From Garden to Weed: Invasive Ornamental Plants in Europe and Emerging Challenges for Biodiversity, Agroecosystems, Agriculture and Management

Ornamental horticulture represents one of the dominant pathways for the introduction of alien plant species and has played a central role in shaping current and future invasion dynamics. Many ornamental plants escape cultivation after long lag phases, driven by high propagule pressure, human-mediated selection of functional traits, and increasing climatic suitability. As a result, ornamental species contribute substantially to Europe's invasion debt, with many future invasions already "locked in" under ongoing global change. In this review, we synthesize current knowledge on the invasive risk of ornamental plants in Europe, examining introduction pathways, biological traits promoting invasiveness, the role of climate change, and the ecological, economic, and social impacts associated with ornamental plant invasions. We highlight that beyond biodiversity loss, invasive ornamental plants pose underappreciated threats to agriculture and related activities, including increased management costs, weed problems in managed landscapes, and disruption of water management and irrigation infrastructure, particularly through invasive aquatic species. We further review tools for risk assessment and prevention, including weed risk assessment frameworks, green lists, horizon scanning, and climate-informed spatial forecasting, emphasizing the importance of proactive, pathway-based approaches. Where prevention fails, management of established invasive ornamentals relies on integrated strategies combining mechanical, chemical, and biological control, often generating large quantities of biomass and long-term economic costs. We discuss the emerging but still limited potential of invasive plant biomass valorization as a complementary management option, highlighting both opportunities and constraints. Finally, we discuss implications for horticultural practices, policy development, and future research, arguing that reconciling ornamental horticulture with biodiversity conservation and sustainable agriculture will require anticipatory governance, stakeholder engagement, and climate-aware decision-making. By aligning horticultural innovation with invasion risk awareness, it may be possible to reduce future invasions while maintaining the social and economic benefits of ornamental plant use in Europe.