Establishment of drought tolerant vegetation is essential for the sustainability of extensive green roofs when irrigation is not installed. This study explored the impacts of watering regimes on drought tolerance potential of green roof plants by examining morphological and physiological data of roots. It was found that deficit watering at the early planting stage tended to induce thinner root size and higher root:shoot ratios in S. lineare. These changes in root traits were likely to positively affect root activity and drought tolerance of plants under drought stress in extensive green roof systems. Although frequent watering tended to produce bigger plant biomass and frondent vegetation, it might also lead to bigger root size and lower root:shoot ratios, which appeared to result in lower root activity, higher root mortality and poorer drought tolerance performance under severe drought stress on a rooftop. An appropriate deficit watering regime at the early planting stage may contribute to better drought tolerance of green roof vegetation. From a practical viewpoint, what one is really doing is acclimating plants to drought conditions, but the irrigation frequency should be curtailed slowly and not all at once.