This thesis explores how plants are perceived and categorised as alien, invasive and native respectively at individual, professional, and political levels. The thesis demonstrates how perceptions of and interactions with plants happen in ways that do not always correspond to the environmental authorities definitions of alienness and nativeness. As alienness and nativeness are concepts that are spatiotemporal in character, the labelling of plants as alien or native often involve value-laden discussions over belonging, as well as demarcations of wanted from unwanted nature. The questions this thesis seeks to address, in essence, are the following; how are alienness, invasiveness and nativeness perceived and expressed at individual, professional, and political levels? And, how is the categorisation of invasive alien species, as applied by the environmental management sector, perceived at individual and professional levels? Empirically, the thesis is based on two qualitative case studies: (1) a study of individual domestic gardeners in selected locations in the county of Oppland, Norway; and (2) a study of professionals, namely planners, landscape architects and environmentalists, and their disagreements over what plants to categorise as alien or native at the time when the former airport of Fornebu, in Oslo, was developed into a site for recreation, housing, and business. Both studies were researched through talking–whilst–walking interviews which allowed investigation of how categorisations and perceptions of species related to interactions with plants. The level of international and national environmental politics and management, where official definitions of alienness, nativeness and invasiveness are formulated, was investigated through a literature review serving as a crucial context for the two qualitative studies. Analytically, the thesis rests on a combination of different theoretical perspectives that enable an investigation of alienness, nativeness and invasiveness at individual, professional, and political levels. At the individual level, Ingold’s (2000) notion of dwelling as a mode of being-in-the-world has been combined with insights from more-than-human geography (for example Wolch & Emel 1998; Whatmore 1999; Matless 2000; Philo & Wilbert 2000; Jones & Cloke 2002; Whatmore 2002, 2003; Cloke & Jones 2004; Whatmore 2006), and in particular studies of human-plant relationships which focus on social agency as both a human and a non-human capacity (see for example Head & Atchison 2009). In this thesis, human plant relationships in domestic gardens have been studied to illuminate how domestic gardeners’ embodied experiences with plants correspond to the terminology and policies on alien, invasive species applied by the environmental management sector. At the professionals’ level, I am inspired by the intellectual historian Quentin Skinner’s (2002) perspective of concepts as speech acts, in order to investigate how the landscape architects, planners and environmentalists at Fornebu perceived and related to plants as well as the concepts alienness, nativeness and invasiveness. One refereed book chapter and two peer-reviewed journal papers have been written based in the empirical investigations. The first paper (paper 1) investigates how alienness, nativeness and invasiveness are practiced in domestic gardens in Oppland. The concepts alien and native as applied by environmental authorities did not correspond to the gardeners’ embodied experiences with plants. Most of the gardeners in the Oppland study were aware of the environmental authorities’ warnings about alien species, but still did not necessarily associate the concept of alien with something negative. The domestic garden can be compared to a private laboratory, where gardeners feel autonomous and in control when experimenting with invasive or climatically unsuited plants. However, as the negative focus on invasive alien species is increasing in scientific, political, and public arenas, the perception of alien species as unwanted has emerged in the context of gardens. The Oppland study revealed that gardeners changed their behaviour and attitudes when they recognised certain species as problematic to control, i.e. they are invasive. As domestic gardens are products of both nature and culture, and are largely controlled environments in the semi–private domain, the gardeners’ perspectives and experiences have largely been overlooked in Norwegian environmental policy–making. It is however a well-studied fact that domestic gardens can severely affect the composition of biodiversity through alien species spreads (see for example Fremstad & Elven 1997a; Zagorski et al. 2004; Dehnen-Schmutz et al. 2007). Consequently, it is important to be aware of how alienness, nativeness and invasiveness work in domestic gardens in order to improve communication between environmental authorities and garden owners. In the Fornebu study, Skinner (2002) enabled an investigation of how concepts were used as rhetorical tools in the conflict at Fornebu. The conflicting groups of professionals (i.e. planners and landscape architects versus the environmentalists) agreed on how the former airport at Fornebu was to be developed according to the approved landscape plans, but disagreed over which plants should be termed alien or native. Paper 2 discusses whether the planting of alien species at Fornebu can be termed environmental criminality. While the alien plantings at Fornebu did not classify as an environmental crime at the time (in 2007), the Fornebu case raises questions related to what type of environmental problem the introductions of invasive, alien plant species represent and how such plantings should be categorized judicially and morally. While the value-based aspects of alienness and nativeness are debated in academic literature, less focus has been centred on the closely related concepts of ‘black listed’ and ‘red listed’ species. Through the Fornebu study, paper 3 raises concern over how not only alienness and nativeness but also red lists and black lists were used rhetorically by interest groups to argue for an ideal nature at Fornebu. Paper 3 demonstrates that the black listing of alien species and red listing of endangered species are management tools which are founded on value-laden and constructed temporal thresholds (i.e. the year 1800) which largely implies idealising and ‘freezing’ past nature conditions serving as a measuring-stick for the future. The conflict at Fornebu with landscape architects and planners on the one side and environmentalists on the other demonstrates how labels such as alienness, nativeness, black listed and red listed contribute to ascribe species a particular status as wanted or unwanted. The different opinions about what counts as wanted and unwanted nature across scientific disciplines such as ecology and landscape architecture is a serious challenge if environmental authorities wish to succeed in halting the spread of alien, invasive species. In sum, the Fornebu study demonstrates that nativeness and alienness, as well as black lists and red lists, may be used as rhetorical tools to strengthen the positions and actions of different groups. It is therefore important to pay attention to the intentions of utterances (as argued by Skinner 2002), and to realise that red lists and black lists are not merely objective tools for policymakers, but can be used rhetorically in ways that may influence the actual composition of biological diversity as well as actual planning and management. Reflections over nature–culture relationships in human geography (for example Fitzsimmons 1989; Cronon 1995; Braun & Castree 1998; Coates 1998; Castree & Braun 2001; Whatmore 2002; Castree 2003) have stimulated a critical questioning of the rigid aliennative dichotomy in this thesis. This thesis demonstrates that the myriad ways that humans and plants interact in social, cultural, and historical contexts are of importance to how categorisations of alienness and nativeness are expressed and perceived. In environmental politics, humans are clearly part of the solution as well as the problem associated with invasive alien species spreads. As alien species policies and legal frameworks are currently in the making in Norway, there is still an open question how legal regulations of species introductions will be formulated as well as what social, cultural, economic and ecologic implications such regulations will have. The thesis contributes to the general societal debate on the management of species by demonstrating that alienness and nativeness are not ’objective’ scientific categories, but rather ambiguous concepts influencing the shifting and contingent status of species as wanted or unwanted. Moreover, the thesis brings a social science contribution from the discipline of human geography to a debate largely dominated by natural science. Empirically, the thesis provides qualitative cases from Norway, which is a country where little prior research has been undertaken from a social science perspective. In particular questions concerning how domestic gardeners relate to their plants have been largely ignored by Norwegian environmental authorities. Within the scientific discipline of geography the thesis adds to the more-than-human debate where studies on human-animal relationships have dominated, with two empirical case studies of human-plant interactions. Theoretically, the thesis further contributes by combining perspectives related to dwelling and more-than-human geography with Skinner’s linguistic focus on what can be done with discursive categories and the associated material consequences. On a more general level the thesis can be considered as a contribution to fundamental societal and academic debates regarding what nature is and should be, and moreover, a reminder that such debates are permeated with values that have implications for our green environments as well as the composition of biodiversity