Why Plants Are Showing Up On Roofs and Parking Lots

Have you ever noticed a beautifully landscaped median? A rooftop with grass on it? Shrubs and trees in the middle of the parking lot?

Greenery in urban areas is strategic; it was either left or put there to manage something called urban runoff. But what exactly is that?

In addition to many buildings, suburban and urban areas have walking and driving surfaces covered by pavement and concrete. The problem is that these hard surfaces don’t absorb rain and snow. Instead, the volume and velocity of stormwater increases tremendously and is a major source of flooding and pollution. Due to nonporous surfaces like pavement and rooftops, a typical city block generates more than five times more runoff than a woodland area of the same size.

As stormwater flows, it takes pollutants with it, including sediment, oil, grease, toxic chemicals from vehicles, pesticides, viruses and bacteria from pet waste and failing septic systems, road salts and heavy metals. All of these pollutants harm habitats, fish and wildlife populations, drinking water and recreational areas.

So how can a green roof and some shrubbery in the middle of a parking lot prevent this from happening?  Let’s start with green roofs.

The green roof on NC State’s Engineering Building III.

A green roof (also known as a rooftop garden) is a vegetative layer grown on a rooftop. The popularity of green roofs has increased in the United States due to many benefits, including reduced energy pollution, reduced air pollution and enhanced management of urban runoff. A research team in the College of Architecture and Urban Studies at Virginia Tech tested the effects of green roofs on controlling urban runoff and found that green roofs significantly reduced and slowed runoff. The vegetation on a green roof acts as a sponge, absorbing and filtering water that would normally rush down gutters and streets polluting waterways with everything it picked up along the way. That’s one reason NC State has green roofs at Engineering Building III, JC Raulston Arboretum, James B. Hunt Jr. Library, Talley Student Union and Wolf Ridge Student Apartments.

So what about those strategically-placed trees in parking lots? Landscaping around streets and parking lots is not just aesthetically pleasing; it plays a role in managing and preventing urban runoff. Trees capture and store rainfall in their canopies, releasing water into the atmosphere and keeping it off streets. Tree roots promote the infiltration of rainwater into the soil as well as slowing down and reducing pollutants. Trees also have the amazing ability to transform pollutants into less harmful substances.

Both green roofs and urban landscaping are part of what is called a green stormwater system.

A student-designed and -built raingarden outside of Lee Residence Hall on NC State’s campus.

Green stormwater systems manage stormwater and protect water quality by creating areas that mimic nature such as the Design-Build projects that NC State students have developed on campus. The use of low impact development and conservation of natural areas allows for maximized surface roughness, infiltration opportunities and flow paths. Green infrastructure is also cost-effective and has also been proven to increase property values. Grey stormwater systems, on the other hand, use curbs, gutters, drains and pipes to move water quickly away from buildings, roads and other instructure.

NC State’s restored Rocky Branch Creek.

One of NC State’s best models of managing urban runoff is the restoration of Rocky Branch Creek, which snakes through campus paralleling Sullivan Drive and the Rocky Branch Greenway Trail. Once labeled one of the state’s most polluted urban streams, the three-phased, multi million-dollar restoration project stabilized the creek, improved water quality as well as aquatic and wildlife habitat, and integrated the creek into the campus environment through greenway trails.

In addition to these major urban runoff projects, homeowners can play a role in preventing urban runoff. Porous materials are available for driveways, sidewalks and patios. Strive to use native vegetation in your yard and use fertilizers and pesticides sparingly. Sweeping a driveway is better than hosing it down, cars parked in your driveway should be regularly serviced and maintained, and pet waste should always be disposed of properly. Everyone plays a part in preventing the impacts of urban runoff.