A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 45 Manuscript received December 2018 Invasion potential of herbaceous ornamental perennials in northern climate conditions Timo Kaukoranta1, Sirkka Juhanoja2, Eeva-Maria Tuhkanen2 and Terho Hyvönen1 1Natural Resources Institute Finland (Luke), Tietotie 4, FI-31600 Jokioinen, Finland 2Natural Resources Institute Finland (Luke), Itäinen pitkäkatu 4 A, 20520 Turku, Finland e-mail: terho.hyvonen@luke.fi Ornamental plants comprise a great share of alien plant species worldwide. In northern regions, harsh climate con- ditions limit their invasiveness. We studied invasion potential of perennial herbaceous ornamental plants in Finland (between 60 and 65° N) by using species ranking based on their reproduction success as an indicator. Data were col- lected from four-year common garden and moisture-controlled field experiments which were completed with ger- mination 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 seed- ling production (73%) was found in the semi-dry moisture regime, followed by the dry (40%) and the moist (27%) re- gimes. 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° N. Several perennial herbaceous ornamentals have a potential for northward range expansion. Key words: alien species, Finland, garden plants, invasive plants, species distribution Introduction In search for new ornamental species, alien species have long been widely introduced outside their native ranges. The wide and repeated introductions increase the likelihood of them escaping from cultivation (Dehnen-Schmutz et al. 2007a, 2007b, Hanspach et al. 2008, Ööpik et al. 2013). Thus, it is not surprising that ornamental plants com- prise of a great share of alien plant species worldwide. For example, 17% of alien plant species in Europe (Lamb- don et al. 2008) and around 70% of weeds in Australia have been imported for gardening (Virtue et al. 2004). Also in northern regions the share of ornamentals of introduced plants is high ranging from over half of the vascular plants regarded as problem species in Norway (Gederaas et al. 2012), 74% of 232 naturalized alien species in Es- tonia (Ööpik et al. 2008) to 18 out of 23 invasive species in Finland (MMM 2012). Ornamentals have been selected for rapid and often vigorous growth, large size, long flowering time, tolerance of competition and survival in physically and biologically adverse environments. These traits also characterize suc- cessful alien invasive plant species (Klironomos 2002, Lake and Leishman 2004, Anderson et al. 2006, Colautti et al. 2006, Pyšek and Richardson 2007, Hanspach et al. 2008, Küster et al. 2008, Fenesi and Botta-Dukát 2010, van Kleunen et al. 2010). Species traits, that generally are not actively selected for in perennial ornamentals, but have been associated with invasiveness in several studies, are the abundant production of seed and rapid germination (Callaway and Josselyn 1992, Pérez-Fernández et al. 2000, Goergen and Daehler 2001, van Kleunen and Johnson 2007, Küster et al. 2008, Perglová et al. 2009, Schlaepfer et al. 2010, Chrobock et al. 2011). Seed production is not a prerequisite for a species to escape from cultivation, which is shown by the fact that some of the globally worst invasive terrestrial plant species, such as Arundo donax L. (Saltonstall et al. 2010) and Fallopia japonica (Houtt.) Ronse Decr. (Engler et al. 2011, Bailey et al. 2009, Weber 2003), propagate rarely by seed in invaded regions. This may facilitate the invasion of perennial ornamentals in northern regions where harsh climate conditions limit seed production. A necessary precondition for a species to survive and become naturalized is climatic match with the source and tar- get regions (see however Milbau and Stout 2008). The match is more likely for species that tolerate a wide range of climates and often predicts naturalization and consequent invasion (Goodwin et al. 1999, Thuiller et al. 2005, Hanspach et al. 2008, Pyšek et al. 2009, Alexander et al. 2011, Dostál et al. 2013), though not in all target areas (Milbau and Stout 2008). Climate matching has usually been studied by modelling (e.g. Thuiller et al. 2005, Araújo A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 46 and Peterson 2012), which provide information on potentially suitable climate conditions in new regions. How- ever, actual reproduction success must be studied by field experiments. For high latitude and cool climates data for comparison of the relationship between the invasiveness and reproduction of ornamental plants is sparse. In this study, we used data from experiments that were planned for testing hardiness of a wide range of herba- ceous ornamental perennials at northern edge of their range. Useful metrics that were observed were establish- ment of new seedlings and rhizomes in open air and emergence of seedlings in the greenhouse. Our goals were 1) to assess the proportion of species that can reproduce by seed among those that can survive over winter, 