Plants for Green Roofs in the Southeastern U.S
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PLANTS FOR G REEN ROOFS IN THE SOUTHEASTERN U.S. Kirk Laminack Research Horticulturist Quick Summary Green roofs are expanding throughout the United States at a prolific rate. A large portion of this expansion is occurring in the southeastern climate zones. Currently, there is a gap in industry standards and academic literature pertaining to plant selection and trials. This investigation is meant to address these needs by trialing plants for their use on green roofs in a southeastern climate. Plants were selected based on a number of criteria, with considerations for being cost effective and easily obtainable in the nursery trade. Twelve 10 ft. by 15 ft. (150 ft2) structures elevated 3 feet from the ground designed to mimic a green roof were constructed in April 2015. These structures were comprised of layers typically found in a conventional green roof assembly. All structures contain the same arrangement of irrigation, drainage, plant species, and sun exposure, with six structures containing 4 inches of media and six structures containing 6 inches of media. In total, twenty-two species were trialed, with each structure containing 2 of each species randomly planted on an individual basis. Field measurements were taken monthly for the first year then measurements were reduced to every 3 months thereafter. Most species had a mortality rate below 10% except for Ruellia elegans (96%), Salvia leucantha ‘Santa Barbara’ (88%), and Talinum ‘Kingswood Gold’ (83%), Chrysogonum virginianum (42%), and Phlox subulata (29%). An overall performance rating is calculated for each species by averaging an appearance rating and plant canopy density rating. This yields a single rating that accurately characterizes the performance of each species on a scale of 0-5. The top 5 best performing species are Nepeta racemosa ‘Walker’s Low’, Ilex vomitoria ‘Nana’, Nandina domestica ‘Gulf Stream’, Penstemon barbatus, Rosmarinus officinalis ‘Huntington Carpet’. The 5 worst performing species in this trial were Ruellia elegans, Baptisia ‘Carolina Moonlight’, Salvia leucantha ‘Santa Barbara’, Talinum ‘Kingswood Gold’, Lagerstroemia indica ‘Gamad I’. There was not a difference in performance ratings of species planted in 4 inches of media versus species planted in 6 inches of media except for Chrysogonum virginianum and Rudbeckia maxima. Introduction Green roofs in the southeastern U.S. have major potential to influence building envelopes by reducing pressures put on cooling systems during warm seasons as well as provide many other of their well-documented services and benefits. Other countries and regions such as Germany, the United Kingdom, other European countries as well as Canada and several northern U.S. cities have been at the leading front of green roof technology. In the southeastern portions of the U.S., less green roof research has been conducted. Throughout this region, warm temperate climates are the norm, categorized by hot and humid days and relatively warm nighttime temperatures. In addition to aesthetics and beneficial functions, plant species selection greatly depends on the climatic conditions of a green roof’s location and is crucial to its performance. Due to the prevalence of green roofs in cooler climates, research has been primarily focused on plant species that are better suited for these locations. Results from these studies, however, do not translate well to warmer and more humid climates. As the popularity of green roofs continues to rise, there is an immediate need for plants that perform well and are aesthetically pleasing on green roofs in southeastern climates. Multiple techniques, characteristics, and plant species must be tailored to this type of climate rather than following common practices specific to green roofs in cooler climates. The aim of this investigation is to determine which plant species perform well in the southeastern portion of the U.S. 1 Methodology Location The green roof trial structures are located on site at Moore Farms Botanical Garden in Lake City, SC located in the Pee Dee region of the state. Characterized by hot and humid summers with mild winters, Lake City, SC is located in USDA hardiness zone 8a. Average annual precipitation is 46 inches per year. Growing season can sometimes extend from March through November without observed freezing temperatures. Trial structures Twelve 10 ft. by 15 ft. (150 ft2) structures designed to mimic a green roof were completed in April 2015. They comprised all layers of a conventional green roof assembly minus insulation and were constructed from wood (Figure 1). All structures were raised 3 feet above ground in full sun with open air beneath, and all have a 2% slope. On the slope side of