Urban green infrastructures offer thermal regulation to mitigate urban heat island effects. To gain a better understanding of the cooling ability of transpiring plants at the leaf level, we developed a method to measure the time series of thermal data with a miniaturized, uncalibrated thermal infrared camera. We examined the canopy temperature of four characteristic living wall plants (Heuchera x cultorum, Bergenia cordifolia, Geranium sanguineum, and Brunnera macrophylla) under increasing drought stress and compared them with a well-watered control group. The method proved suitable to evaluate differences in canopy temperature between the different treatments. Leaf temperatures of water-stressed plants were 6 to 8 °C higher than those well-watered, with differences among species. In order to cool through transpiration, vegetation in green infrastructures must be sufficiently supplied with water. Thermal cameras were found to be useful to monitor vertical greening because leaf surface temperature is closely related to drought stress. The usage of thermal cameras mounted on unmanned aerial vehicles could be a rapid and easy monitoring system to cover large façades.

Water is the key resource in fulfilling the cooling function of plants in urban environments and needs to be supplied reliably and adequately, especially during dry periods. To avoid an unsustainable use of high-quality drinking water for irrigation, the reuse of greywater should be implemented for Green Infrastructure irrigation in the sense of the circular economy. In this study, the influence of greywater irrigation on vitality of two trees species, Tilia cordata and Acer pseudoplatanus, was determined by investigating the effect of irrigation with raw or treated greywater in comparison to municipal tap water. Plant growth parameters were measured, including leaf area, number of leaves, average leaf area and annual growth. In addition, the relative chlorophyll content was determined and image analysis was used to identify vital and necrotic leaf parts. While treatment did not affect growth after one growing season A. pseudoplatanus had significantly higher leaf necrosis (34.8%) when irrigated with raw greywater compared to treated greywater (15.5%) and tap water (5.8%). Relative chlorophyll content of T. cordata irrigated with tap water decreased over time until it was significantly lower (28.5) then the greywater treatments (34.5 and 35). Image analysis of leaves to quantify necrosis proved to be a sensitive method to quantify plant health and showed negative effects earlier than an analysis of growth. Anionic surfactants and electrical conductivity had a significant influence on plant vitality. Therefore, plant selection should take these parameters into account, when planning green infrastructure irrigated with greywater.

Climate change forecasts for Central Europe predict increased drought and reduced summer precipitation, potentially endangering established forest ecosystems. Reliable detection of drought-stressed forests could be helpful to plan and initiate mitigation measures. This study evaluates the effectiveness of thermal infrared imaging in detecting drought stress in coniferous species, Picea abies (Norway spruce) and Abies alba (Silver fir), under controlled conditions. We conducted a 45-day experiment in a controlled environment using low-cost, high-resolution thermal cameras to monitor differences in leaf surface temperatures (TDIFF) of drought-stressed and well-watered potted plants. We also measured transpiration rates using weight cells to understand the link between leaf temperature and water loss. Our findings indicate that drought stress is detectable 17–25 days earlier in the thermal infrared than through visual symptoms, such as leaf desiccation. For A. alba, TDIFF increased significantly by Day 9 of drought stress imposition, while for P. abies, the first signs appeared at Day 13. Maximum mean TDIFF values reached up to 1.5°C for A. alba and P. abies, demonstrating a clear thermal response to drought stress. The correlation between decreasing transpiration rates and increasing TDIFF highlights the utility of thermal infrared imaging for the early detection of drought stress in coniferous species. This research supports the potential of thermal infrared imaging for monitoring plant water stress while noting the challenges of controlling environmental variables in practice.