2) to assess their effectiveness for producing viable seeds and seedlings and 3) based on reproduction success in the climates of the experimental sites, to identify the species that have the highest potential for spreading to the bo- real climate zone. We expected the majority of the species to succeed in the sexual reproduction since the plant species represented herbaceous perennial ornamentals that had been found to tolerate cold winters, spring frosts, and short growing seasons (Tuhkanen and Juhanoja 2010). However, reproduction could be also confounded by other factors that were not systematically recorded: pests and diseases or short day requirement (19–22 hours day-length in the study region), self-incompatibility or sterility. Methods Plant material and experiments The data consisted of observations on seedling and rhizome production in (1) common garden experiments in five cities, (2) a field experiment with controlled minimum soil moisture, and (3) a greenhouse test of emergence rate and germination speed of seeds collected from the field experiment. In the cities and in the field experiment, ob- servations were made in 2006–2010. These three types of data sources represented different geographic scales and measurement accuracies. The common gardens covered a wide climatic range and many years, but the obser- vations were only semi-quantitative. In the moisture-controlled field experiment, quantitative observations were made over years. The greenhouse experiment allowed accurate, repeated observations. Using the experimental data, each clones’ potential geographic ranges of reproduction by seed were assessed by mapping over Finland and Baltic Sea region temperature sums (ETS) of recent climate and comparing it with the ETS values the clones required for the production of viable seed in the experiments. Originally, the experimental planning aimed at finding hardy perennials that require low maintenance and have high ornamental value. The perennials were vigorous wild types that have not been intensively bred. They were expected to compete effectively for space by being either tall and large plant types, or effectively vertically spread- ing types. Where several origins were available for a species, the clones obtained from southern nurseries were used in the northern experimental sites. However, even the southern experimental sites can be considered to be high in the north (60 to 64°N) relative to the original growing areas of non-native species. Rooted cuttings for planting were obtained from six Finnish commercial nurseries, some from botanical gar- dens of the Universities of Helsinki, Joensuu, Oulu and Turku, and a few from private collections (Juhanoja and Lukkala 2008). Altogether 220 clones from 166 species were included in the study (Table 1, Appendix 1). Table 1. Number of species and clones observed in common gardens, in moisture controlled field experiment and in greenhouse experiment Status Experiment Location Irrigation threshold No of species No of clones Native Alien found in Finland Alien not found in Finland Common garden Helsinki 112 117 18 37 61 Tampere 39 41 4 18 19 Turku 69 73 14 19 40 Kuopio 38 41 5 16 20 Oulu 69 72 9 23 39 Field experiment Piikkiö –100 52 61 7 22 31 –150 83 96 12 29 54 –200 70 82 11 27 43 Greenhouse Piikkiö 79 84 9 29 44 A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 47 Nineteen species were native to Finland and 37 were aliens reportedly growing in Finland, the others have been grown or tested in parks and gardens but not reported to have spread to landscape (Lampinen et al. 2012). Three species – Solidago canadensis L., Fallopia japonica and Vinca minor L. – that are considered invasive in Finland (MMM 2012), the Baltic Sea region or climatically nearly analogous areas of North America (Global Invasive Spe- cies Database 2014, EPPO 2019, Invasive Plant Atlas of the United States 2014, National Invasive Species Infor- mation Center 2014, DAISIE European Invasive Alien Species Gateway 2014, NOBANIS 2019) were included in the experiments. Common garden experiments Experiments were established in 2005 in five city areas: Oulu (65°01′N, 25°28′E) (four sites), Kuopio (62°53′N, 27°40″E) (two sites), Tampere (61°29′N, 023°45′E) (three sites), Turku (60°27′N, 022°16′E) (one site), and Helsin- ki (60°10′N, 024°56′E) with its adjacent cities Espoo, Vantaa and Kerava (23 sites) (Fig. 1). Rooted cuttings were planted in 1373 observations plots. All species were not tested in all sites and all cities, and the species composi- tion for each site was selected for expected tolerance to local climate and site type (park, cemetery, shade, sun). In the common gardens, the size of the planted areas varied from about 10 m2 to over 100 m2. Their shapes were adjusted to match planting sites. A homogenous, weed free 30–50 cm thick soil bed (Kekkilän perennamulta, Kek- kilä, Tuusula, Finland; pH 6, organic matter 14%, nitrogen-phosphorus-potassium 100-2-100 mg kg-1 of dry matter) was used