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Substrate Composition and Fertility

In the green roof industry, there is a need to define growing substrates that are lightweight, permanent, and can sustain plant health without leaching nutrients that may harm the environment. High levels of substrate organic matter is not recommended because it will decompose resulting in substrate shrinkage and can leach nutrients such as nitrogen (N) and phosphorus (P) in the runoff.  The same runoff problems can occur when fertilizer is applied. Also, in the midwestern U.S., there is a great deal of interest in utilizing native species and recreating natural prairies on rooftops.  Since most of these native species are not succulents, it is not known if they can survive on shallow extensive green roofs without irrigation.

Five planting substrate compositions containing 60, 70, 80, 90, and 100% of heat-expanded slate (PermaTill; Carolina Stalite; Salisbury, N.C.) were used to evaluate the establishment, growth, and survival of two stonecrops (Sedum spp.) and six non-succulent natives to the midwestern U.S. prairie over a period of 3 years.  A second study evaluated these same plant types that were supplied with four levels of controlled-release fertilizer. Both studies were conducted in interlocking modular units (36 x 36 inches) designed for green roof applications containing 10 cm of substrate.

Higher levels of heat-expanded slate in the substrate generally resulted in slightly less growth and lower visual ratings across all species. By May 2004, all plants of smooth aster (Aster laevis), horsemint (Monarda punctata), black-eyed susan (Rudbeckia hirta), and showy goldenrod (Solidago speciosa) were dead. To a lesser degree, half of the lanceleaf coreopsis (Coreopsis lanceolata) survived in 60 and 70% heat-expanded slate, but only a third of the plants survived in 80, 90, or 100%.  Regardless of substrate composition, both ‘Diffusum’ stonecrop (Sedum middendorffianum ‘Diffusum’) and ‘Royal Pink’ stonecrop (Sedum spurium ‘Royal Pink’) achieved 100% coverage by June 2002 and maintained this coverage into 2004. In the fertility study, plants that received low fertilizer rates generally produced the least amount of growth. However, water availability was a key factor. A greater number of smooth aster, junegrass (Koeleria macrantha), and showy goldenrod survived when they were not fertilized.  Presumably, these plants could survive drought conditions for a longer period of time since they had less biomass to maintain.  However, by the end of three growing seasons, all three non-succulent natives also were dead.

Overall results suggest that a moderately high level of heat-expanded slate (approximately 80%) and a relatively low level of controlled-release fertilizer (50 g×m-2 per year) and can be utilized for green roof applications when growing succulents such as stonecrop.  However, the non-succulents used in this study require deeper substrates, additional organic matter, or supplemental irrigation.  By reducing the amount of organic matter in the substrate and by applying the minimal amount of fertilizer to maintain plant health, potential contaminated discharge of N, P, and other nutrients from green roofs is likely to be reduced considerably while still maintaining plant health.

Complete results are published in:
Rowe, D.B., M.A. Monterusso, and C.L. Rugh.  2006. Assessment of heat-expanded slate and fertility requirements in green roof substrates.  HortTechnology 16(3):471-477.

several containers on the ground with substrates and plants in them
Modules comparing substrate composition

Development of Green Roof Substrates Utilizing Recycled Materials

Our objective is to develop more environmentally friendly light-weight green roof substrates using recycled materials that can supplement or replace the energy intensive heat expanded slate, shale, and clay that are currently the industry standard.  In addition, development of lighter weight substrates will allow a greater number of roofs to qualify for vegetation cover and allow for greater depths without expensive structural modifications to buildings.

Substrates containing recycles materials (brick mulch, foamed glass, and crushed porcelain) are being tested on green roof platforms to see how various plant types perform compared to a commercially available green roof substrate.  Because mixes considered must meet FLL and ASTM standards for green roof substrates, physical properties of each mix will be analyzed for granulometric distribution, bulk density, pore volume, water permeability, pH, and nutrient values. During the experiment, measurements of survival and growth index (height x width x width/3) are being recorded. Plant stress is being quantified by periodically measuring chlorophyll fluorescence, a technique frequently used to quantify the efficiency of the photosynthetic apparatus. In addition, volumetric moisture content of the substrates is being monitored throughout the experiment to determine the rate at which various substrates dry out.  At the end of the growing period, stems and leaves (top growth) and roots (root growth) will be harvested, dried, and weighed to provide dry weights.  Data will used to determine biomass accumulation and total growth.

