AbstractDrought stress is one of the critical environmental factors in determining growth and survival of herbage grasses. In this study, by using a hydroponic culture system including different amounts of polyethylene glycol (PEG), responses of plant shoots to water stress in four different intensities (0 Mpa, −0.6 Mpa, −1.2 Mpa and −1.8 Mpa) were examined in diploid and tetraploid cultivars of Italian ryegrass (Lolium multiflorum) and perennial ryegrass (L. perenne). Since freezing injury is caused by cell dehydration, freezing tolerance was also examined for six subzero temperatures (−11, −12, −14, −16, −18 and −20°C) in both species. L. multiflorum had a larger shoot biomass at all stress intensities and a lower survival rate under severe water stress and freezing stress conditions than L. perenne. Thus, there was a trade‐off (negative correlation) between potential growth under a stress‐free condition and survival under a severe stress condition in diploid and tetraploid cultivars of both species. This trade‐off was mediated by tissue water content. High water content led to a high growth rate through increasing specific leaf area, while low water content resulted in a high tissue osmotic potential that could confer high cell dehydration tolerance.