the structure is a green roof edging component meant to keep the media and plants in place while allowing water to drain (Green Roof Solutions®). Six structures contain 4 inches of media while the remaining six structures contain 6 inches of media. All structures received the same semi-intensive green roof growing media engineered by Erth Products® (FLL Guidelines). Every structure is irrigated at the same rate using an overhead irrigation system installed during construction. During the growing season irrigation is applied 2-3 times per day with each application totaling 0.05”. A rain sensor delay temporarily halts irrigation for 24 hours when rainfall equals or exceeds 0.05”. Irrigation is turned off around September through March with an occasional irrigation application if a dry period occurs during this time. Each structure is outfitted with 3 soil moisture sensors and 1 soil temperature sensor (Irrometer®). Figure 1: Construction layers of the green roof trial plots installed at Moore Farms Botanical Gardens Plant selection The species selection process involved multiple criteria. First, plants were selected based on expected aesthetic performance. Therefore, factors such as nativity, pollination, wildlife habitat, and evergreen characteristics were not a priority. Second, they had to be commercially available in the industry because trialing species that are difficult to acquire does little to benefit the green roof industry. Lastly, all plants were expected to withstand full sun and low soil moisture. Plant container size was not a major factor. The majority of green roofs are planted 2 with plugs or small potted plants. However, this trial was meant to take advantage of what is commonly found in the industry. Some plants were planted as plugs or small pots while others were planted as 3 gallon containers. The benefit of planting larger plants is that they are planted further apart and fewer plants can be installed. Since the installed plants are larger it provides a more effective and instant effect. Twenty-two species were chosen for the investigation (Table 1). While each structure was planted with the same species of plants, they each had a different randomized planting layout. Each species is replicated twice per structure totaling 44 plants per structure. Table 1: Species list of plants included in the green roof trials along with their mature size, pot size at time of planting, and planting cost. The planting cost is for the entire research trials plot. The green roof trial project was planted in May 2015. Species Common name Mature size Pot size Planting cost Baptisia 'Carolina Moonlight' False Indigo 3' T x 3' W 1 gallon (3L) $78.65 Chrysogonum virginianum Green and Gold 1' T x 1.5' W 1 quart (1 L) $100.56 Cuphea glutinosa Mexican Heather 6" T x spread 2.5" pot (L) $40.18 Eragrostis chloromelas Love Grass 3' T x 3' W 2.5" pot (L) $127.29 Ilex vomitoria 'Nana' Dwarf Yaupon Holly 4' T x 4' W 3 gallon (10 L) $236.25 Lagerstroemia 'Gamad I' Dwarf Crape Myrtle 4' T x 4' W 3 gallon (10 L) $337.50 Lantana camara 'Chapel Hill Yellow' Lantana 2' T x 3' W 1 gallon (3 L) $101.25 Liriope 'John Burch' Liriope 1' T x 1.5' W 4" pot (0.5 L) $38.28 Muhlenbergia 'Pink Flamingo' Muhly Grass 3' T x 3' W 2.5" pot (L) $127.29 Nandina domestica 'Gulf Stream'' Nandina 3' T x 3' W 1 gallon (3 L) $297.00 Nepeta racemosa 'Walker's Low' Catmint 2' T x 3.5' W 1 gallon (3 L) $109.93 Panicum virgatum 'Northwind' Switchgrass 5' T x 2' W 2.5" pot (L) $127.29 Penstemon barbatus Beardlip Penstemon 2' T x 1.5' W plugs $31.44 Phlox subulata Moss Phlox 6" T x spread 2.5" pot (L) $40.18 Rosemarinus officinalis 'Huntington Carpet' Rosemary 2' T x 6' W 1 gallon (3 L) $104.36 Rudbeckia maxima Great Coneflower 6' T x 1.5' W 2.5" pot (L) $40.18 Ruellia elegans Wild Petunia 3' T x 3' W 1 gallon (3 L) $80.00 Salvia leucantha 'Santa Barbara' Mexican Bush Sage 3.5' T x 2' W 1 gallon (3 L) $78.65 Schizachyrium scoparium 'The Blues' Little Bluestem 3' T x 2' W 1 quart (1 L) $127.29 Spiraea japonica 'Goldmound' Japanese Spiraea 3' T x 3' W 1 gallon (3 L) $229.50 Talinum 'Kingswood Gold' Jewels of Opar 1.5' T x 1' W 2.5" pot (L) $40.18 Vitex agnus-castus 'SMVACBD' Dwarf Chaste Tree 4' T x 5' W 4" pot (0.5 L) $287.76 Total $2,781.01 Planting area (ft2) 1,800 Cost/ft2 $1.55 Data collection and analysis Field measurements were taken monthly for the first year in order to thoroughly record establishment performance. Measurements were then reduced to every 3 months thereafter. All data was collected solely by the research horticulturist. Weather data is collected from an onsite weather station, recording humidity, rainfall, temperature, and wind speed. An overall performance rating is calculated for each species by averaging the appearance and plant canopy density ratings (Table 2).