Biological and human-mediated mechanisms in invasions of alien plant species are interdependent, and as a result, it is difficult to predict where and why such invasions will occur. However, most plant naturalisations and invaders nowadays originate from and in the future will derive from deliberate importations (Mack and Erneberg 2002, Kowarik 2003, Mack 2003). Specifically, the study of the economic uses of plant species that are invasive in various parts of the world has established that most plant invaders are ornamentals (Weber 2003). These species have been introduced for horticultural use by nurseries, botanical gardens, and individuals (Reichard and White 2001). Of course, invasive alien species can be introduced unintentionally (Pyšek et al. 2011) or by other deliberate pathways developed by humans, e.g., for forestry and agricultural purposes (Mack and Lonsdale 2001, Starfinger et al. 2003). Nevertheless, the expansion and globalisation of the horticultural trade is widely accepted as a principal reason for the increase in alien species (e.g., Dehnen-Schmutz et al. 2007a, 2007b, 2010, Barbier et al. 2011) and it is also widely recognised that many invasions begin in urban domestic gardens (e.g., Smith et al. 2006, Hanspach et al. 2008). On a general macroeconomic scale, income growth is the primary driver of globalisation and there is a clear association between Gross Domestic Product and the richness of alien floras (Hulme 2009). In addition, when the variations in invasion rates were partitioned according to a range of explanatory variables, it was found that only national wealth and population density were statistically significant (Pyšek et al. 2010). These predictors simply reflect the intensity of human activities, in which the horticultural trade is also a relevant component.

There is now an extensive literature that analyses the reasons why certain species become invasive (e.g., Pyšek and Richardson 2007, Küster et al. 2008, Milbau and Stout 2008). Many papers identify particular traits that cause an ornamental alien species to become invasive. For example, Marco et al. (2010) showed that when an ornamental perennial alien species escapes from cultivation, pre-adaptation to the local environment and its potential for spreading vegetatively play important roles, along with gardening practices. Most authors conclude that successful invasion cannot be explained by a single trait or characteristic, but is determined in combination with climatic and environmental factors, species traits, and human uses (Thuiller et al. 2006, Milbau and Stout 2008, Beans et al. 2012, Richardson and Pyšek 2012). The phrase “human-mediated propagule pressure” is often used to explain processes, where propagule pressure is defined as the frequency with which a species is introduced to a site, combined with the number of individuals in each introduction event (Simberloff 2009; see also arguments in Lockwood et al. 2009). Human-mediation multiplies this process and it is connected both to human population size and density and to economic characteristics (Pyšek et al. 2010, Trueman et al. 2010). Species residence time gives another dimension to the propagule pressure: the longer the species is present in an area, the more propagules are spread and the probability of establishing new populations increases (Rejmánek et al. 2005, Milbau and Stout 2008, Trueman et al. 2010, etc.).

Human-mediated propagule pressure from ornamental horticulture itself plays both a direct and implicit role for species to become naturalised and even invasive. At a local level it is particularly important, because it promotes invasions in variety of ways (see, e.g. Reichard 2011): 1) the introduction and reintroduction of new plants; 2) the selection of species and cultivars for characteristics such as climatic suitability and rapid propagation; and 3) the creation of propagules due to long residence time, the pressure of gardening fashions and the great number of garden centres. In Britain, market presence, prices and dates of introduction are among the socio-economic factors that have influenced invasions (Dehnen-Schmutz et al. 2007a). Pemberton and Liu (2009) have also stressed the importance to successful naturalisation of the number of years a plant was sold. The current naturalisation success of North American woody species in Europe is determined by introduction history and particularly by planting frequency (Bucharova and van Kleunen 2009). Selective introduction and human-mediated selection of ornamental plants with fast and abundant germination might increase the risk of species ultimately becoming invasive (Chrobock et al. 2011). Kowarik (2003) suggested that secondary releases of alien species might mimic demographic and dispersal processes that influence population growth and expansion of range, and provide opportunities to species whose propagules are not able to move naturally over long distances. In addition, cultivation (e.g., protection from predators and parasites, drought, cold) can facilitate naturalisation by protecting alien populations from environmental stochasticity (Mack 2000).

