Sustainable Cities Collective has re-launched as Smart Cities Dive! Click here to learn more!

A New Way to Plant Urban Trees


At the 2010 GreenBuild, Peter MacDonagh, the Kestler Design Group, James Urban, FASLA, Urban Trees + Soils, and Peter Schaudt, FASLA, Hoerr Schaudt Landscape Architects, argued that without new tree planting techniques that use healthy loam soils, major "one million" urban tree planting campaigns will fail, wasting lots of money in the process. MacDonagh said "urban forestry is broken. We need to remake with a different approach."

Finding out What Works for Urban Trees

Urban trees are now understood to be a central part of green infrastructure systems and provide a range of benefits. They reduce the urban heat island effect, manage stormwater, and provide shade that lengthens the life of materials. In the summer, shadier streets also means lower neighborhood temperatures, which can reduce air pollution that increase asthma rates. "All of these benefits are great, but they won't happen if we keep planting like we have. It will be a mirage," argued MacDonagh.

MacDonagh said larger, older trees are far more valuable than youger ones, so work needs to be done to preserve these and use new techniques to enable younger trees to stay in place longer. Citing data, he argued that a 30-inch diameter breast height (DBH) tree provides 70 times the ecological benefits of a 3-inch DBH tree. For example, a large tree intercepts 79 percent of rain hitting the ground, providing the "best green infrastructure you can find."

The key to preserving larger older trees and keeping younger ones in place up to 50 years or more is to use large amounts of loam or bioretention soils that are 65 percent sand, 20 percent compost and 15 clay silt. These soils are not only the best growing mix for trees, but also filter out heavy metals, phosphorous, and nitrogen most efficiently. Nitrogen runoff can cause algae blooms and kill other life if it's allowed to get to the watershed in large amounts.

The rule needs to be two cubic feet of loam for one square feet of tree canopy. So, for a tree that provides a 700 square foot canopy a designer needs to use 1,400 cubic feet of high-quality soil. These soils can be combined with "silva cells" that prevent soil compaction to enable the growth of tall, healthy trees. To prove this, MacDonagh showed the work of Bartlett Tree Lab's Urban Plaza study, which demonstrated that loam soil grew trees had 300 times more leaves and were 1.7 times taller than those grown in compacted soils. "This is important because the average street tree only lasts 13 years."

To sum up, MacDonough said "codify minimum loam soil volumes, diversify tree species to prevent devastating blights, set minimum canopy targets, and plant small trees properly." Otherwise, "those million tree campaigns will be exercises in futility." 

Overcoming Obstacles in the Built Environment and Dealing with Increased CO2 Emissions  

James Urban, FASLA, said structural soils, which combine broken up rock and soil, have issues so urban tree planters came up with a new idea: suspended pavements. In a new project, Queens Quay, along the Toronto waterfront, these suspended pavements use 48-inch deep silva cells, which kind of look like rubber packing crates, and 1,680 cubic feet of loam per tree. Within the combined loam and silva cells are irrigation systems that move water to the trees. Given the Toronto government was concerned that this system wouldn't work, Urban says they first tested in a small strip and demonstrated that the approach works.  

Here Urban complained about one major obstacle: low tree quality. He argued the "American nursery business isn't doing its job" and one firm tried to deliver trees with "girdling roots, a fatal flaw that would have killed the tree in five years." He added that the current nursery "stock of trees is horrible." If we are going to do million tree campaigns, he asked, "How can we check each one?"

In another project, The Bosque on the new Lincoln Center roof in New York City, Urban worked with Diller, Scofidio + Renfro on planting 30 trees on a roof deck. "There were lots of obstacles — everything is going on in the built urban environment." Urban navigated the shallow roof, elaborate lighting systems, and thin paving on top of the deck. He ended up adding in "geogrids" and gravel that helped ensure the new platform could provide a safe growing environment for trees and also bear the weight of a light pick-up truck or ambulance.

Urban added that in addition to the CO2 emissions created from hauling in those soils, there were also tons of emissions released from the trailers and cranes that were used to install the huge trees. "Are urban trees really sustainable? Our carbon footprint was so large that these trees will never be able to sequester the amount we just put into the atmosphere." He argued landscape architects must stop pretending urban trees sequester carbon when they are actually net-producers of carbon if you factor in transportation and installation. 

Still, many progressive city governments including New York City see massive tree planting campaigns as a core part of their climate adaptation plans. Perhaps the questions are: Is there a way to mitigate uban trees' installation and transportation-related emissions in the short-term with a greener installation technique? If not, does improved long-term resiliency to climate change somehow make up for increased short-term CO2 emissions?

Urban Trees Are Key to Successful Public Spaces

Peter Schaudt outlined his firm's well-regarded Uptown Normal traffic circle (see earlier post) in Normal, Illinois, which was funded by federal, state, and local governments, "so you can imagine the number of meetings." Schaudt decided to create a "people space in the center of a roundabout," which some government officials didn't think would be safe.

Schaudt thinks the new space, which features a set of urban trees, outer lawn, bog water infiltration sytsem, and circular stream filled with cleansed water, represents the "park of the future." Instead of seeming dangerous, the circle interior offers a safe space in large part due to the trees, which separate the cars from the social space. Trees in the traffic circle and nearby streets were also supported by silva cells, loam and drip irrigation, using Urban's approach but on a smaller scale.

The circle's trees were set-up to live a long time – Schaudt says he plans for the "4th dimension — time," and likes to show clients what the site will look like in 25 years.

Lastly, MacDonagh added that well-planted trees are not only more cost-efficient, they also provide more ecosystem benefits. To demonstrate cost-efficiency, he pointed to research conduced by Minneapolis' government, which found that they could either spend $3.5 million on new stormwater conveyance pipes to deal with runoff or spend $1.5 million on silva cell systems. On ecosystem service benefits, another study showed that 13-year old trees planted in standard structured soils had a net cost of $3,000, while a 50-year old tree planted in bioretention soils and silva cells offered $9,000 in benefits over its total lifecycle.