Invasion potential of herbaceous ornamental perennials in northern climate conditions

Ornamental plants comprise a great share of alien plant species worldwide. In northern regions, harsh climate conditions limit their invasiveness. We studied invasion potential of perennial herbaceous ornamental plants in Finland (between 60 and 65 degrees N) by using species ranking based on their reproduction success as an indicator. Data were collected from four-year common garden and moisture-controlled field experiments which were completed with germination tests of seeds in a greenhouse. Altogether, 220 clones from 166 species were included in the studies. In common gardens, 50% of the species were found to produce seedlings and 75% rhizomes, respectively. Twelve of the clones produced neither seedlings nor rhizomes. Rankings of the invasion potential based on the two reproduction modes did not correlate. The species known for their invasion potential in temperate or cool climates appeared to be among the highest ranking seedling producers in the common gardens. In the field experiment, the highest seedling production (73%) was found in the semi-dry moisture regime, followed by the dry (40%) and the moist (27%) regimes. In the greenhouse experiment, 83% of the studied clones and 84% of the species emerged. Temperature sum required for the production of viable seeds for one third of the studied species is reached at least every second year in latitudes 62-63 degrees N. Several perennial herbaceous ornamentals have a potential for northward range expansion.