Tree Boxes

Finding the Right Green Solution

Capitol Flexi-pave workers putting down the new sidewalk along 7th Street, SE, near Eastern Market. Photo: Rindy O’Brien

Joni Mitchell’s ballad, “Big Yellow Taxi,” about paving paradise has been the anthem for the environmental community since 1970. But the chorus, “Don't it always seem to go, you don't know what you've got till it's gone,” seems to fit the current debate swirling around the effort to use new materials to replace soil in our public tree box spaces.

The sidewalk in front of Woven History and Fairy Godmother on Seventh Street near Eastern Market is well traveled by residents and the many hundreds of weekend visitors. The tall mature trees give the street character and provide shade on those really hot summer days. Yet, the tree roots have pushed the sidewalk up in a crazy pattern that has tripped up many a walker over the years.

Thus city planners have been searching for a solution that would keep the sidewalk even and safe while finding a way for the tree roots to spread out.

They were quite optimistic a few years ago when they installed the current porous paver panels which they thought would help keep the trees growing, while making the sidewalk a little less bumpy. The current pavers were chosen because they expand to let the tree roots have space and let water seep through, leading to fewer cracked sidewalks. Sadly, the pavers didn’t perform up to expectations, and the sidewalk became even more difficult for pedestrians to navigate.

On October 11, 2013, the city sent its contractor, Capitol Flexi-pave, out to begin work on tearing out the tree boxes and leveling the sidewalk using a new porous material called flexi-pave to once again try and level the sidewalk.

Upon learning that the work involved tearing out the tree boxes, members of the Market Row Association, an association of merchants and owners bordering on Historic Eastern Market, immediately expressed their concerns. An agreement was reached to stop DC’s Department of Transportation from digging up the tree boxes for now. The flexi-pave was poured onto the sidewalk and the situation with the tree boxes will be reevaluated in six months. It was never the city’s intention to remove the trees, as early reports on the situation suggested. But the project as proposed would remove the soil and extra plantings around the trees.

What is Flexi-pave?

Capitol Flexi-Pave says that flexi-pave is an innovative construction paving material that is “dynamic in its physical nature, so it flexes during thermal expansion and use, is immune to freeze and thaw, and sediments do not clog the internal pores, but pass through it during a rain event.” It is made in a process that combines recycled tire and granite chips with an elastomeric binding agent to end up with a highly porous, insulating, flexible paving material. It sounds like something dreamed up by a Star Wars team.

Dr. Jessica Saunders, Director of Technical Services and Research, for Casey Trees, says that the city has been using flexi-pave throughout the city for sidewalks and new playgrounds and she thinks it may be a really good material for the urban environment. “We have found, so far, it doesn’t harm the trees at all,” Jessica said, “and the city has had good success in switching out the old tree soil boxes with the new materials.”

“The city is under new DC water regulations that really don’t give them any choice but to find new ways to address storm-water runoff,” she said. The new regulations started this summer, and are part of an order to meet the federal requirements set by the Environmental Protection Agency in accordance with the Clean Water Act.

Forty-three percent of DC’s land area is impervious, meaning that when it rains, the rain isn’t absorbed into the ground, but rather it rolls off the surface and has to go somewhere. For DC that rain goes into our storm water drains and into the local rivers. Part of the Mayor’s Vision for Sustainable DC released April 2012 aims for DC’s waterways to be swimmable and fishable in the future. To get there, 75% of DC lands must filter or capture rain by 2032.

Flexi-pave is one way that will allow rain be absorbed below the surface where the trees are able to use it. According to staff at EPA, a number of products are starting to be used around the country with great results in allowing plant materials to grow, and letting storm water infiltrate beneath the surface, reducing storm water runoff. The US Green Building Council has endorsed the use of materials like flexi-pave and has a number of projects in Massachusetts and Florida that have received gold ratings for their use of flexi-pave.

The National Zoo used the materials in 2012 at their elephant area after conducting very thorough research on such products and best practices. This spring, tree box projects were completed in Georgetown, and to date the reports on trees and sidewalk have been very positive.

Will it Work?

There is ample research that shows it is an excellent choice if you are choosing to pave a parking lot or put in new sidewalks that don’t have well-established trees and boxes. There seems to be a good track record indicating that new trees which are planted with flexi-pave and receive the special nutrient treatment do well. But it is harder to find research about the ability of mature trees to adapt to this new “soil.”

Dr. Saunders of Casey Trees feels that the trees will do okay as long as trained arborists are part of the team working with the construction workers when laying the new materials. Another tree expert suggested that, given the awful clay soil most of the DC trees have to survive in and the lack of care that most of the city tree boxes get, this new approach couldn’t be any worse.

One thing that does get lost in moving to a total flexi-pave sidewalk is the ability to plant flowers and other greenery around the base of the tree. One compliant about the new look is that it looks unnatural to see a tree just sprouting out of pavement.

Certainly, tree plantings make our sidewalks much more beautiful and allow us to have a seasonal touch to the street. Planting in the tree boxes can be okay if done sparingly, but when too many plants share the box, the trees can lose the competition for water and nutrients.

Six-Month Review

Switching from the tree boxes we have today to city trees that become part of the seamless sidewalk is an effort underway across Washington, DC, and is part of the Sustainable DC vision that Mayor Gray unveiled a year ago. The motive behind the project is a good one.  But, the questions remains whether there has been enough experience with the porous materials to know whether it supports the health of DC’s beautiful trees.

It is good to know that the experts at Casey Trees think it will work.  But prudence may dictate proceeding with some caution to see how the pilot projects in Georgetown and other parts of the city go. There may be a window of opportunity to do so given that the Hine redevelopment project will soon disrupt our streets anyway.  

If you are interested in expressing your thoughts about this new effort or want to be placed on an advisory list, you should contact DC Department of Environment, Natural Resources Administration or DC Urban Forestry Division under the DC Department of Transportation. Both departments can be reached through The local Advisory Neighborhood Commissions (ANC) may also take a specific look at this new trend in upcoming meetings, so you should check their agendas in the days ahead. 

Rindy O’Brien has written for many years on Hill gardening and urban landscaping issues for the Hill Rag. She continues to monitor issues relating to her environmental interest. She serves on a board of directors in Missouri that operates a sustainable working forest. For comments contact

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