Posted by sustainable_hort on February 27, 2010 | No Comments
Every green roof requires plants of some kind. While various succulents have proven relatively dependable in a roof’s harsh environment, there are many research projects looking at many other plants. As the results come in, new opportunities for the nursery industry and growers will appear.
But, at this point, there still seems to be a rather limited view of what plants might be best on our roofs. This post is part one of a two part article on green roof plants and environments. The second section will look at the basic types of green roofs we now see being built. This post also contains an extensive reference list.
Plants for Roofs
When reviewing plant use on green roofs, it is useful to first divide the discussion into “extensive” or “intensive” use.
“Extensive” roofs are generally installed for environmental/ecological reasons, not as additional human living space. If the roof is visible from surrounding buildings, the plant material may be irrigated, but often it is not watered. So, only tougher varieties that can go dormant for dry periods are chosen.
Define “intensive”…Moves beyond the purely environmental uses to creating livable spaces…multi-use spaces defined and enclosed in almost traditional landscape plantings though many of the plants are at a smaller scale. Use of containers, raised beds, and deeper soils allow for larger and vertical varieties. This concept is more developed in
Europe, though the re-urbanization movement with more concentrated condo living may demand more of these spaces be developed.
Yet, even within the more limited “extensive” options, the plant palette is still a work in progress. As Nigel Dunnett and Noel Kingsbury wrote:
“The horticultural potential of green roofs has yet to be fully realized. The majority of extensive green roof rely on a small number of species and cultivars that are use ubiquitously.”…”Widening the range of plant species used beyond the widely used sedum carpets has many potential benefits.” (b)
Broadening the Research on Plants for Green Roofs
Part of what Snodgrass called being “discovered” was all the research on the plants and soils for these roofs that has exploded in the last several years. After some notable green roof failures (see below), specific plant research is acutely needed so regional collections can be developed that will be successful in most situations.
Soil Depth Key to Plant Variety Success
Soils are one area of active research. The proceedings of the Green Roofs for Healthy Cities annual conference contain many reports and papers on soil make-up and depths relating to successful plant establishment on green roofs.
Add decades of practical experience, especially in Europe, and there are some basic parameters on soil depth and successful plants. The shallowest soils, with only 2 – 3 cm (0.8 – 1.2 inches) of medium, will grow only sedums and mosses. Substrate depths of 5 – 8 cm (2 –3.2 inches) support a wider range of succulent species, grasses, and herbaceous plants. Depths of 10 – 20 cm (4 – 8 inches) allows for a wide range of drought-tolerant perennials and grasses to be grown, as well as tough sub-shrubs. These depths will also support turf grass and lawns. Depths of 30 – 50 cm (12 – 20 inches) will support many perennials and shrubs to be grown. Trees generally need depths of 80 – 130 cm (32 – 52 inches).
The soil media composition is just as important as media depth to plant variety success. The percentage of organic to mineral matter, and the organic source material’s nutrient providing capacity, both play roles in the plants’ success. There are many ongoing research projects that will help determine which of the various media mixes work best in specific climates and situations. Soils need regional versions, similar to the choices with plants.
The Regional Need…
It is also generally accepted that one plant, or even one plant family, will not fit all the varied roof environments, due to both climate and differences in roof height and configuration. As mentioned above, nurseries like [Snodgrass] recognized the trend early, began with plants already successful in the regional climate, and began supplying contractors.
Nigel Dunnett discussed the same issue, stating…”because roof greening originated in central and northern Europe, the limestone meadow flora of this region has tended to dominate the plant-selection…partly because of its eminent suitability and partly because a variety of seed mixes are readily available. Yet, outside this region this limestone meadow flora may be regarded as inappropriate or potentially unsuccessful, particularly in areas with longer and hotter summers, making it important that local mixtures are investigated for their suitability instead.” (e)
The Northwest climate is a perfect example of a region that is probably not well understood. It has an image of grey skies and rain, yet is has a Mediterranean climate with little summer rain. This is different than much of the US that can have significant summer rains.
Early Northwest Project Teaches Hard Lessons
In fact, an early eco-roof in Portland provided a lesson in what not to do. The project, a green roof on the new Brewery Blocks development in northwest Portland was designed by Gerding Edlen Development. They eventually faced challenges at three points: drainage, soil and plants selection.