Recycled, mostly white porcelain being used as a substrate. Plants are mostly yreen and yellow in color.Recycled Porcelain from MSU's Campus

a man working on a point frame and research platforms
Using an elevated point frame to determine plant growth and coverage

Comparison of Compost Formulation on Plant Performance and Runoff Quality

Six composts in combination with two plant species (Sedum floriferum and the sedge Carex eburnea) will be tested for plant performance and runoff water quality. Substrates consist of 25% each of Haydite A and Haydite B heat-expanded shale aggregate (Haydite Hydraulic Brick Company, Brooklyn, IN), 30% 2NS sand, and 20% of each of the following compost types: (1) Municipal yard waste (Granger Recycling, Lansing, MI), (2) municipal yard waste (Tuthill Farms and Composting, Inc, South Lyon, MI), (3) MSU Student Organic Farm (SOF) standard substrate composed of wood chips, straw, hay, and peat,( 4) SOF standard with the addition of liquid digestate consisting of treated cafeteria food wastes from the MSU Anaerobic Digester Research and Education Center, (5) SOF food waste mixed with horse bedding, and (6) SOF food waste vermicompost that was composted with red wiggler worms.  All food wastes consisted of pre-consumer scraps from the MSU dining halls.  The study is being conducted in the Plant Science Greenhouses in 54 bulb crates. Plant performance is being quantified by recording plant survival, growth index, chlorophyll fluorescence, substrate moisture content, and harvest dry weights of roots and shoots. Initial and periodic runoff samples are being collected and analyzed for nitrate N and phosphate P.

Several baskets contqaining compost substrates and plants in a greenhouse.
Overview of greenhouse compost study (May 2013)

Changes in Organic Matter Over Time and its Effect on Plant Performance

The objective of this study is to quantify plant establishment, long-term performance, and substrate shrinkage dependent on the percentage of organic matter in the original substrate and plant community. Three aggregate blends container either 0%, 20%, or 40% compost will be seeded with two plant communities (sedum mix and native perennial mix).  Substrate depth is 10 cm. Sedum species used include S. acre, S. album, S. floriferum, S. kamtschaticum, S. pulchellum, S. reflexum, and S. spurium ‘Summer Glory’.  Native herbaceous perennials and grasses include Allium cernuum (nodding pink onion), Aster laevis (smooth aster), Coreopsis lanceolata (lanceleaf coreopsis), Dalea purpurea (purple prairie clover), Liatris aspera (rough blazingstar), Monarda punctata (dotted mint), Ruellia humilis (wild petunia), Schizachyrium scoparium (little bluestem), Tradescantai ohiensis (spiderwort), and Verbena stricta (hoary vervain). Seed was obtained from Jelitto Staudensamen GmbH, Schwarmstedt, Germany (sedum) and Prairie Nursery, Westfield, WI (native plants).

Initial and changes in plant coverage and diversity will be determined annually with a point frame transect. In addition, substrate samples will be analyzed annually to determine organic content. The study will answer the question as to how the percentage of organic matter in green roof substrates changes over time and how this change influences plant communities.  The study will continue for a minimum of five years.

herbacious perennial germination
Germinating seeds of herbaceous perennials and grasses (June 2013)

Effect of Substrate Organic Matter Content on Vegetable Production

The objective of this study is to quantify plant health and yields of sweet peppers (Capsicum annuum) and cucumbers (Cucumis sativus) dependent on the percentage of organic matter in the substrate. Six aggregate blends container either 0%, 20%, 40%, 60%, 80% or 100% finished compost from landscape wastes (Hammond Farms, Dimondale, MI).  Substrate depth is 15 cm.

Peppers and Cucumbers growing on research platforms
Sweet peppers and cucumbers growing in substrates composed of various percentages of compost

several containers on the ground with substrates and plants in them
Modules comparing fertility levels

Rececled substrates on research platforms.  3 boxes per platform.
Recycled Substrates Study

Haydite substrate with green and grassy plants

Light grey growstone with small green and yellow plants.
Growstone Foamed Glass

Compost substrate in a basket with 6 plants growing
Sedum Floriferum

Compost substrate with 6 plants growing
Carex Ebumean

basil growing in compost substrate in a basket