at all sites. In early spring, withered plants were cut back and plant residues were mostly left to serve as a source of nutrients (Juhanoja and Lukkala 2008). Starting one year after planting, reproduction of the clones by seed and rhizomes or runners was scored early in September in 2006–2010. The scale was (0) no, (1) 1–5, (2) >5 seedlings per m2 emerged or rhizomes produced. Though at each site the clones were planted in several plots, only one seedling and one rhizome score were re- corded for each clone and site in each year. To get a largely comparable selection of species for every city, the data were pooled over observation sites and years in each city by calculating means and maximums of the scores for each species in a city. Thus, in further analysis there was one observation for the seedling emergence and one observation for the rhizome production for each species in a city. The clones were ranked in each city both by the scores of the seedling emergence and the rhizome production. In each city, (1) Pearson correlation was used for testing for the association of the mean and maximum scores of the seedling emergence/ rhizome production. (2) Consistency of the results over the cities was tested by computing Turku Piikkiö Helsinki Oulu Kuopio Fig. 1. Median of degree-days (ETS) 1999–2018 (base temperature 5 °C) A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 48 correlations between the cities: mean/max seedling emerge and min/max rhizome production. (3) Kendall’s tau-b correlation was applied for testing for the association of the status of alien species (not found in Finland by 2012 / alien found in Finland) with mean score of seedling emergence. The correlation was computed for all clones and for the clones which had maximum seedling emergence scores greater than 0.2. Moisture-controlled field experiment An experiment with similar objectives as the common garden experiment was established in Piikkiö (60°23’N, 22°33’E) with 108 clones representing 95 species (Juhanoja and Lukkala 2008). In contrast to the common garden experiment, the soil moisture was accurately controlled and observations on seedling emergence were quantitative. Three soil moisture regimes were set up to provide optimal conditions for plant species adapted to moist, semi-dry or dry conditions with irrigation thresholds –100, –150, –200 hPa, respectively. Each regime was divided in three blocks to create three replicates of the plots. A plot of one square meter for a clone consisted of a double row of two to 16 plants, depending on the expected final size of plants. Pathways between the rows were covered with hardwood chips. Soil bed was made of 30–50 cm thick layer of weed free soil mixture (Kekkilän perennamulta, Kekkilä, Tuusula, Finland). Plants were grown in full sunlight, except shade requiring species that were covered by a green screen that reduced radiation by 50% (Juhanoja and Tuhkanen 2010, Juhanoja and Tuhkanen 2013, Tuh- kanen and Juhanoja 2010, 2013). Soil moistures were monitored by tensiometers and maintained by drip irrigation. Observations on seedling emergence were made in 2006–2010. Most seedlings emerged and could be scored only in the first two or three years after planting when the canopies were not yet fully closed. Seedlings that had emerged during a growing season were removed in late autumn, they were identified and their numbers counted. The experiment was continued until 2012 when seeds were collected for a greenhouse experiment. Abundance of seed production by the clones was assessed qualitatively on the following percentage scale: 0, 25, 50, 75, 100. The clones were ranked by the number of seedlings per plot (1 m2). The rank orders of the clones were compared pair-wise by computing least square differences of the rank orders with proc GLM of the SAS statistical software. Rankings of the soil moisture regimes were compared using Kendall’s tau-b correlation. In each regime-pair com- parison, only the clones that existed in both regimes were included. Emergence rate and germination speed experiment in greenhouse A greenhouse experiment was conducted to assess the capacity of clones for producing viable seed and to estimate their germination rate. Seeds were collected in the moisture controlled field experiment in 2012 after a cool, rainy growing season. The season lasted 183 days above 5 °C and had an effective temperature sum (ETS) of 1385 degree-days (dd) with a base temperature 5 °C. Seeds were stored in paper bags at 5 °C until sowing into the greenhouse from mid-December to mid-March. Seeds of each clone were sown in four one-litre pots containing half a litre of soil mixture (Kekkilän perennamulta, Kekkilä, Tuusula, Finland). The pots were irrigated three times a week from above. Heating, ventilation and shading screen were used for controlling air temperature. It was al- lowed to freely fluctuate between 10 °C and 25 °C, which corresponds approximately to mid-latitude climate from spring to autumn in Western