First, they misjudged the need for some slope in the drainage pattern. Since run-off was to be slow, they felt the level roof should not prevent water from eventually moving off. But, the roof was not completely level, and their drainage boards were too thin, leading to a pooling of water. Many of the plants did not like their “feet” wet and died.
Then, the soil had too much organic material. Most successful green roof soils have a small percentage (no more than 10%) organic material and are mostly made of inert materials such a perlite, ground lava rock, and other light-weight materials. In fact, many of the successful genera and species thrive in low-nutrient, sandy conditions, as mentioned above.
Finally, there was the plant selection. When designed in 2002, there was limited information on green roof plants. GED looked to Europe, borrowed many of the German succulents. But, they did not survive Oregon’s wet winters and very dry summers.
These problems were solved several years into the project by just starting over. Lessons learned, more information available, and hiring several experience consultants helped turn it around.
GED partner Dennis Wilde summed up the experience:
“First, hire landscape architects that have vegetated roof experience,” he stated in a 2008 Eco-Structure magazine article. “(Then), develop a deep understanding of the drainage, soil and plant issues you need for a successful installation.” (r)
At this point, most regional research continues to concentrate on plants that evolved in exposed soils, often mountainous areas, with rocky, sandy or xeric conditions. Adapted to cope with a lack of water, the plants have a morphology (see below) that stores water and reduces water loss. Many have fleshy leaves and stems that help deal with drought and high temperatures, with Sedums being the dominant choice. (n)
Dunnett presented research that showed there runoff was not generally affected by vegetation complexity or taxonomic composition of the communities. He noted seasonal differences. The winter months’ high precipitation quickly saturated the soil run-off was similar from all treatments. In the summer, with intermittent rains, run-off differed in relation to the plant canopy structure, where precipitation is lost to interception, stem flow and evaporation, along with transpiration.” The thicker the vegetative cover during low rain periods (summer), the more effective the roof is at capturing rain.(z)
Heavy Use of Sedums (Succulents)
As mentioned above, the heavy use of sedums is based on their survival rate in generally harsh environments. These plants will both store water and have a special type of metabolism called ‘Crassulacean Acid Metabolism’, CAM for short. CAM plants are unique in that under drought conditions their stomata (leaf pores) are open at night rather then during the day, as is the case with most plants. CAM plants exchange gasses (oxygen and carbon dioxide) in the dark when it is cooler and less windy. CAM plants are up to ten times more efficient with water conservation than non-CAM plants.
Many of these same plants will also go dormant, particularly during the drier periods. This is particularly important in the west coast’s Mediterranean climate. While some regions actually have a “wet” reputation, these states can be relatively dry from late spring until mid-Fall. This is in direct contrast to much of the US where summer rains are common.
This gives them a decided advantage when green roof conditions dictate certain limitations. Thus, the majority of green roofs you visit will have at least some sedums and succulents. Some roofs will be nothing but these plants.
Research Confirms Sedum’s Strengths
2006 research from Michigan State University tested sedum’s repudiated superior performance in low water conditions, common on green roofs. 25 Sedums were compared to several Michigan natives. The plants were watered at varying intervals. The non-sedum natives only survived if they received water every two days. In contrast, several sedum varieties were alive after 89 with no water. (a).
These plants have been used in Europe for decades, so early U.S. designers borrowed the palette without necessarily having the same climate. Results have been less than perfect. This may be due to the fact that much of Germany receives significant rain amounts in the summer and different winter conditions than many U.S. regions.
Nurseries Find Success with Succulents
Emory Knoll Farms, now possibly the largest U.S. green roof plant nursery, currently grow more than 100 different varieties for green roofs. Snodgrass’ choices are based on European sedums, selections from the Denver Botanical Garden, and many plants from South Africa, particularly the Ice Plant varieties. (d)
Green Living Technologies lists nearly 50 Sedum cultivars that are used in their hybrid mat-layered systems (see more below).
