Gardens of the Cross Timbers: Green roofs

Becky Emerson Carlberg
Contributing writer
Green Roof on McKeon Center for Creativity at TCC

The Hanging Gardens of Babylon were constructed 500 BC. Stone arches were covered in layers of tar and reeds before trees and plants were added. Some of the most famous of the Green Roofs.

The #5 presentation in the Oklahoma State University Shackleford Series was held May 13. Brad Rowe, director of the Michigan State University Green Roof Project, showed up on my computer screen and presented “The Role of Green Roofs in Sustainable Development.” The Zoom presentation went off without a hitch at my house. It could have as well been plagued with low band width, interrupted internet connections or a cat running over the computer keyboard.

Two days before the lecture, the two toilets at my home went from slow to flood stage. The plumber came with his gigantic roto-rooter, pulled out a few tree roots and problem solved. The utility room flooded that night. The back flow from our calcium magnesium cylinder went everywhere in the room since the septic tank pipe was again blocked. Problem back. During the emptying of the septic tank next day, a dead squirrel was found. It may have blocked the line from the house to the tank. The little guy had to have been investigating the roof vent pipe and somehow tumbled down to the outflow pipe. A case of curiosity killed the squirrel. After a few shocked exclamations and shaking of heads, the small squirrel was buried between two pine trees. Problem solved.

If we just had a green roof. The squirrel might not have even seen the pipe. A living roof with grasses, shrubs or even trees. When we lived in Germany, we saw several houses and businesses with roofs not of tile, but low-growing plants. I wondered about the weight of such a covering as well as maintenance. Coming from Oklahoma, keeping things alive in summer requires water. Granted, the German climate is more temperate, warmed by the North Atlantic Current, and rains fall throughout the year. In Oklahoma, furious bouts of rain may be interspersed with lengthy droughts. A weird balance between humid, subtropical eastern Oklahoma and semi-arid western Oklahoma.

Germany is the leader in green roofs, which fall into two categories: Intensive and Extensive. The Intensive roof design is meant to include people, has deeper soils, larger trees and shrubs limited to flat roofs or terraces. The Extensive green roof usually has shallow soil (4”-6”) which limits plants to succulents and drought tolerant types.

Tulsa Community College (TCC) Green Roof Media

In Vancouver, Canada, one hotel roof supports a natural forest. The Church of Latter-Day Saints in Utah is topped by a natural prairie with surrounding terraces of trees. The Chicago City Hall semi-extensive green roof (half an acre) has been proven to mitigate urban heat island effect and improve air quality. Installed in 2001, 20,000 herbaceous plants, including a Cockspur hawthorn and Prairie Crabapple, are up there. Chicago has 200 green roofs.

The High Line was an elevated railway system in Manhattan. The non-profit Friends of the High Line planted and now maintain the 1.45 miles of herbaceous perennials living in 18” of soil. The High Line has turned into the #1 tourist attraction.

Roofs are wasted areas of space. One reason for green roofs is control of storm water. The flow and instantaneous runoff are constant battles, especially in urban areas. Options are available to deal with excess rainfall. Swales (shallow depression with low slopes), phytoremediation (using plants to control water and contaminants) and retention ponds are just a few ways to remedy excess water.

With a green roof, peak overflow is spread over a longer time, not the sharp discharge of rainfall in a short period of minutes as what happens to conventional roofs. Substantial amounts of rainfall are retained and used by plants. An asphalt roof lasts 20 years, wood shake roof perhaps 30 years, and a properly installed green roof over 40 years. Green roofs offer considerable energy savings depending on location and building type. They are living insulation layers.

Carrabba’s Italian Grill Green Roof

The plant cover is aesthetically pleasing, deadens sound, offers a boost to health and provides food. Carrabba’s Italian Grill in Tulsa has trees and shrubs growing on the porch roof. Other restaurants grow fresh seasonal produce for salads on roof tops. Brooklyn Grange at the Navy Yard Farm has three green roofs in New York City that cover 5.6 acres. They harvest over 100,000 pounds of organic produce per year! The apiary bee hives on roofs and other refuges in NYC assure pollinators.

Roof gardens temper the urban heat buildup and decrease air pollution. They reduce vandalism by people and varmints but offer a refuge for native plants and enhance biodiversity. One green roof has been in existence since 1914 in Zurich, Switzerland with orchids that exist nowhere else in the world. The surrounding natural areas are long gone.

Pioneers built sod houses. The sod roofs often supported plant life. The turf roof was traditional in Scandinavia. Even now many Norwegian roofs are covered in grass.

If ready to rescue the loss of green space where you live, consider a green roof. Dr. Rowe’s dog house has a green roof. His two dogs approve. Define your intent and level of expected maintenance. The roof should be flat or have a slight slope and be structurally sound. Base layer is a waterproofing membrane, topped by a root barrier, drainage layer and growing media. Use light weight, porous, well drained growing media such as heat expanded shale and clay and organic planter mixes. Plants, trees, shrubs, plugs, seeds, and cuttings can be placed in the growing media as well as modules and pre-vegetated mats.

Consider the environmental conditions of the roof (wind, rainfall, temperature). Choose plant species that can handle the roof environment. Sedums are often planted on green roofs since they are slow-growing and can withstand drought. Native prairie plants and herbaceous perennials are options.

Let the roof go wild or weed it or mow it. Burn it? Ducks Unlimited Interpretive Centre in southern Manitoba conducts a prescribed burn on the 1.10 acre roof of native grasses and wildflowers every four to five years!

Becky Emerson Carlberg

Becky Emerson Carlberg, graduate of Oklahoma State (Plant Pathology) is a teacher, artist, writer as well as certified Oklahoma Master Gardener and Master Naturalist. Contact her at Becscience@att.net.