Experiments were devised to establish suitable techniques of screening seedling material of Lolium perenne for cold tolerance under controlled conditions: these techniques were then used to distinguish between varieties and ecotypes from contrasting climatic regions. For the wide range of material used, greatest discrimination was achieved by 14 days hardening at 2 °C and 8 h photoperiod, followed by 2–4 days freezing at -8 °C in the dark. There were indications that the light intensity and/or temperature before and during hardening could considerably modify the cold tolerance.
The most useful criterion of cold tolerance was the percentage of tillers surviving 14 days after thawing. In some instances, death of tillers and whole plants continued after this time, and this tendency appeared to be related to growth habit; southern ecotypes were particularly susceptible.
Varieties from north Europe were generally more cold tolerant than ecotypes from the Mediterranean region, but tolerance depended also on the altitude of the original habitat. The cold tolerance of a population could, in part, be related to the average lowest temperature in the coldest month and in part to environmental conditions of solar radiation and/or temperature during the autumn in its place of origin.
Along with the experiments under controlled conditions, spaced plants of the same populations were grown in the field in order to evaluate winter hardiness under local conditions. The overall correlation between winter hardiness in the field and cold tolerance as assessed in the growth rooms was not high. Results from one year's field testing showed that indigenous varieties such as S. 23 and S. 24 were more winter hardy than foreign ecotypes and varieties, even if these came from colder climates and showed greater cold tolerance in the growth room.