Intrinsic Perennial Garden is another supplier of green roof plants, They even have their own blend of native plants called the BIO-diver-CITY™ Blend These are Midwest native plants selected by Intrinsic Perennial Gardens, Inc. after trialing over 100 plants for the tough conditions of a green roof environment in the Midwest. (x)
Green roof varieties drop off quickly after the sedums and related plants. A more limited range of grasses have also been successful, including many Festuca and Carex choices/
Grasses usefulness in green roof planting depends significantly on soil depth. Research indicates that very shallow soils (5 cm and less) will only support a couple small Festuca species and a restricted range of short sedges (b). Thus, the Festuca species appear on many green roof plant lists.
Yet, grasses continue to be popular choice since many can go dormant and revive vigorously when watered, or the rains return. Yet, several authors have noted a concern that they might create a fire hazard. This may represent another area of research.
Still, other varieties are suggested for use on green roofs. These include the following grasses and grass-like plants.
• Agrostis pallens (Bent grass)
• Calamagrostis stricta (Slimstem reedgrass)
• Panicum virgatum (Switch grass)
Meanwhile, the more aggressive nurseries are now offering the following plants: ferns, Alliums, many herbaceous perennials, and even mosses. The research below contains many newer choices now being tested.
It was surprising that research showed mosses might play a more important role in establishing a green roof than has been recognized. A recent paper discusses the BRYOTECH Process, a method developed for the industrial production of pioneering plant mosses associated with microorganisms. The authors provide brief summaries of results of studies that explored the effects of soils with a “biological crust” on plant growth and performance. According to the authors, mosses and other symbiotic plants and soil micro-organisms can create green roofs that immediately incorporate the living elements that nature would introduce over a much longer period of time: micro-organisms associated with mosses, and wild seeds of dependent xerophilous plants. The need for maintenance of plants and the use of fertilizers is greatly reduced. The various studies and works carried out by the firm MCK Environment demonstrate that the use of mosses is key in re-vegetating land where colonization is particularly difficult. This principle is also applicable to green roofs. (t)
Those interested in plants for green roof should get a subscription to “Eco-Structure” magazine (www.eco-structure.com); and “Living Architecture Monitor,” published by Green Roof for Healthy Cities, (www.greenroofs.org).
(a) Evaluation of Crassulaceae Species on Extensive Green Roofs. Durhman|VanWoert|Row|Rugh|Ebert-May, Angela K. |Nicholaus|D. Bradley|Clayton L.|Diane. Proceedings: Greening Rooftops for Sustainable Communities 2004, 2004, 3.6.
(b) Plant Options for Extensive and Semi-Extensive Green Roofs. Nigel Dunnett and Noel Kingsbury. Proceedings: Greening Rooftops for Sustainable Communities. 2004 conference proceedings, 2.2.
(c) Native Coastal Plants for Northeastern Extensive and Semi-Intensive Green Roof Trays: Substrates, Fabrics and Plant Selection: Jeff Licht, EdD, LLC, Jeremy Lundholm, PhD Wayland, MA, Roof Garden Consultant / St. Mary’s University, Halifax, Nova Scotia, Canada Rooftops for Sustainable Communities 2006, conference proceedings, 4.3.
(d) Green Roof Plants. A Resource and Planting Guide. Edmund C. and Lucie L. Snodgrass. Timber Press. 2006
(e) Planting Green Roofs and Living Walls. Nigel Dunnett and Noel Kingsbury. Timber Press. 2004.
(f) Rain Gardens. Nigel Dunnett and Andy Clayden. Timber Press. 2007.
(g) Plant Options for Extensive and Semi-Extensive Green Roofs. Nigel Dunnett and Noel Kingsbury. Proceedings: Green Rooftops for Sustainable Communities 2004 conference proceedings, p 16.
(h) Water-Efficient Plants for the Willamette Valley. Regional Water Providers Consortium publication. Information at www.conserveh2o.org.
(i) The Green Fuse: Using Plants to Provide Ecosystem Services. Sustainable Plant Research and Outreach (SPRout) publication. A literature review. Rene Kane. 2004
(j) Evaluation of Sedum ternatum in a Shaded Green Roof System. C. Hise, V. Jost, K. Luckett, S. Morgan, T.Yan, and W. Retzlaff. Southern Illinois University Edwardsville, 2006.
