Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Plants, Soils, and Climate

Committee Chair(s)

Youping Sun (Co-Chair), Kelly Kopp (Co-Chair)


Youping Sun


Kelly Kopp


Scott B. Jones


Lawrence Hipps


Lorence R. Oki


Increases in urban population and inadequate rainfall result in imbalanced water budgets in urban and sub-urban regions. Water conservation becomes important in urban landscapes because of increased water demands. Modern landscape designs require drought-resistant plants to maintain urban landscape greenness during water scarcity. Irrigating plants at their irrigation requirements, which is the minimum irrigation rate that can achieve acceptable aesthetic quality, can also conserve water. However, the drought tolerance and irrigation requrements of most landscape plants have not been widely evaluated. Shepherdia ×utahensis ‘Torrey’ (‘Torrey’ buffaloberry) and Penstemon species (beardtongues) are low-water-use landscape plants, but their drought resistance mechanisms are largely unknown. Despite garden roses (Rosa ×hybrida L.) being widely used in residential landscapes, their responses to reduced irrigation frequency and irrigation requirements are unclear. The objectives of this research were to determine the plant growth, morphology, and physiology of ‘Torrey’ buffaloberry, four penstemon species, and five garden rose cultivars under drought stress resulting from reduced substrate water content or irrigation frequency. Shepherdia ×utahensis ‘Torrey’ and the four penstemons were grown using an automated irrigation system to preciously control their substrate volumetric water contents at their setpoints between 0.40 to 0.05 m3·m-3 in a greenhouse for 50 days, whereas the five rose cultivars were irrigated at three irrigation frequencies (high, medium, and low) in an open field, where the irrigation frequency at the high, medium, and low levels was controlled using 80%, 50%, and 20% reference evapotranspiration (ETO), respectively. Under reduced substrate moisture levels, S. ×utahensis promoted root growth, closed stomata, and increased leaf trichomes density to regulate canopy temperature. Penstemon species native to arid regions, such as P. barbatus (Cav.) Roth ‘Novapenblu’ (Rock Candy Blue® penstemon) and P. strictus Benth. (Rocky Mountain penstemon), showed greater leaf reflectance, a higher volume of root system, and cooler canopy temperatures than the mesic penstemon species, including P. digitalis Nutt. ex Sims ‘TNPENDB’ (Dakota™ Burgundy beardtongue) and P. ×mexicali Mitch. ‘P007S’ (Pikes Peak Purple® penstemon). Rose cultivars were able to partially close their stomata to reduce water loss when irrigation frequency dereased or air temperatures increased. ‘MEIRIFTDAY’ and ‘Meibenbino’ roses, which showed reductions in the dry weights of leaves and stems and exhibited unacceptable overall appearance, were less tolerant to reduced irrigation compared with ‘ChewPatout’, ‘Overedclimb’, and ‘Radbeauty’ roses when irrigation events decreased from eight to one time during the growing season. Our research showed that landscape plants might change their morphology and physiology to allow them to tolerate water deficit and leaf overheating under drought conditions. Using plants with drought resilience could maintain acceptable visual quality with reduced irrigation water.