Water use and drought responses of eight native herbaceous perennials for living wall systems

Vertical greenery provides a number of ecosystem services, yet its potential remains underexploited in urban areas. Further, existing designs use a narrow range of species chosen for ornamental rather than environmental traits. Knowledge of species options is limited, but its importance is mounting as stormwater irrigation technologies evolve and vertical greenery's potential as an evapotranspiration mechanism grows. To identify additional species that can maintain high water evapotranspiration in wellwatered conditions and low evapotranspiration in drought regimes, we selected eight native, herbaceous perennials from similar habitats, placed them in vertical, "green wall trays" under greenhouse conditions, and compared their growth responses to water shortages ranging from one to six weeks. Two species, Knautia arvensis and Geranium sanguineum, presented both of the desired water-use traits. More generally, species with a degree of succulence of root, shoot, or leaves dealt best with drought conditions. Armeria maritima, Campanula persicifolia and Saxifraga granulata survived the best under extended drought. The performance of Dianthus deltoides, Hypericum perforatum and Leucanthenum vulgare was inadequate for both water use and drought survival. The study confirms that the trait relevant for absolute volume of water transpiration is plant biomass. Further, irrespective of species' drought strategy, degree of succulence is shown to have the greatest impact on a plant's ability to survive drought.