The ‘urban renaissance,’ a term used to describe the comeback of many city centers in the US, coincides with the revival of green roofs, or maybe the two are intertwined. Either way, the demand for increased green infrastructure and green space continues to grow. And why not – many, many studies have shown the benefits of plants, from human health benefits such as stimulating a relaxation response in humans to environmental benefits, such as providing ecosystem services, and more.
As one might imagine, green roofs vary greatly in the construction and composition, and they are more complex than they appear. Green roof environments tend to be more susceptible to extreme temperatures and drought that lead to high evaporative demands; so, a great deal of resources has been dedicated to optimizing substrate composition and depth. In fact, the selected substrate guides the plant material to be used, maintenance needs, and the ultimate success of the planting.
Various sedums dominate the plant palette for green roofs; they’re drought-tolerant and can thrive in shallow substrates. A recent study looking at just how shallow a planting of succulents can tolerate was published in the Horticultural Research Institute’s (HRI) publication, the Journal of Environmental Horticulture (JEH). The study, led by Dr. Rolston St. Hilaire at New Mexico State University, evaluated three depths (3.9, 5.9, and 7.9 inches) to support a planting of hens and chicks, Sempervivum calcareum, in arid-type climates common to New Mexico. Environments on green roofs are extreme in any climate and especially so in arid climates.
The New Mexico State group concluded that shallow depths are acceptable for green roofs established in arid climates, especially with the inclusion of subsurface irrigation. This enables plant success due to the reduction of evaporative losses, when compared with surface irrigation. To read the full study, visit the Journal of Environmental Horticulture.
In 2017, HRI funded a study at Michigan State University, led by Dr. Bradley Rowe, evaluating different substrate depths and changes to organic matter content over a period of several years. This study is ongoing and plans to conclude later this year.
Dr. Rowe explained a bit about his project, “In our HRI funded study at MSU, 17 different herbaceous perennials and grasses were planted on a campus green roof in substrate depths of either 4 or 8 inches and were irrigated when needed during the first two years but had to rely on natural rainfall thereafter. Plant diversity decreased over time at both depths. Only seven species were still present after seven years when grown in 8 inches of media, and only three survived at a media depth of 4 inches.”
“This study emphasizes the need for long-term studies to avoid making premature conclusions, the importance of substrate depth on soil moisture and plant survival, and that irrigation during drought periods may be necessary if herbaceous perennials and grasses are to be successful on green roofs.”
While care must be taken in the design and construction phases, maintenance is key to longevity and presents some unique challenges to landscape managers.
“Visibility of some sites is very high; so, the maintenance needs can be intense. Many of our green roof contracts require weekly site visits. These are mostly integrated with mixed use buildings of residential, office, and retail spaces; so, expectations are high for beautiful, manicured green areas,” explained Chris Bixler, Branch Manager at Ruppert Landscape. Ruppert Landscape specializes in high-quality and large-scale commercial landscape construction and provides landscape management services to property owners and managers throughout the eastern US. They have a sister company, Ruppert Nurseries, a wholesale nursery production facility.
For one, weed control can be difficult, since most herbicides do not list green roofs as an approved site on labels. Plus, very few herbicides are safe to use as over-the-top, broad-spectrum applications on sedum. Finally, many green roofs collect at least some runoff in cisterns and recycle it as irrigation water. This further restricts herbicide use, as residues in irrigation water could potentially injure the plants.
In the summer, cities commonly report daily temperatures above surrounding suburban or rural areas, called the urban heat island effect. More paved surfaces associated with cities translate to increased absorption of solar radiation that is then re-radiated as heat. Conventional black roofs are part of this problem, as they notoriously absorb significant amounts of energy from sunlight. Green roofs, in comparison, also absorb solar radiation but use it to evaporate water from the substrate by way of the plants (through roots to the leaves), lowering temperatures in the process. One study at Penn State found that on a day where the air temperature was 90°F, an exposed black plastic surface reached 145°F, while the surface of a sedum planting was only 82°F.
All those paved surfaces in urban areas also create problems for storm water management. Too much runoff can overwhelm a city’s water treatment facility or sewage system, leading not only to increased costs but pollution as well. Green roofs can help. They capture significantly more rainfall during a storm than a conventional roof, 80% vs. 24% according to a study by Penn State.
Other documented benefits include increased energy savings as the vegetation and media act as an additional insulation layer, extended lifespans of roof membranes, neutralization of acid rain, a reduction in air pollution as they trap dust and airborne particulate matter, habitat for wildlife, and provide space for local production of fruits and vegetables in cities.
Green roofs have clearly grown up from being a simple sod roof but still have a ways to go. Luckily, researchers continue to find ways to improve them not only for the public’s enjoyment but also to in terms of environmental services provided and ease of maintenance. Who knows – maybe goats will be incorporated into them again too!