Fall 2006 Sep 20.qxp - Green Roofs for Healthy Cities

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The Media is the Message Sandra McCullough is a LEED Accredited Professional working with GreenGrid® green roofing systems, a business of Weston Solutions®, Inc. There are scores of articles written on the benefits and features of green roofing systems. As further studies are conducted, we can expect even more reports to come out detailing the many pluses of green roofs. The growing media or ‘engineered soil’ used on a green roof is also a hot topic. The plants selected— usually sedums, grasses, perennials, some native plants, and other succulents—are always a major consideration when installing a green roof because climate conditions can vary so markedly. The plants chosen invariably must be hearty, drought-tolerant, and lowmaintenance, requiring little or no irrigation. “Low growing sedum species have proven to be the most tolerant plants, especially in shallow soils that are not irrigated. Even with extreme fluctuations in temperature, they survive and grow very well on the roof,” says Grace Koehler, sales manager with the nursery Midwest Groundcovers, in St. Charles, IL. However, much less is discussed of the growing media used for green roofing systems. And remarkably, it is the new technologies and advances in engineered soil that have made green roofs, as well as other forms of urban landscaping, so successful. Indeed, without proper growing media selection, it might be difficult for a green roof — and the Green Roofing Industry — to be flourishing as it is today. Engineered Soil Many cities in California are famous for their palmtree-lined thoroughfares. Chicago’s Michigan Avenue and surrounding street areas have also become wellknown for their ample urban landscaping. However, historically, many of these trees and plants — usually transplanted from nurseries — fail to thrive in their new environments, and many die in a relatively short time. In fact, for years in California, determining if a newly planted palm tree would survive more than a few years was almost like tossing dice, even after employing the best fertilizers, irrigation systems and care. These plant failures could be costly and often damaged relations between landscapers and their customers. Eventually, a closer examination of the problem revealed that one of the keys to plant health and increased life expectancy, especially in urban settings, had a great deal to do with the soil used. Successful plant transplanting requires rapid root growth, and the development of engineered soil — soil that helps optimize the interaction of roots with air, water, and soil nutrients — has helped accomplish this. Midwest Trading’s computerized system for mixing the growth media (Courtesy Midwest Trading) A similar soil challenge has confronted segments of the Green Roof Industry. According to Koehler, “The soil used for green roofs is very important. For instance, it is crucial to make sure there is enough air space in the soil so water and oxygen can move through the growth media.” In addition, the soil must contain all the nutrients for the plant to not only survive but also thrive in most types of climates and conditions. Here again, engineered soil helps address these issues. What is the proper mix of materials for engineered soil? “In the United States, there really are no standards or guidelines as to how to formulate engineered soil for extensive green roof systems,” says Robert Cenar, of Midwest Trading Horticultural Supplies, Inc. in Virgil, IL. “About the only thing that comes close are the FLL standards developed in Germany. These include specific characteristics, both physical and chemical, that a green roof soil should include and are often used as a basis or a guide in the US.” According to Cenar, much of the engineered growing media now produced at Midwest Trading for green roofing systems was developed with proper pH balance, air space, nutrients, drainage, and several 12 The Green Roof Infrastructure Monitor - www.greenroofs.org Volume 8, No. 1
other components in mind. All help to maximize root development; improve plant growth, quality, and survival; and reduce costs for the end customer. In fact, the company has devised one of the most sophisticated mixing centers in the United States and serves the GreenGrid® Green Roof System projects in the Midwest, among other system providers. The computer-controlled system brings a level of precision to the growing media mixing process that is found at few other locations in the country. Because the growing media is so important, it is essential that professionals in the industry have a good overall knowledge of its characteristics. “The growth media is literally the foundation of a green roof system, and for its success, a basic understanding of the engineered soil’s characteristics are necessary,” adds Cenar. Engineered Soil Components With a modular green roof system, as with all extensive green roofs, the growing media must anchor the plants and provide the chemicals and nutrients necessary for plant growth. The growing media must also provide a healthy setting for the plant while remaining lightweight, water-permeable, and resistant to rot. Natural soils may be too heavy for use in a green roof system, lack necessary plant growth nutrients, or have a tendency to become compacted when moved or transported. Instead, a blend of nutrients, organic and mineral materials of varying sizes, grains, and compost should be employed, along with pore space to allow air and water movement into the soil. In addition to carbon, hydrogen, and oxygen, plants need several different essential elements in relatively large amounts for survival and optimum growth. Some of these are referred to as macronutrients and include: 1. Nitrogen 2. Phosphorous 3. Potassium 4. Calcium 5. Magnesium 6. Sulfur Plants also need several other elements, called micronutrients, such as sulfur, iron, manganese, copper, boron, and zinc. These are needed in only very small quantities but are every bit as essential as macronutrients to normal plant growth. Often the compost initially used in the growth media will provide all of these nutrients, though it may be necessary to augment it with fertilizers later on to keep nutrient levels high. The organic materials used in the compost will be the initial source of nutrients for plant growth and should have a fairly low carbon-to-nitrogen ratio. They also help bond the soil structure and make the soil more easily pulverized. However, Koehler advises using less organic matter and more aggregate pieces—clusters or crumbs of soil particles stuck together. “This is because organic materials break down quickly,” she says, “and engineered soils are designed for long life.” It is the mineral material that most distinguishes green roof growth media from natural soils. Synthetically produced slate, clay, and/or shale are often used. Naturally occurring materials, such as pumice and scoria, may also be found in growing media Weight is an important consideration with mineral materials. For example, some inexpensive mineral components, such as sand, may be heavier than comparable materials. The added weight could pose installation problems, limiting where the green roof can be installed. GreenGrid Pre-growing bed (courtesy GreenGrid) As the interest in green roofs grows and they become more commonplace, green roof manufacturers and soil and nursery professionals will likely continue to reevaluate the growing media used, looking for more ways to promote healthy, optimum plant growth in even more varied climates and settings. And as these are introduced, it is essential that landscape professionals, architects, and others in the industry stay attuned to the latest developments and technologies. Engineered soils, those already available as well as new ones that will likely be developed, can often determine the success or failure of a green roof installation. Fall 2006 The Green Roof Infrastructure Monitor - www.greenroofs.org 13
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Fall 2006 Sep 20.qxp - Green Roofs for Healthy Cities

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