Less than half of the approximately 2630 plant species found in Norway occur naturally in this country. Of the species that have immigrated, 593 have become a permanent part of the Norwegian vegetation. More than 110 of these are thought to be expanding and some of them may have a significant impact on the vegetation.

Alien species have come here from all parts of the world. They have arrived on ships, by road and rail transport, in seed mixtures and grass, as agricultural, horticultural and forestry plants, as garden plants, through tourism, by being sown on roadside verges, and with imports of grain and other consumer goods. Botanical gardens and gardening activities are responsible for the introduction of approximately 45 per cent of the species now spreading in Norway. 

Alien species of vascular* plants that are spreading in Norway (excluding Svalbard)

* Vascular plants are plants with special tissues for transporting fluids, i.e. all ordinary plants with stems, roots and leaves. Mosses, lichens and algae are not vascular plants.

Source: Fremstad, E. & R. Elven 1997: Alien plants in Norway; a review.
Norsk geografisk tidsskrift Vol. 51. 199-218.