Assessment of heat-expanded slate and fertility requirements in green roof substrates

• Published in: HortTechnology (Alexandria, Va.) Va.), 2006, Vol.16 (3), p.471-477
• Main Authors: Rowe, D.B, Monterusso, M.A, Rugh, C.L
• Format: Article
• Language: English
• Subjects: cacti and succulents; forbs; grasses; green roofs; growing media; heat-expanded slate; indigenous species; leaching; metamorphic rocks; mortality; organic matter; ornamental plants; plant establishment; plant growth; roofs; slow-release fertilizers; soilless culture; water holding capacity
• ISSN: 1063-0198; 1943-7714
• DOI: 10.21273/horttech.16.3.0471

Green roof technology in the United States is in the early development stage and several issues must be addressed before green roofs become more wide-spread in the U.S. Among these issues is the 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 are not recommended because the organic matter 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 (Perma Till) were used to evaluate the establishment, growth, and survival of two stonecrops (Sedum spp.) and six nonsucculent 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 at ground level 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 (S. middendorffianum) and 'Royal Pink' stonecrop (S. spurium) 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 plants survived when they were not fertilized. Presumably, these plants could