Response of Landscape Groundcovers to Deficit Irrigation: An Assessment Based on Normalized Difference Vegetation Index and Visual Quality Rating

Abstract

Developing water conservation strategies for urban landscape groundcovers grown in hot and dry summers like inland Southern California, USA, is crucial because they are one of the largest residential water users. A 2-year (2020-21) study was conducted in Riverside, CA, to assess the effect of irrigation rates on the growth of landscape groundcovers as evaluated by visual quality ratings (VR) and normalized difference vegetation index (NDVI). Relationships between VR and NDVI were also established to obtain the minimum threshold values of NDVI for each groundcover. Lastly, the groundcover water response function was developed to estimate groundcover response to irrigation rates over time. Four reference evapotranspiration (ETo)-based irrigation treatments ranging from 24% to 99% ETo and 10 landscape groundcovers were laid in a randomized complete block design and replicated three times. Data were collected from May to October in 2020 and 2021. The irrigation controller overirrigated the plots on average by 7.7% and 4.7% in 2020 and 2021, respectively. A significant relationship (P < 0.05, 0.35 # R2 # 0.82) between NDVI and VR for each landscape groundcovers was found. On the basis of the NDVI values and VR, it was found that three landscape groundcovers, including Rhadogia spinescens, Baccharis × 'Starn' Thompson, Eriogonum fasciculatum 'Warriner Lytle' can withstand water stress and can maintain their growth and visual quality at 24% ETo irrigation. Groundcovers Ruschia lineolate nana, Rosmarinus officinalis 'Roman Beauty', and Eremphila glabra showed the potential to perform well with as low as 49% ETo irrigation, whereas Lantana montevidensis, Oenothera stubbei, and Lonicera japonica required 75% ETo or more.