Europe. Seedling emergence was observed over 221 days after the sowing (d.a.s.), first every couple of days, then weekly and at the end biweekly. From the data were computed final emergence at 221 d.a.s. (FE221) and durations from sowing to 1% (D1d), 10% (D10d) and 30% (D30d). Transformation SQRT(FE221+0.5) was applied to stabilize variance of FE221 and then significance of pair-wise least square differences (LSD) of FE221 were computed with proc GLM of SAS statistical software. Significance (p < 0.05) of pair-wise LSD of D10d and D30d were computed with proc GENMOD of SAS statistical software using gamma distribution with inverse (power(-1)) link function. All clones were tested pair-wise despite this leading to post-hoc testing. Association for emergence speed and final emergence was analyzed by computing Pearson cor- relations between the final emergence and D10d and D30d. The clones were ranked by their ability to produce seedlings in the common gardens and controlled moisture ex- periment to five groups. The groups were assigned coefficients 0, 25, 50, 75, and 100 that reflected the abundance of seed production (ASP). ASP and FE221 were used to get a qualitative measure of the potential for spread by seed (PSeed): PSeed = ASP × FE221. The clones in the experiments were ranked by PSeed, but their differences were not tested statistically. A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 49 Comparing experiments The clonal compositions of the three experiment types were partially overlapping. Of the clones in the common garden experiment 49% and 38% were included in the moisture controlled and greenhouse experiment, respec- tively. 41% of the clones in the greenhouse experiment were included in the moisture controlled experiment. To test the similarity of species rankings in different types of experiments, Kendall’s tau-b rank correlation was applied. When comparing experiments, only the clones present in all compared experiments were included in the analyses: (1) Experiments in the common gardens were compared with each other for the rank of mean and maximum score of seedling emergence, (2) Common gardens (rank of maximum score of seedling emergence) were compared with the moisture controlled field experiment (rank of number of seedlings per square meter), (3) The greenhouse experiment (FE221 and PSeed) was compared with the common garden experiments (rank of maximum score of seedling emergence) and (4) with the moisture controlled field experiment (rank of number of seedlings per square meter). Geographic range of seed production limited by ETS To estimate potential geographic range of spread by seed as limited by warmness of summer in the Baltic Sea region, ETS requirements for the seedling emergence and production of viable seed were compared with the median ETS in the region. For the experimental sites ETS above a base temperature of 5 °C was computed using daily gridded temperature data 2005–2010 from Finnish Meteorological Institute (FMI), except for the Piikkiö field where data from a FMI weather station located in the field. In the common garden experiment and in the mois- ture experiment, it was not known in which year the seeds that gave rise to seedlings had grown. Therefore, the highest ETS value of the three years preceding an observation of a seedling emergence was attached to that ob- servation. For the clones tested in the Piikkiö greenhouse in 2013, exact ETS was known because the seeds had been collected in 2012. The median ETS for years 1999–2018 was mapped using gridded (0.1° resolution) daily mean temperature from E-OBS v. 19.0e dataset (Cornes et al. 2018). Results Reproduction by seeds and rhizomes in common gardens The majority of the species and clones tested in common gardens produced seedlings in Helsinki, about half of species in three cities, and a third of species in Kuopio (Table 2). In all cities about half of the species and clones produced rhizomes. Over all of the gardens, 60% of the clones and 50% of the species were found to produce seedlings, and 73% and 75% rhizomes, respectively (Appendix 1). Twelve of the clones were observed to produce neither seedlings nor rhizomes. The maximum seedling emergence scores were moderately and significantly cor- related between all cities, except ranks in Turku with the ranks in the inland cities of Kuopio and Tampere (Table 3). The mean and maximum scores of the seedling emergence in each city were strongly related, having significant Pearson correlation coefficients above 0.9 (result not shown). Table 2. Percentage of species and clones that were found to produce seedlings and rhizomes in common gardens and moisture controlled field experiment. In greenhouse experiment percentage of species and clones that emerged during 221 days after sowing. Seedlings (%) Rhizomes (%) Experiment Location Irrigation threshold Species Clones Species Clones Common garden Helsinki 59 77 55 51 Tampere 46 41 54 50 Turku 45 29 54 53 Kuopio 32 22 48 44 Oulu 49 