Trees are a special quality of DC’s urban environment, but the city’s tree canopy has been shrinking in recent years. A new set of design standards ensures new construction on the roads and in public space includes enough soil so that trees can thrive.

Photo by Dewita Soeharjono on Flickr.

In April 2014, the District Department of Transportation released new green infrastructure design standards for its projects. A major goal is to capture rain directly where it falls rather than dumping it into sewers or rivers, which causes flooding, spreads pollution, taxes sewer systems, and wastes an increasingly valuable resource.

The standards push for green areas to retain stormwater in heavily-paved areas like sidewalks, plazas, and streets. With them, DC joined a growing group of cities like New York, Philadelphia, and Chicago that use “low-impact design” features to help restore ecological function to urban areas.

But the District is distinguishing itself by also requiring a minimum soil volume for street trees, which must now get 600 to 1,500 cubic feet of soil (depending on how large they can grow).

Trees play an important role in our lives

Many of us have fond memories linked to trees. Perhaps you had a favorite climbing spot as a kid or you planted one in the yard of the first house you bought. Maybe you regularly cool off in a tree’s shade during a heat wave or have called the parks department when one was vandalized in your neighborhood.

People instinctively want to be around trees, which makes sense when you consider how good they are for human health. Trees make cities cleaner, more inviting, safer, and more profitable.

According to Casey Trees, a local nonprofit, DC’s tree canopy currently covers 35 percent of its land, down from 50 percent in 1950. Advocates want to to turn that around and reach 40 percent by 2035. An increase of five percent may sound small, but to meet this goal the District will need to add more than 2,000 acres of tree canopy — an estimated 216,000 trees.

It’s not just about the number of trees. While trees are part of almost every DC street, mature urban trees are rare, meaning the city frequently loses out on the value and utility they bring to public spaces.

Mature trees are significantly more valuable than young ones to the ecosystem. A tree with a 30-inch trunk circumference delivers 70 times the air quality benefits of a tree with a 3-inch trunk. And mature trees “intercept,” or prevent from hitting the ground, far more rainwater per year than young ones. That reduces the amount of stormwater that flows into sewers and rivers, which frequently causes flooding and carries pollutants. One model found a 40-year-old hackberry tree intercepted 5,387 gallons of rainfall per year while a 5-year-old one intercepted only 133 gallons — a 40-fold difference.

Soil is key to healthy trees

The long-term success of a tree is fundamentally linked to the quantity and quality of the soil it grows in. To make a real impact the District needs to not just plant trees, but give them the space and nutrients they need to grow to maturity. , soil retains rainwater directly where it falls, which is good because diverted rainwater leads to flooding, spreads pollution, taxes sewer systems, and wastes an increasingly valuable resource.

For context, the average street tree in a typical 4’ x 4’ space can have as little as 75 cubic feet of soil. This is less than one tenth of what experts recommend for long-term tree health. The new policy will guarantee space for roots, and coupled with proper care and maintenence, will enable trees to grow and thrive for decades to come.

There are an increasing number of ways to put soil below streets, parking lots, and plazas to give trees soil even in dense or pedestrian-heavy areas. For example, the new American Veterans Disabled for Life Memorial at Washington Avenue and 2nd and C streets SW, which my company worked on, suspends the pavement with soil cells, modular, stacking units that leave space for soil and roots for 30 trees.

Soil cells at the American Veterans Disabled for Life Memorial. Image from DeepRoot.

DC’s renewed commitment to trees will pay off in the long term

These new green infrastructure standards arise from years of discussion, public comment, and pilot projects. “DDOT has attempted to change the culture of urban stormwater management by applying these standards to all capital improvement and private sector projects in the public space,” said (former) DDOT Chief Engineer Ronaldo Nicholson in a press release in April about the new standards.

Residents might not notice all that much change immediately. After all, trees take years to grow and reach their potential. But our children will.