<p>Heatwaves are an increasing threat to urban health and comfort, and evapotranspiration by urban lawns and trees offers a potential solution. However, their distinct effects and mechanisms remain unclear. Using ten years of observations, we investigate the evapotranspiration responses of urban lawns and trees to 54 heatwave events in a subtropical city. We hypothesize that urban trees and lawns exhibit distinct evapotranspiration response patterns during heatwaves due to different water-use strategies and stomatal regulations. Our results show that (1) lawns, with high canopy stomatal conductance, rapidly increase evapotranspiration (+ 37.65%), providing significant cooling (7.05 °C m⁻² per day), but at the cost of rapid surface water depletion. (2) Although trees provide a lower cooling effect of 3.5 °C m⁻² per day, they stabilize transpiration during heatwaves by closing their canopy stomata (−35.06%) and accessing deeper soil moisture. This strategy results in a slight decrease in transpiration rates (from 1.77 to 1.66 mm per day), ensuring more stable water use and cooling despite the extreme conditions. These findings highlight the different water-use strategies and cooling capacities of lawns and trees, offering critical insights for optimizing urban vegetation design in regions facing diverse climatic and water-availability challenges.</p>