ASLA Green Roof Gets Second Round of Planting
Less vigorous growth on south wave caused by high temperatures, experimental plants, and vandalism by birds.

Visitors to the ASLA green roof, and those visiting the roof virtually through the ASLA green roof web cam, cannot help but be struck by the different growing patterns between the north wave of the roof, which has been growing at a vigorous pace, and the south wave, which has experienced less vigorous growth since the roof was dedicated in late April. After a full summer season, green roof growth can be summed up as “a tale of two waves.”

Richard Hindle of Michael Van Valkenburgh Associates installs new plants on the ASLA green roof.

Noting this lack of vigor on the south side of the green roof—the poor growth has also affected what has become known as the “south terrace,” a flat area of extensive green roof situated behind the south wave—Michael Van Valkenburgh Associates Inc. (MVVA), has hired Richard Hindle to monitor plant growth and make specific recommendations on spurring growth on the south wave, in consultation with Ed Snodgrass of Emory Knoll Farms, who donated the plants for the green roof.

After several weeks of observation, Hindle, who holds a degree in botany from Cornell University and is currently studying landscape architecture at the Rhode Island School of Design, recommended increasing the overall density of plants and increasing coverage on the south wave to help counteract the extreme heat and evaporation that beset the roof this summer. He also recommended increasing the coverage with typical green roof sedums and succulents to create a more typical green roof archetype. When the south wave was initially planted, native species that were not typically used on green roofs were used. Hindle, along with MVVA staff, implemented this new planting plan last week with mature green roof plants donated to ASLA by Emory Knoll Farms. Specifically, Hindle made the following for the wave in an executive summary of his observations prepared for ASLA:

• Replant using proven green roof sedums S. reflexum, S. spurium, Delosperma nubigenum.
• Increase plant density to encourage more coverage of exposed surfaces. This will ameliorate some of the temperature peaks experienced on the bare surface.
• Continue watering through the fall, spring, and summer. Water more frequently with less water to avoid plant plugs drying out and becoming hydrophobic. Also, water will reduce surface temperatures until plants establish themselves.

Hindle notes that the lack of vigor on the south wave should not be viewed as a failure when compared to the exceptionally strong growth on the north wave. Rather, he notes that all of the plants used on the south wave were experimental, and intended to gauge whether native northeast plants could thrive under the harsh conditions of the green roof. In fact, in the executive summary Hindle notes that “much of what is occurring with the plants on the roof is to be expected. For example, the high temperatures on the southern terrace will decrease with more vegetation, and the sedums planted on the south wave were predicted [by Snodgrass] to die, as they are natives to the area not well suited to green roofs, especially slopes.”

Low density, high temperatures, erratic rainfall
The primary reason for a lack of growth on the south wave has been due to the combined effects of low plant density and extreme temperatures during the Washington, D.C., summer, which fed off each other—the less cover, the more reflective heat is generated—to quickly dehydrate the plants. Hindle notes that this was further exasperated by the fact that when peat moss, which some of the plugs were planted in, dries out, it becomes hydrophobic and nearly impossible to rehydrate. The south wave was lacking plant density because more than 400 native perennials selected for it were not planted due to a lack of availability.

Weather also placed a heavy burden on the south wave during the summer months. Hindle noted that roof temperatures during the summer months reached 136 degrees, with average temperatures reaching well into the 80s. In addition, fluctuations in rainfall—from extremely wet conditions in June to near drought conditions in July, followed by heavy rains from the remnants of Hurricane Ernesto—took its toll on the south mound plants as well.

The birds
One of the more quixotic “enemies” of the south wave turned out to be birds digging for insects in the substrate of the south wave. According to Hindle, the birds would pull up the planting plugs looking for insects, and scatter them around the wave to dry out in the sun. Hindle noted that this vandalism was fairly pervasive, and birds even pulled out the prickly pear cactus plugs that sit at the top of the roof.

Grate for shade
One of the more surprising aspects of the summer growing season was the success of the plantings placed under the grating on the green roof. When this idea was initially proposed in the design process, representatives from MVVA and the ASLA Green Roof task force openly wondered if the grating would heat in the summer sun and effectively “cook” the sedums placed underneath it. To help mitigate this, the landscape architects choose aluminum grating which would absorb less heat.

However, the grating had the opposite effect. Rather than heating the plants, it created shade that kept the plants from getting overheated and created “a nice little climate” for them, Hindle says. In fact, the plants under the grating thrived no matter where they were planted on the roof—south or north. Hindle also noted that the grates were situated so openings would reflect direct sunlight while allowing indirect sunlight to shine through.

While it is still very early in the process, the success of the plants under the grate could open up new applications for green roofs. If a building owner desires an extensive green roof, but there is a need or want for access due to maintenance, or as a public amenity, the grating could provide a cost-effective way to provide access, while at the same time protecting plants from some of the harsh conditions of the green roof.