Growing Medium for Green Roofs
Considerations for Engineered Soil Used in Green Roof Installations
Green roofs are building coverings that use vegetation in their engineered design. They offer multiple environmental benefits, including reducing energy consumption required to heat and cool buildings, minimizing the quantity of water runoff from rooftops, and lowering the heat island effect in cities. As part of a stormwater management system, green roofs can help reduce the amount of water runoff from rooftops, but several studies have shown that the quality of water running off green roofs is not necessarily improved over traditional roofing systems. The pollutants in the water have been attributed to the components in the growing medium.
What is Green Roof Growing Medium?
The plants used on green roofs must be supported in soil, but natural soil is too heavy to use on rooftops. The soil or growing medium used for green roofs is specifically engineered to support the plants while meeting other rooftop requirements. It must be lightweight, able to retain some water while allowing excess water to drain, able to provide nutrients for plant growth and minimize the amount of nutrients leaching into water runoff. To achieve these characteristics, growing medium consists of soil mixed with aggregates (non-organic, absorbent fillers) and organic matter. To replenish the soil nutrients, fertilizer is also applied from time to time.
Aggregates in Green Roof Growing Medium
Non-organic minerals are added to the growing medium to support the plant roots and provide pore space for air, water, and the exchange of gases. These minerals may come from naturally-occurring materials such as pumice or scoria, or they can be synthetically produced, such as expanded slate, clay or shale. Sand, perlite scoria, or pumice fines may also be used. The choice of aggregate used in the growing medium affects the density of the medium, the overall weight, and the ability to absorb water. Aggregates may also affect the quality of the roof top runoff. In their paper, “What’s that Running Off My Green Roof”, presented at the Greening Rooftops for Sustainable Communities conference in 2008, W. Retzlaff et al reported finding increased levels of iron and phosphorus, as well as a pH level over 8 in water running off green roofs that used a growing medium containing recycled glass.
Organic Material in Green Roof Growing Medium
Materials such as peat moss, bark, straw, sawdust, leaves, and so on are often added to provide nutrients to the plants; however, organic materials decompose and can cause problems on green roofs. As the material decomposes, the volume of the growing medium is reduced and plant roots may be exposed. In addition, organic fines from the decomposing material get caught in the filter fabric where it turns into a slimy substance that impedes drainage. Elements from the decomposing material also leaches into the water running off the roof and contributes to poor water quality in the runoff. In their paper, “A North Carolina Field Study to Evaluate Green Roof Runoff Water Quantity, Runoff Quality, and Plant Growth”, presented at the Green Roof for Healthy Cities conference in 2004, Amy Moran et al reported that runoff from rooftops using growing medium with 15% organic matter contained excessive quantities of phosphorus and nitrates. They also cited a smaller study that showed that when the growing medium contained a smaller concentration of organic matter, the levels of these nutrients in the runoff was reduced.
Using Fertilizers on Green Roofs
To replenish the nutrients in the growing medium, fertilizers may be added. These nutrients are necessary for sustained plant growth, but the type of fertilizer used can affect the amount of nutrients that leach into the water runoff. In their conference presentation, Retzlaff et al found that water-soluble fertilizers have greater potential for nutrients to leach into the water runoff while slow-release fertilizers that are coated with a thin permeable membrane allow nutrients to be allocated into the soil in a controlled manner.
Green roofs offer many environmental benefits and the growing medium is a key component for achieving optimal results from a green roof installation. The best blend of materials for a growing medium must consider many factors including the nutrient requirements of the plants, the water retention and drainage capacity of the materials, as well as the impact on the water runoff quality.
Growing Medium
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Green Roof Layer Showing Growing Medium
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