Evaluation of 75 Species for Survival and Vigor On a Rooftop In the Northern US

Covering rooftops with plantings, ‘green roofs’, can provide environmental benefits when compared to traditional, impervious rooftops.  A study was conducted during the 2008-2009 (YR I), and 2009-2010 (YR II) growing seasons on a rooftop at the University of Minnesota (Minneapolis, Minnesota, USA; plant zone 4) to evaluate survival and vigor of 75 plant species with horticultural attributes.  A Minnesota rooftop is drier and can have greater temperature extremes compared to rooftops where much work has been done on species selection in Europe, US, and Canada.  Rooftop temperatures varied in our experiment from -29.2˚ to 36.7˚C.  Plants were planted in containers that were 40cm in length and width and 12.7 cm in depth in a media composed of 1 part expanded shale aggregate, 1 part rockwool, and 2 parts Sunshine SB500 high porosity growing media.  Plants were not fertilized or irrigated during the experiment.  There were 12 plants planted per square meter and plantings were placed in each of four blocks across the rooftop to accommodate for a perceived temperature and irradiance gradient.  Soil moisture and temperature, air temperature and humidity, precipitation, solar radiation, and wind direction and speed data were collected. Plant survival was collected and vigor was assessed by a qualitative measurement of growth and area coverage within a tray for each plant species.  In YR I and YR II, 35 of the 52 species, and 21 of 23 species survived, respectively.  A number of traditional herbaceous perennials were considered unacceptable as all plants died including Rudbeckia fulgida ‘Goldstrum’, Hemerocallis ‘Stella d ‘Oro’, and Ajuga reptans ‘Bronze Beauty’.  In contrast, a number of species thrived (survival score of 3-4 and vigor score of 41-81) such as Allium senescens ‘Glaucum’, Festuca ovina VNS, and Sedum ellecombianum.  Among blocks, species survival and vigor varied and those differences appeared to be associated with soil moisture content.  Implications of this work and the potential implications of periodic irrigation on species survival will be presented.  In addition, the implications of this work with respect to planting time will be discussed.