Hybrid aspen and hybrid poplars have been cultivated in Sweden since the end of the 1930s. The hybrid aspen was expected to do best on moraine soils and hybrid poplar on alluvial soils. Some of these plantations still exist but most are in poor condition. The reason for this is largely the origin of the species involved. Most of them are from Oregon and Washington State in the U.S.A., which is about 1000 km to the south of the location where they are now cultivated. This means that introduced poplar clones start to grow too early in the spring and set buds too late in the autumn. Frost then damages them severely, particularly late spring frosts in the beginning of the growing season and early autumn frost at the end. Intensive breeding and rapid genetic change requires good knowledge about both genetic control and physiology of important traits. Poplars are probably the species most studied in this respect. In recent years, molecular biology has provided tools suitable for rapid and detailed genetic analysis of higher organisms. It is supposed that the frost problem of poplar plantations in Sweden can be solved by advanced breeding and hybridisation between the species Populus trichocarpa and Populus deltoides from appropriate latitudes in the U.S.A. Copyright (C) 1996 Elsevier Science Ltd.