47 54 51 Field1 experiment Piikkiö –100 27 33 – – –150 73 73 – – –200 40 44 – – Greenhouse Piikkiö 84 83 – – 1) Seedlings and rhizomes were not separated. A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 50 The native species Alchemilla alpinus L., Lythrum salicaria L. and Malva moschata L. and non-natives Alchemilla mollis (Buser) Rothm., Heliopsis helianthoides L. var scabra (Dun.) Fern., Hyssopus officinalis L., Salvia nemorosa L., Telekia speciosa (Schreb.) Baumg., and Verbena hastata L. had abundant seedling production most regularly (Appendix 2). The rank correlations of all alien clones (alien not found vs. found in Finland) and the clones which had maximum seedling emergence scores greater than 0.2 (alien not found vs. found in Finland, clone emergence score > 0.2) with the maximum seedling emergence were not significant (result not shown), To test whether the status effect would be found among the alien clones that produced seedlings at least moder- ately, the correlation was computed for the clones which had maximum seedling emergence scores greater than 0.2. Again, the status was not found to be significantly correlated with the emergence (result not shown). The most efficient rhizome producers were not good seedling producers. This can be seen in Table 4, where mean rank of seedling emergence (S rank) is shown in parallel with the rhizome production scores. However, the correla- tions of the city-wise mean/max rhizome production with the mean/max of the seedling emergence were not statisti- cally significant. The mean rank of seedling emergence was computed by averaging the city-wise ranks of the clones. Reproduction in field experiment In all moisture regimes, about 70% of 105 clones and species produced seedlings (Appendix 1). The highest seed- ling production (73%) was found in in the semi-dry moisture regime (–150 hPa irrigation threshold), followed by dry (–200 hPa) and moist regimes (–100 hPa) (Table 2). Fourteen clones started flowering in August or September, which was too late for seeds to mature. The clonal rankings in the dry regime were correlated with the rankings in semi-dry regimes irrigation threshold) every year. Whereas the rankings in the moist regime were not correlated with the rankings in the other regimes (results not shown). Comparisons of species can be done on plot area basis but not on per planted plant basis because the number of plants planted was not equal for all species. Solidago canadensis ‘Goldkind’ was among the highest seedling producers in all soil moisture regimes (Table 5), whereas ‘Leraft’ did not produce any seedlings. Alien Anaphalis margaritacea performed well in all regimes and aliens Heliopsis helianthoides, Inula helenium L. and Telekia spe- ciosa were among the ten highest ranking seedling producers in the two regimes with the highest soil moisture (–100, –150 hPa). Emergency rate and germination speed in greenhouse The overall seed germination rate was high in the greenhouse experiment: 83% of the studied clones and 84% of the species emerged in the experiment (Table 2, Appendix 1). 18–19% of the clones and species had at least 30% emergence within 35 d.a.s. (Table 6). Emergence rate was mostly stable within each clone, as indicated by the fact that the durations from sowing to 1% (D1d), 10% (D10d), and 30% (D30d) emergence were highly and signifi- cantly correlated (results not shown). The emergence speed was not related to the final cumulative emergence at 221 d.a.s. (FE221), which was shown by low, non-significant Pearson correlations of FE221 with D1d, D10d, and D30d (results not shown). Table 3. Kendall’s tau-b correlations of maximum scores in common garden experiments among cities Helsinki Tampere Turku Kuopio Tampere tau-b 0.33 – – – p 0.015 – – – N 35 – – – Turku tau-b 0.35 0.40 – – p 0.001 0.037 – – N 62 22 – – Kuopio tau-b 0.42 0.25 0.07 – p 0.003 0.294 0.656 – N 37 15 30 – Oulu tau-b 0.31 0.46 0.39 0.44 p 0.003 0.004 0.002 0.018 N 59 27 43 23 p=probability of tau-b differing from 0; N=number of clone pairs in correlation test A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 51 The fastest to emerge and also the ones which had the greatest potential for spread by seed (PSeed) were Dian- thus deltoides L. ’Leuchtfunk’, Aconogonon divaricatum (L.) Nakai ex Mori, Solidago canadensis ‘Goldkind’ and Cerastium Tomentosum L. var. columnae Silberteppich. Seven out of eight native species had low emergence rate. An exception was a non-native cultivar ‘Leuchtfunk’ of Dianthus deltoides. The group of alien species found (42 clones) and not found (32 clones) in Finland did not essen- tially differ in emergence speed and final emergence rate. Their mean D1d and mean FE221 were 53 d / 23% and 47 d / 24%, respectively. Their mean PSeed and mean rank by PSeed were 1218 / 60 and 1751 / 65, respectively. Comparing experiments The rankings of the clones in the common gardens, based on mean score of emergence over 2006–2010, were not consistently correlated with