Irrigation and pruning affect growth water use efficiency of two desert-adapted shrubs

Amenity landscapes often require supplemental water application in situations of low soil water availability; such as in root restricted urban settings, during unusual or seasonal periods of low precipitation, or in arid regions where irrigation is essential year round. In the arid western United States, there is growing public concern about the long-term viability of water resources. Recent trends in landscaping include the use of drought-adapted trees and shrubs in low water use landscape designs, and some municipalities restrict the use of turf and offer incentives to convert to xeric designs. Paradoxically, planting schemes and maintenance practices often include excessively dense plantings and over irrigation of xeriphytic species, which must then be frequently pruned to limit over crowding and obstruction of roads and walkways. Water use efficiency (WUE) of two Southwest landscape shrubs, Nerium oleander 'Sister Angus' (oleander) and Leucophyllum frutescens var. green cloud (texas sage), was determined in a field study in response to 2×3 factorial treatment of irrigation volume (high or low volume) and pruning frequency (every 6 weeks, 6 months, or unpruned). WUE was defined as the ratio of total biomass produced to volume of irrigation water applied over one year. Shrub biovolume was measured before and after every pruning event or every 3 months. All biomass pruned from shrubs was weighed. After one year's growth, 12 unpruned control shrubs of each species and irrigation treatment were harvested to determine allo-metric relationships between standing biovolume and standing biomass. Greatest total biomass production occurred for shrubs receiving the high irrigation volumes or shrubs left unpruned. WUE was highest for unpruned shrubs receiving low irrigation volumes and was lowest for the 6-week pruned shrubs receiving high irrigation volumes. These data suggest that frequent pruning lowered WUE of well-watered oleander and texas sage.