urban areas, trees can help restore pre-development water flows and remove pollutants and filter water. Trees act as natural filtration machines, which can hold, release, and clean water through soil and evaporation.

A question often asked regarding urban trees and water capture is, can stormwater runoff from roads be too polluted for the trees to use?

Yes, stormwater can be very polluted, as large amounts of debris, particulates, and rubbish can suffocate a tree and prevent water and nutrients from reaching the tree’s root system for absorption.

Gross pollutants, such as plastic rubbish or vehicle parts, can largely be filtered out often by screens, like stormwater grates.

Smaller than gross pollutants are total suspended solids (TSS). TSS refers to solids suspended in water or wastewater that can be trapped by a filter. TSS can include various materials, such as silt, decaying plant and animal matter, industrial wastes, and sewage. High suspended solids concentrations can cause many problems for stream health and aquatic life.

Then there are soluble or water-borne pollutants, which are difficult to filter out economically. These pollutants can cause severe damage to ecosystems.

Storm water systems can be installed, which prevent these pollutants from accumulating in our water streams. Citygreen offers a revolutionary range of stormwater management solutions that prevent water pollution and make it easier –and more affordable–to manage and re-use stormwater.

Why use Trees for Stormwater Management?

Trees offer a large value add when compared to traditional stormwater management systems. Trees for stormwater management offers numerous advantages in urban areas. Firstly, trees act as natural water managers by absorbing excess rainwater, mitigating the risk of flooding and erosion. Their extensive root systems serve as filters, trapping and breaking down pollutants present in stormwater, which ultimately improves the quality of water entering local water bodies. Additionally, trees reduce the volume of stormwater runoff, easing the burden on municipal drainage systems. Moreover, these green giants contribute to urban cooling by providing shade and releasing moisture through transpiration, enhancing the overall urban microclimate and conserving water by reducing evaporation from impermeable surfaces.

Case Study: Urban Trees for Shade & Stormwater Management at Kinsmen Sports Centre in Edmonton

Furthermore, the aesthetic and social benefits of urban trees are noteworthy. Well-maintained green spaces with trees enhance the quality of life for urban residents and promote community well-being. Trees also create habitats for wildlife, contributing to urban biodiversity and ecological balance.

Engaging communities in tree planting and maintenance fosters a sense of ownership and responsibility. From an economic perspective, effective stormwater management with trees can lead to cost savings by reducing the need for extensive stormwater infrastructure. Lastly, utilizing trees that align with regulatory compliance, as many cities and regions have specific stormwater management requirements where integrating trees into stormwater management strategies not only addresses practical concerns but also contributes to the creation of attractive, sustainable, and resilient urban environments.

How Do Trees Clean Stormwater?

Trees play a vital role in filtering water through a series of natural mechanisms. Their root systems, for instance, engage in a process called root uptake, where they absorb water from the soil, including groundwater and rainwater. This not only helps in managing excess water in urban areas, reducing the risk of flooding and waterlogging, but also contributes to water purification. As water moves through the soil surrounding tree roots, it undergoes natural filtration. The soil acts as a powerful filter, capturing impurities, sediments, and pollutants present in the water, thereby improving water quality. Microorganisms in the soil and on tree roots further aid in this process by breaking down organic matter and pollutants into less harmful substances.

Additionally, trees are proficient at nutrient uptake, extracting essential nutrients from water for their growth. In doing so, trees indirectly remove excess nutrients like nitrogen and phosphorus, common water pollutants. Some tree species excel at phytoremediation, absorbing and storing pollutants such as heavy metals and chemicals, thus contributing to water purification and helping prevent water pollution.

Trees release water vapor through transpiration into the atmosphere, mitigating local flooding risks by reducing runoff volume. Trees also help retain sediments and prevent erosion, which can lead to waterbody sedimentation and, consequently, improved water clarity. Altogether, trees collectively enhance water quality by naturally reducing contaminant levels, pollutants, and sediments, making a significant positive impact on the health of water systems.

Case Study: Pelican Waters

Pelican Waters, a residential estate located on the Sunshine Coast of Queensland, Australia, has been trialling Citygreen’s Strataflow™ system with so far great success.

This new development aimed to use the advanced water-sensitive urban design (WSUD) and improve sales of lots near bioretention basin. Research has shown that preserving natural features in residential developments can increase the value and sale price of lots.

Instead of a traditional bioretention basin, Citygreen’s Strataflow™ uses an underground structural soil cell system, which delivers a high standard of stormwater treatment with a completely natural look.

To any passer-by, what you see is a healthy, flourishing tree, surrounded by a grassy verge, but beneath the ground is an advanced WSUD.

The Strataflow™ is a specialised design ‘hybrid’ tree pit, combining the best urban forestry for sustained and healthy tree growth with fully functional stormwater management – including filtration and flow management.

These designs may start with the Strataflow Kerb Inlet. This device sits in the road kerb alignment, retaining the inherent structure of the concrete kerb. The inlet has a grate (acting as a screen), to stop larger-sized pollutants from entering the system, which inhibits healthy tree growth.

The inlet lets water from the road carriageway flow through the front grate of the drain at a capacity of up to 18 litres/ 5 gallons per second. This allows the inlet to minimise pollutants entering waterways and reduce flood risks by controlling the stormwater flow entering our city’s underground drains.

When the water flows through the street, it enters through the inlet and flows underground. From there, the stormwater reaches the stratavault system, where the stormwater is stored, filtered and distributed effectively for the benefit of urban trees and for proper stormwater management.

The inlet ensures the water drains down at the correct optimal depth beneath the pavement height. From there, the stormwater reaches the structural soil cell system and the trees’ root system, where the stormwater is stored, filtered and distributed effectively for the benefit of urban trees and for proper stormwater management.

Essentially, Strataflow™ utilises readily available stormwater rather than potable water to irrigate street trees, which improves the vitality of trees and reduces the impact of stormwater on the local environment, all while maintaining a high natural presentation.

pelican2 Citygreen — *Growth comparison of trees planted in Strataflow (left) and trees planted in grass verge (right)*

Green space for shading and mitigation of heat island effect

Soil Used for Stormwater Filtration

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Looking for a cost-effective and sustainable stormwater solution? Contact our friendly Citygreen Team now by clicking here.

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