the mean rankings in the moisture controlled field experiment (Table 7). Most- ly, the rank correlations were positive but they were at least moderately significant (p < 0.1) only in five of the 15 correlation pairs. Telekia speciosa, which was the most successful seedling producer in the common gardens (see Table 3), appeared also in the top ten in the moist and semi-dry regimes in the moisture controlled field experi- ment (Table 5). Hyssopus officinalis, Euphorbia epithymoides L. and Dracocephalum sibiricum L. that produced well seedlings in the common gardens were in the top ten in the dry and semi-dry regimes in the moisture controlled field experiment. Solidago canadensis, which appeared in the top ten at all moisture regimes in the moisture con- trolled field experiment (Table 5), produced seedlings in the two warmest common garden sites, Helsinki and Turku. The rankings of the final emergence 221 d.a.s. (FE221) and the potential for spread by seed (PSeed) derived from the greenhouse germination experiment were not correlated with the rankings of mean seedling emergence in the common garden experiments and the Piikkiö field experiment (Kendall’s tau-b correlation, results not shown). Table 4. Mean rhizome production in common garden experiments in 2006–2010. In the righthand column, the S rank is the mean of common garden maximum ranks of seedling emergence. Means were calculated over cities. Fifteen clones ranked highest by rhizome production or having a mean score higher than 0.5 in any garden are shown. Below are also results for other clones that are considered potentially invasive (MMM 2012). Clones are sorted by mean rank of rhizome production over cities. Cities where clones were not tested are marked by dash. Species or clone Helsinki Tampere Turku Kuopio Oulu S rank Echinops ritro 0.48 – – – – >87 Geranium macrorrhizum 0.50 – 0.78 – 0.13 66 Vinca minor 0.50 – 0.00 – 0.88 >87 Lamium galeobdolon ‘Florentinum’ – – 0.48 – 0.35 87 Hosta Tarhafunkia ‘Ginko Craig’ 0.25 – 0.63 0.27 – 38 Euphorbia palustris 0.38 – – – – 65 Veronica austriaca ssp. teucrium ‚Königsblau’ 0.33 0.38 – – 0.38 35 Bistorta officinalis 0.12 – 0.68 0.00 0.50 81 Agastache ‘Blue Fortune’ 0.00 – – 0.80 0.17 59 Hosta sieboldiana ‘Elegans’ 0.32 – – – – >87 Monarda didyma ‘Scorpion’ – – – – 0.31 61 Epimedium x youngianum – 0.31 – – – >87 Hosta Fortunei ‘Hyacinthina’ 0.00 – 0.50 0.41 – 39 Calamagrostis x acutifolia ‘Karl Foerster’ 0.30 – – – – >87 Saponaria ocymoides – – – – 0.30 >87 Cerastium Tomentosum ‘Silberteppich’ 0.06 0.10 0.50 – 0.52 51 Known potentially invasive Solidago canadensis unidentified cultivar 0.00 0.25 0.00 0.42 0.43 50 Solidago Canadensis ’Leraft’ – 0.00 – 0.00 0.45 50 Solidago Canadensis ’Golden Mosa’ 0.10 – – – – 50 Fallopia japonica var. compacta – – – – 0.02 >87 A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 52 Geographic range of seed production limited by ETS For the production of viable seed ETS of 1220–1385 dd appeared to be enough for most clones. Forty-four clones tested in the common gardens were able to produce viable seed with 1300–1400 dd (result not shown). ETS for the 15 highest ranked clones from Appendix 2 is presented in Table 8. The 1300–1400 dd is reached at least every second year up to the latitude 62° N in western Finland, and up to 63° N in eastern Finland (Fig. 1). Every four years this ETS occurs approximately 1.5° higher in the north (results not shown). Table 5. Number of observed seedlings for ten species that produced highest number of seedling in moist, semi-dry and dry blocks in the moisture controlled field experiment. Species are sorted by sum of seedlings over years 2006 to 2010. None of the species differed in post-hoc pair-wise comparison. Year 2006 2007 2008 2009 2010 ETS 1680 1507 1397 1370 1500 Moist Euphorbia palustris 13 9 2 9 14 Lythrum salicaria 20 6 3 2 3 Eupatorium maculatum 0 14 6 2 8 Heliopsis helianthoides 5 5 3 6 3 Aruncus dioicus 0 1 0 10 9 Inula helenium 0 1 0 2 7 Solidago canadensis 4 1 3 0 2 Trollius sp. 2 0 3 1 4 Pulmonaria saccharata 0 0 2 1 3 Telekia speciosa 1 0 0 0 5 Semi-dry Solidago canadensis 182 94 71 71 126 Achillea millefolium 30 139 60 36 45 Anemonidium canadense 16 0 62 77 109 Malva sp. 63 23 18 26 48 Dracocehalum sibiricum 71 27 14 39 25 Heliopsis helianthoides 34 49 28 23 30 Anaphalis margaritacea 58 9 11 12 27 Telekia speciosa 15 5 3 20 37 Inula helenium 0 14 2 19 38 Eupatorium maculatum 0 15 2 9 35 Dry Achillea millefolium 20 27 32 30 34 Anaphalis margaritacea 24 6 10 31 19 Solidago canadensis 16 7 16 18 29 Malva sp. 10 3 8 19 22 Hyssopus officinalis 14 2 6 8 10 Artemisia pontica 15 0 9 7 0 Armeria maritima 9 3 2 3 7 Euphorbia polychroma 0 0 1 6 13 Veronica prostrata 1 0 2 2 15 Veronica virginica 2 0 3 4 5 ETS= effective temperature sum (5 °C threshold) of seasons 2006 to 2010 A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 53 Table 6. Germination of the seeds in the germination experiment. Duration in days from sowing to 10% emergence (D10d) and