It has previously been shown that in the case of the Estonian Alien Flora (Ööpik et al. 2008), the establishment and naturalisation of alien species in a given area depended on the level and type of human mediation. The majority (74%) of 232 naturalised alien species in Estonia originates from deliberate introductions. Overall, approximately half of the alien flora consists of short-lived species, but only 24% of the naturalised species belong to this group. Moreover, cultivation in a cold temperate region has tended to favour perennial species with propagative advantages for attaining greater abundance and higher invasive status; especially in natural and semi-natural habitats. The pattern of invasive species is more diverse, but among the most problematic species there are also deliberately introduced perennials, which have escaped from cultivation and are now spreading aggressively, forming monocultures, are toxic, and causing other conservation or human health problems. Hence, through selection and cultivation, humans increase propagule pressure, while invasion success also increases with alien species’ residence time. Consequently, at a national level in countries such as Estonia, it is necessary to examine the influence of local factors more deeply, so that measures can be taken to reduce the probability of invasions.

In this paper, the following questions are raised in order to examine the situation in Estonia in further detail:

(i) What is the number of herbaceous perennial species offered for sale, both to horticultural professionals and amateurs? What is the composition of these species?

(ii) Is availability (i.e., lower prices at certain outlets) related to the current status of these species, i.e., the presence of the alien species in the spontaneous flora and/or their abundance in the wild, and how is this connected to species’ residence time in Estonia?

(iii) Does this list include problematic or even legally prohibited alien species that may cause nature conservation problems and therefore needs national attention?

It is generally accepted that only a limited fraction of introduced species in a given area become naturalised and an even smaller number eventually cause problems (Williamson and Fitter 1996, see also Jeschke et al. 2012). However, initially it is not usually possible to speculate about the potential magnitude of these introductions. The results of the present study demonstrate that the total introduced number of 789 alien herbaceous perennial species available in Estonian commercial nurseries compares with the 787 alien species in the entire Estonian Alien Flora (Ööpik et al. 2008). Species chosen for their ornamental attraction do not necessarily have more invasive potential than other comparable plants, but the proportionally large number of all introduced species increases the likelihood that a fraction will become naturalised and even invasive (see also Mack 2003). The situation should be therefore monitored carefully.

Invasions have a highly dynamic nature, and many of the most problematic alien species are not recent arrivals, but they reflect historical human activities, a phenomenon termed an invasion debt (Essl et al. 2011). One part of this reflection is species residence time in the given area: the longer the species is present in the area, the more propagules are spread and the probability of naturalisation increases (Rejmánek et al. 2005). Unfortunately, in real-life situations the actual introduction time is usually unknown or not determined and for this study were available only years of introduction for alien species in alien flora (Ööpik et al. 2008). Nevertheless, the data in the present paper support the generalisation that the residence time affects the range and frequency, as well as the invasive status, of alien species, at least in cases where the species have already reached a status in spontaneous flora. Also, quite often the most effective predictors of invasiveness were “invasive elsewhere” (Herron et al. 2007). Our data show that the invasive status in Estonia, abundance in the wild and of course, the year of introduction into Estonia correlates with current invasiveness of the species in the whole region. Thus, the regional cooperation in this field should be favoured.

Recently, it has been shown that residence time is a pivotal factor in the spatial patterns of alien species and human pressure has a greater influence on species that have been introduced more recently (Dainese and Poldini 2012). For the future studies we hypothesise here that exponential growth of trade and travel and various pressures from current consumption could decrease the importance of residence time; further, naturalisations and even invasions will take place quicker and be more dependent on other human-mediations than at present.

Propagule pressure created by the historic factors of supply and demand is also an important part of the explanation for the invasiveness of ornamental species (Dehnen-Schmutz et al. 2007b, Pemberton and Liu 2009). Hence, the current lists have the present levels of socio-economic activity as a background and suggest the possibility of future invasions. Nevertheless, according to the study data, native and expanding wild alien species have today a similar market profile, as they are both inexpensive and widely available. This could be a robust reflection from the previous planting history of alien species, because today’s price or availability does not differ between groups of casual, naturalised or invasive aliens. Also, among species in alien flora the introduction time does not predict today’s lower price or wider availability. All the species which have currently reached status in spontaneous flora are similarly favoured on the market as “easy-to-grow” species. Such plants are often popular, because they may be readily propagated, and hence are usually less expensive. However, a history of active planting and repeated introductions creates additional pressure and may result in rapid rates of spread, successful escape of species from cultivation and subsequent naturalisation.