(k) Plant Species Evaluation for Extensive Green Roof Applications in the Midwestern United States. S. Kaufman1, V. Jost2, K. Luckett3, S. Morgan1, T. Yan1 and W. Retzlaff1. 1Southern Illinois University Edwardsville. 2006.
(l) Evaluating Plants in Green Roof Systems Following Establishment. Sydow M.1, K. Forrester2, V. Jost3, K. Luckett4, S. Morgan5, and W. Retzlaff1, 2. 1Department of Biological Sciences; 2Environmental Sciences Program; 3Jost Greenhouses; 4Green Roof Blocks, St. Louis MetalWorks, Inc.; 5Department of Civil Engineering; Southern Illinois University Edwardsville. 2006.
(m) Evaluating Green Paks Green Roof Systems. R. Lucas1, H. Luckie1, V. Jost2, K. Luckett3, S. Morgan1, T. Yan1 and W. Retzlaff1. 1Southern Illinois University Edwardsville, Ill. 2006.
(n) Native Survivors. By Ron M. Wik, Living Architecture Monitor, pages 24 – 25. Winter 2008.
(o) Hot and Humid, What Plans Work Best in Tropical and Subtropical Situations? By Steve Skinner, Living Architecture Monitor, pages 26 –27. Winter 2008.
(p) Drought…Evaluating the Performance of Green Roof Plants and Growing Medium. By Dr. Bill Retzlaff, Dr. Susan Morgan, Kelly Luckett and Vic Jost. Living Architecture Monitor, page 29. Winter 2008.
(q) Oregon State University Stater, Winter 2008, pages 21 – 26.
(r) Best Intentions, article by Jim Schneider, Eco-Structure magazine, April 2008, p. 44 – 47.
(s) Establishment and persistence of Sedum spp. and Native Taxa for Green Roof Applications, By Monterusso, Michael A., Rowe, D. Bradley, Rugh, Clayton L., Michigan State University. HortScience 40(2):391-396, April 2005.
(t) Mosses, A Necessary Step for Perennial Plant Dynamics. Chiaffredo|Denayer, Michel|Franck-Olivier.. Greening Rooftops for Sustainable Communities, Portland, OR, June 2-4, 2004, p. 9.
(u) 100 Extensive Green Roofs: Lessons Learned. Snodgrass, Ed. Proceedings: Greening Rooftops for Sustainable Communities, May 4-6, 2005, p. 6.
(v) White|Snodgrass, John W.|Edmund. Extensive Greenroof Plant Selection and Characteristics. Greening Rooftops for Sustainable Communities, p. 14.
(w) Whitlow|Compton, Thomas|Jeannette S.. 11-MAY-2006. A Zero Discharge Green Roof System and Species Selection to Optimize Evapotranspiration and Water Retention. Greening Rooftops for Sustainable Communities, Boston, MA, May 11-12, 2006, p. 12.
(x) Hauth|Liptan, Emily|Tom. 30-MAY-2003. Plant Survival Findings in the Pacific Northwest. Greening Rooftops for Sustainable Communities, Chicago, Illinois, May 30, 2003, p. 13.
(y) Rowe|Monterusso|Rugh, Bradley|Michael|Clayton. 01-JAN-2007. Evaluation of Sedum Species and Michigan Native Taxa for Green Roof Applications. Greening Rooftops for Sustainable Communities, Washington, DC , May 2-4, 2005, p. 13.
(z) Dunnett|Nagase|Booth|Grime, Nigel|Ayako|Rosemary|Philip. Vegetation Composition and Structure Significantly Influence Green Roof Performance. Greening Rooftops for Sustainable Communities Greening Rooftops for Sustainable Communities, Washington, DC, May 4-6, 2005, p. 10.
(aa) Taken from Blackdown Horticultural Consultants Limited’s PDF brochure, available at www.greenroof.co.uk.
(bb) Article “Just Add Water: Wetlands Green Roofs for Enhance Performance” in The Green Roof Infrastructure Monitor, Fall 2007, p. 6 – 9.
(cc) Houghten, F. C. et al. 1940. Summer cooling load as affected by heat gain through dry, sprinkled and water covered roofs. ASHVE Transactions. 46:231-242.