Foreign plants have been introduced into the new environment for various reasons, such as nutritional use, usefulness of plant parts, such as wood, ameliorative abilities, aesthetic value, research interest or other reasons. The first documented introduction of a considerable number of non-native tree species to the territory of present-day Slovenia took place at the end of the 18th and the beginning of the 19th century. At first, species were planted in specialized plant collections, and their planting frequency was low. However, the planting frequency of some species was high since they were widely used for afforestation of degraded soils. In forests non-native tree species were more widely introduced in the period between the two world wars, and already at that time there were considerations of the suitability and possibilities of planting of larger number of non-native tree species.