The objective of breeding plants to encourage specific traits differs between agriculture and ornamental horticulture. Obviously, bred ornamental cultivars are genetically distinct from the wild genotype of the species, but usually the change in genetic composition is minor and has little to do with the traits that lead species to become naturalised and invasive (Reichard 2011). Nevertheless, for a few species, recent molecular evidence suggests that continuing propagule pressure aids the spreading of an invasion by introducing genetic variation adaptive for new areas and habitats (Simberloff 2009). In this paper, selection strategies to reduce the invasive potential in introduced plants (Anderson et al. 2006) are not considered. Instead, it is suggested that the number of cultivars per species is likely to be strongly correlated with the number of nurseries where that species is available and will provide an additional opportunity to escape from cultivation. In this case there is another dimension of generated human-mediated propagule pressure. When more cultivars come on the market, then inevitably there will be more specimens in more sites to act as potential dispersers, which is the classical explanation of propagule pressure (Simberloff 2009).

The financial costs to countries of controlling the major invasive species are very high and pose serious problems (Pimentel et al. 2005); hence, regulations are needed. In theory, the control of international release should be straightforward. However, such control is practically difficult due to the financial pressure resulting from the modern global trade in plants. In general, an appropriate framework should enable the trends to be monitored and accept the control of future introductions (e.g., Hulme et al. 2008). There are also difficulties with the negotiation of international agreements. Such difficulties occur within the European Union and are even more pronounced on a global scale.

The one pro-active possibility is that green lists should be compiled. These lists would be especially useful in large-scale projects and would not contradict any trade agreements (Dehnen-Schmutz 2011). In Estonia, The Nature Conservation Act forbids the release of alien species into the wild and The List of Invasive Alien Species prohibits the import of specimens and all transactions with live plants. Currently this list includes 13 plant species, which differ significantly - some species are well-established and some are not yet introduced into Estonia, but have caused problems in countries with similar environments. Unfortunately, the results of this paper show that the compiled list of more problematic species in Estonia and in neighbouring countries include altogether 38 species (Table 3). Three regionally problematic species, Solidago canadensis, Fallopia japonica, Fallopia sachalinensis, are available on the Estonian market and are at the same time on the legally prohibited list. Thus, even though the country has quite strict existing regulations, they are not entirely enforced; currently Estonia is only beginning systematic work in this field. It has been proposed that voluntarily implemented Codes of Conduct (Heywood and Brunel 2006) which are addressed to governments, to the horticultural industry and trade, to local environment and conservation agencies, societies and associations, botanic gardens, etc., can be better alternatives to regulate this situation comprehensively. At a national level, a few European countries have addressed the issues of invasive alien species and horticulture and developed a strategy (see references in Heywood and Brunel 2011, p. 19-20).

Why should the horticultural industry care about alien species? This question arises because the problems caused by alien species do not have a substantial impact on the industry. For this reason, it is probable that further regulation will be difficult. An economical study by Knowler and Barbier (2005) has suggested the possibility of employing market-based instruments consistent with the concept of ‘introducers-pay’, to regulate the nursery industry. Yokomizo et al. (2012) proposed a cost-benefit analysis for determining plant introduction that incorporates probability to escape, expected economic cost after escape, expected commercial benefits, and the efficiency and cost of containment. Details of the structural changes required, are given by Dehnen-Schmutz et al. (2010), who also emphasise the complexity of the situation. Drew et al. (2010) and Peters et al. (2006) have both suggested that the involvement of consumers and also professionals, as well as education, may yield better results by addressing the moral problem of the risks caused by alien species and understanding the characteristics of the industry. Different voluntary initiatives and regulations are often quite effective and should be increased to limit horticultural introductions of invasive plants (Burt et al. 2007, Niemiera and Von Holle 2009). Some results suggested that merely labelling the plants as invasive or native could be a viable strategy for changes in customer behaviour (Yue et al. 2011). Control measures should at least be part of any overall framework, if there is agreement to implement regulations. However, in nature as in economics, there are always cost-benefit trade-offs.

1) The availability of plants from the horticultural trade is a major source of alien species, including locally and regionally naturalised and invasive plants.

2) Supply and demand factors create new and reflect previous propagule pressure and are an important component of the reasons behind the invasiveness of ornamental species. In general, native and expanding wild alien species have a very similar profile on the market - both are inexpensive and widely available, and more cultivars per species are also available.

3) There are urgent requirements for regional and national authorities to regulate and control the ornamental plant trade in order to diminish the risk of new invasions.

4) It would be helpful to involve the public in finding alternatives and encouraging best practices for both horticultural professionals and amateurs.