to 30% emergence (D30d), final emergence 221 days after sowing (FE221), seed production abundance (ASP) and potential for spread by seed (PSeed (PSeed = FE221 × S)) in 2013. Species with the same superscripted letter in columns D10d, D30d and FE221 do not differ statistically in post-hoc pair-wise comparison. Fifteen species which had at least 30% emergence rate in 35 d.a.s. are shown and are ranked by D30d. Species D10d D30d FE221 ASP PSeed Dianthus deltoides ‘Leuchtfunk’ 12a 12a 72ab 100 7200 Aster conspicuus 12a 12a 51bc 75 3825 Salvia x sylvestris 22a 13ab 20ef 50 1000 Salvia nemorosa 12a 15ab 40cd 75 3000 Heliopsis helianthoides var. scabra 12a 15ab 39cd 50 1950 Aconogonon divaricatum 14a 17ab 61b 100 6100 Helenium hoopesii 15a 18b 36cd 75 2700 Aconogonon weyrichii 15a 19b 36cd 75 2700 Cerastium tomentosum ‘Silberteppich’ 17a 21bc 50bc 75 3750 Potentilla megalantha 20a 21bc 30de 25 750 Solidago canadensis ‘Goldkind’ 16a 22bc 61b 100 6100 Nepeta subsessilis 12a 30c 70ab 25 1750 Thymus serpyllum 16a 30c 37cd 25 925 Doronicum orientale 20a 33cd 31d 50 1550 Thymus praecox 20a 33cd 13ef 25 325 Table 7. Kendall’s tau-b on mean seedling emergence (1 m-2) in moisture controlled field experiment in dry, semi- dry and moist blocks with mean scores of seedling emergence each common garden city in 2005–2010 Helsinki Tampere Turku Kuopio Oulu Piikkiö tau-b 0.16704 –0.08146 0.29483 –0.05103 0.36898 Dry block P 0.1961 0.6775 0.0747 0.8504 0.0211 N 37 18 26 11 26 Piikkiö tau-b 0.17918 –0.09311 0.1328 0.35441 0.02627 Semi-dry block P 0.0813 0.5612 0.2922 0.0785 0.8341 N 56 24 39 17 40 Piikkiö tau-b 0.05356 0.20612 0.11605 –0.27779 –0.43202 Moist block P 0.6803 0.3385 0.4869 0.3009 0.0116 N 37 15 25 11 25 Table 8. Maximum effective temperature sum (ETS) of the three years preceding occurrence of seedlings in spring or autumn for the 15 highest ranked species and Solidago canadensis. In the Greenhouse (GH) column, ETS is for the species grown in Piikkiö field and tested in greenhouse. Species Helsinki Tampere Turku Kuopio Oulu Piikkiö field GH Telekia speciosa 1350–1730 1410–1580 1390–1520 – – 1496–1680 1385 Salvia nemorosa ‘Ostfriesland’ 1570–1730 1390–1610 – – 1220–1380 – 1385 Tellima grandiflora 1600–1660 1410–1580 – – 1220–1380 1680 1385 Malva moschata 1570–1650 – 1620–1620 1590–1590 1380–1380 – – Lavatera thuringiaca 1560–1650 – 1620–1620 1590–1660 1220–1380 – 1385 Verbena hastata ’Rosea’ – – 1620–1620 – 1320–1380 1496 – Sanguisorba officinalis 1620–1730 – 1520–1620 – 1220–1380 1680 – Hyssopus officinalis 1490–1730 - - - 1220–1380 1496–1680 1385 Veronica longifolia 1620–1690 – 1620–1620 – 1220–1380 1507 – Euphorbia epithymoides – 1390–1610 – – 1220–1320 1507–1680 1385 Geranium platypetalum – – – – 1220–1380 – – Alchemilla mollis 1570–1694 1410–1580 1520–1620 1440–1550 1220–1320 1507–1680 1385 Aconogonon weyrichii 1510–1730 – – 1340–1490 – – 1385 Dianthus deltoides 1530–1730 – – – – 1496–1680 1385 Dracocephalum sibiricum 1640–1640 – 1490–1620 1590–1590 1380–1380 1507–1680 – Solidago canadensis 1350–17301 – 1390–15201 – – 1496–16801 13852 Cultivar: 1Unspecified, 2Goldkind A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 54 Discussion As expected, the majority of the species included in the study were found to be capable of producing viable seeds or rhizomes in the northern climate conditions. While the common garden experiments showed mostly similar rankings for the full set of the clones for all cities, the three different types of experiments gave different rankings for the set of clones that were common for all experiments. Most of the species, which are reported to be inva- sive in some regions with cool climate, reproduced better by seed than the rest of species. One reason for the success in the reproduction of the species may lie in the species selection. The studied species represent about 30% of the 500 species of herbaceous perennial ornamentals that are currently listed in Finnish nursery catalogues. The species selected for the study had the basic requirement of being vigorous and appar- ently winter hardy. Very few died in winter within two years without producing seedlings or rhizomes. A majority, 61% of the species could produce seedlings in the common gardens when effective temperature sum (ETS) was 1220–1730 dd, and 84% of the species produced viable seed at the climatically mildest site (Piikkiö) with 1385 dd. Based on the observed ETS in the experiments, about one third of all tested species would be able to reproduce by seed at least every second year up to latitudes 62–63° N and every four years up to 63.5–64° N. These findings indicate potential for the northward range expansion of the studied species, which will be further enhanced by warming climate (Hyvönen et al. 2012). Twelve of the clones were found neither to produce seedlings nor spread by rhizomes in common gardens. Six of these clones were included in other experiments and was found to produce seedlings, which indicates sub-opti- mal moisture or temperature conditions or seedling death caused by pest or diseases. Seed production of some species may also have been limited by day-length (Saikkonen et al. 2012). Long days in the outdoor experiments, maximally 19–22 hours, may have delayed the onset of flowering of day-length sensitive species too much for seed maturation to succeed. Further, pollinators of insect-pollinated species may have been absent at flowering time, and some clones could be sterile due to genetic hindrances (Müller-Schärer et al. 2004). These factors are also likely causes for the divergent results between the experiments regarding all species. 