Turf Magazine - June, 2009

CENTRAL FEATURES

Green Roof Maintenance

Photos by Jeremiah Johnson. The sloped green roof research study installed. The green cloth is a shade cloth to ensure the seed germinates/establishes.

Nearly a decade ago, at a new assembly plant in Dearborn, Mich., Ford Motor Company faced investing millions of dollars in stormwater infrastructure. To help offset this cost, as well as lower energy consumption within the building, Ford partnered with Michigan State University to turn 10.4 acres of roof into green space. Today, it is one of the largest extensive green roof expanses in the country.

Landscaping an area 20 feet (or higher) off the ground can be seen as a real maintenance challenge. The reality, say green roof experts, is that a properly installed green roof is fairly easy to maintain.

There are three types of green roofs: extensive, intensive and semi-intensive. Extensive roofs are lower maintenance and are most popular in areas where roof access is difficult or limited. This type of green roof has soil substrate of 6 inches or less. Semi-intensive green roofs have a variety of species and depths for roots. These systems could have irrigation systems and include plants that require trimming. Intensive systems most resemble traditional gardens and ground landscaping. The root levels usually start at 8 inches and can go as deep as 15 feet, allowing for shrubs and trees and can also include architectural additions like gazebos or ponds. The amount of general maintenance for the green roof will correspond directly with the type of system used. The extensive system is set up for minimal long-term maintenance, while the intensive system will need a great deal of care.

Maintenance also depends on how the roof is installed, explains Jeremiah Johnson of Advanced Green Architecture: “Are you coming in with 100 percent coverage? Are you going to seed it or plug it? Maintenance is dependent on these choices. For plugs, I’d recommend adding irrigation, but if it is 100 percent live growth, you can put it up there and be done.”

Green roofs should be planted in either the fall or the spring. Greg Raymond, managing member of EcoGardens, recommends spring installation from late March/early April through June and fall planting between mid-September through the end of October. These times of the year, he says, allow the plant systems to best utilize natural rainfall and cooler temperatures.

The green roof prior to the installation of the research study on the Old Mission Peninsula in Traverse City, Mich.

Before installing the green roof, Raymond suggests doing a thorough inspection of the original roof. “We do an inspection of the roof membrane, the exposed membrane, the counter flashing, all the details that could deteriorate over time.” He adds that this inspection is something that continues as part of the general maintenance of the roof.

To prevent leaks, Sara Loveland, interim executive director of DC Greenworks, recommends installing a new roof membrane as a first step. Also consider adding gravel or large river rocks around the drainage areas and borders. “That way, if there is a leak, you are only moving rocks, not plants and dirt,” she says.

The type of topsoil is important, as well. According to Greenroofs.com, green roof soils need to be lightweight to meet roof loading weight restrictions and have good drainage while maintaining sufficient rainwater retention. Regular garden soil can’t be used on the roof.

The first two years after installation are the most critical. “That’s how long it takes the sedum to grow to 90 percent coverage,” says Loveland. “However, if you are using pre-vegetated mats, it eliminates the more intensive maintenance period.”

During those years, Loveland says, it is important to assess the roof at least twice a year, checking for plant health and drainage issues. “There might even be spotty sun or shade issues that need to be kept in check,” she says.

Sedum is the most popular plant choice, because of its ability to go for periods of time without water and for its shallow roots.

“Sedum does quite well across the vast majority of the country,” says Raymond. “They are extremely drought tolerant, they are efficient in terms of absorbing and getting rid of water quickly, and most respond well to propagation through cuttings. If you want to thicken your roof, you could snap off cuttings and redistribute them. Sedum is diverse and durable and does a good job on roofs.”

Native plants with shallow roots also work well. “Native plants tend to survive and thrive on their own, which is what you want for green roofs,” says Loveland.

Plants that should be avoided are shrubs and ground cover that are invasive. “You want to have a diverse culture,” says Raymond. “Then, if you have a declining roof, it is just a portion to worry about, not all of it.”

One major maintenance concern is keeping the roof green during drought conditions. In general, roof irrigation systems are expensive. Because of this, most green roof systems are designed to withstand long periods of drought. The roofs are designed with good drainage systems, and most of the plants used are drought tolerant.

“The roof might not look as good during a drought period, but it would still be unusual if the plants die off, as long as they are selected appropriately for the location,” Loveland says.

Loveland keeps a sedum in a pot in her backyard as a control sample. “I use it as a gauge,” she says. When she goes on site visits, she has a good idea of what the roof sedum should look like, based on her backyard plant.

Once the green roof is established, the most critical maintenance will be weeding. Weed pressure can be significant, says Raymond, depending on issues like nearby open fields or bird population.

Photos Courtesy of Dr. Brad Rowe.
	The Ford Rouge green roof in Detroit is roughly 500,000 square feet.

During the first two years, weeding should take place at least four times. After that, a spring weeding and a fall weeding should be enough. Johnson also recommends once a year trimming and general removal of biomass that would be done in ground landscaping.

Experts are mixed on the use of fertilizer. Some feel that fertilizer goes against the other eco benefits of a green roof, but others believe fertilizer help the roof establish itself.

A green roof challenge is bird life. “Pigeons are a huge issue,” says Raymond. “We try to deter them with spikes, bio-safe chemicals or by introducing natural predators.”

Safety is always an issue with green roof maintenance. The experts agree that building owners should hire professional landscapers with green roof experience to do the basic maintenance.

“No green roof is maintenance-free,” Raymond says. “When they mature, it becomes a little less, but you’ll need at least one maintenance per season.”

The author is a freelance writer who writes for a variety of trade and B2B publications, covering topics from sustainable living to construction and landscaping/lawn care. She resides in State College, Pa.