Moisture Content of Extensive Green Roof Growing Media and Growth Response of Fifteen Plant Species During Dry

Success of an extensive green roof is primarily dependent on plant species ability to survive the low moisture content of the growing media.  Due to the well-drained nature of the growing media, plants adaptable to dry, porous soils are primarily used in extensive green roof applications.  Although Sedum species have dominated the plant palette for extensive green roofs, there is growing interest in expanding the plant list for extensive green roof systems.  In order to effectively select suitable plants, species need to be evaluated in terms of their response to gradual and prolonged dry down of the growing media.  A study to determine the relative rates of dry down for fifteen species was conducted in greenhouse and outdoor trials.  During dry downs that extended over five months, succulent and herbaceous species dried down at different rates.  Although, not all succulent or herbaceous plants had consistent moisture contents during the initial 18 days of dry down.  Despite differences in dry down, the succulent species, as a group, maintained viable foliage for over five times longer than the herbaceous species.  The revival rates of the succulent species were nearly double those of the herbaceous species.  Therefore, not only are succulent species more likely to be longer-lived during periods of drought, but these species are more likely to resume growth soon after water is made available.  Based on these results, irrigation frequency is recommended for succulent species at a maximum of 28 day intervals and herbaceous species at a maximum of 14 day intervals.