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. Seven clones ranked in the top 40 seedling producers had earlier been reported to be invasive or potentially invasive in some region with cool climate, whereas among the rest of 128 clones there were two invasive species that did not produce seedlings or viable seed: Vinca minor and Fallopia japonica. They both use rhizomes for spreading (Weber 2003). F. japonica established new plants but it was ranked low for rhizome spread among the species, whereas V. minor spread rapidly in soil beds. V. minor is locally invasive in the Baltic States (NOBANIS 2014) and North-Western USA (Invasive Plant Atlas of the United States 2019) and potentially invasive in Finland where it has competed out other shrubs in a deciduous forest area in the south-western coast of Finland. Most of the alien species that were ranked high for their reproduction have not yet established widely in Finland. A possible reason is that most of the studied species have been introduced in Finland very recently, in the last 30–60 years. A longer exposure time is expected to increase the likelihood of the escape (Dehnen-Schmutz et al. 2007a, 2007b, Pyšek et al. 2009). Another known successful invasive species, Solidago canadensis, ranked high in seedling production also our experiments. However, the success appeared partly to be dependent on the cultivar. ‘Goldkind’ was a very good seedling producer in the common gardens and emerged very rapidly in the greenhouse, but ‘Leraft’ produced sterile seed in the moisture-controlled field experiment. Furthermore, S. canadensis spreads well by rhizomes, too. In common gardens, it was among the most efficient rhizome producers at latitudes 63 and 65° N. For these reasons, S. canadensis is invasive in temperate and cool climates in Europe and Asia (Zwölfer 1976, Dong et al. 2006). In Finland, it has commonly escaped from gardens, mainly below the latitude 62° N and along the Baltic Sea coast up to the latitude 65° N (Lampinen et al. 2012). Among other alien species, the common garden and moisture controlled field experiments brought Telekia spe- ciosa to the top. It produced seedlings in moist and semi-dry soil but not in dry soil in the moisture controlled experiment with 1370–1680 dd. Though, the greenhouse germination experiment showed that seeds grown in 1385 dd had low viability, which indicates that current climate is not yet optimal for rapid spread of T. specio- sa. In Denmark, with milder oceanic climate, it is classified as highly invasive (EPPO 2019, DAISIE European Inva- sive Alien Species Gateway 2019). In Norway, it is considered to have high invasion potential but not expected to have significant ecological impact (Gederaas et al. 2012). The recent spread of T. speciosa in the Central Europe- an mountains by seed (Zająz and Zająz 2009, Pyšek et al. 2012, Zelnik 2012) and spread in Central Russia (Notov et al. 2011), naturalization in Finland, and its capacity for seedling emergence in the experiments indicate that it can spread further in Finland in warmer years. A G R I C U LT U R A L A N D F O O D S C I E N C E T. Kaukoranta et al. (2019) 28: 45–57 55 The findings of our study showed that numerous herbaceous ornamental perennials are capable of producing viable seeds or rhizomes at high latitudes. Previous studies have shown seed production to be associated with invasion success (Callaway and Josselyn 1992, Pérez-Fernández et al. 2000, Goergen and Daehler 2001, Hamilton et al. 2005, van Kleunen and Johnson 2007, Küster et al. 2008, Perglová et al. 2009, Schlaepfer et al. 2010, Chrob- ock et al. 2011). Regarding perennials, rhizome production alone can be enough for successful invasion (Bailey et al. 2009). 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Invasion potential of herbaceous ornamental perennials innorthern climate conditions Introduction Methods Plant material and experiments Common garden experiments Moisture-controlled field experiment Emergence rate and germination speed experiment in greenhouse Comparing experiments Geographic range of seed production limited by ETS Results Reproduction by seeds and rhizomes in common gardens Reproduction in field experiment Emergency rate and germination speed in greenhouse Comparing experiments Geographic range of seed production limited by ETS Discussion Acknowledgements References