The number of alien plants escaping from cultivation into native ecosystems is increasing steadily. We provide an overview of the historical, contemporary and potential future roles of ornamental horticulture in plant invasions. We show that currently at least 75% and 93% of the global naturalised alien flora is grown in domestic and botanical gardens, respectively. Species grown in gardens also have a larger naturalised range than those that are not. After the Middle Ages, particularly in the 18th and 19th centuries, a global trade network in plants emerged. Since then, cultivated alien species also started to appear in the wild more frequently than non-cultivated aliens globally, particularly during the 19th century. Horticulture still plays a prominent role in current plant introduction, and the monetary value of live-plant imports in different parts of the world is steadily increasing. Historically, botanical gardens - an important component of horticulture - played a major role in displaying, cultivating and distributing new plant discoveries. While the role of botanical gardens in the horticultural supply chain has declined, they are still a significant link, with one-third of institutions involved in retail-plant sales and horticultural research. However, botanical gardens have also become more dependent on commercial nurseries as plant sources, particularly in North America. Plants selected for ornamental purposes are not a random selection of the global flora, and some of the plant characteristics promoted through horticulture, such as fast growth, also promote invasion. Efforts to breed non-invasive plant cultivars are still rare. Socio-economical, technological, and environmental changes will lead to novel patterns of plant introductions and invasion opportunities for the species that are already cultivated. We describe the role that horticulture could play in mediating these changes. We identify current research challenges, and call for more research efforts on the past and current role of horticulture in plant invasions. This is required to develop science-based regulatory frameworks to prevent further plant invasions.

Bio Protection Research Centre Lincoln University 7648 Canterbury New Zealand

Biodiversity Macroecology and Biogeography University of Goettingen Büsgenweg 1 37077 Göttingen Germany

Biodiversity Research University of Potsdam Maulbeerallee 1 Potsdam D 14469 Germany

Botanical Garden Meise Bouchout Domain Nieuwelaan 38 1860 Meise Belgium

CABI Bakeham Lane Egham TW20 9TY U K

Centre for Agroecology Water and Resilience Coventry University Ryton Gardens Coventry CV8 3LG U K

Centre for Ecology and Conservation University of Exeter Penryn Campus Penryn U K

Centre for Ecology Evolution and Environmental Changes Faculty of Sciences University of Lisbon Campo Grande 1749 016 Lisbon Portugal

Department of Agriculture University of Sassari Viale Italia 39 07100 Sassari Italy

Department of Biosciences Durham University South Road Durham DH1 3LE U K

Department of Botany and Biodiversity Research University of Vienna Rennweg 14 1030 Vienna Austria

Department of Community Ecology Helmholtz Centre for Environmental Research UFZ Theodor Lieser Street 4 06120 Halle Germany German Centre for Integrative Biodiversity Research Halle Jena Leipzig Deutscher Platz 5e 04103 Leipzig Germany

Ecology Department of Biology University of Konstanz Universitätsstrasse 10 D 78457 Konstanz Germany

Environment Department University of York Wentworth Way Heslington YO10 5NG York U K

European and Mediterranean Plant Protection Organization 21 boulevard Richard Lenoir 75011 Paris France

German Centre for Integrative Biodiversity Research Halle Jena Leipzig Deutscher Platz 5e 04103 Leipzig Germany

Institute for Science in Society Radboud University PO Box 9010 6500 GL Nijmegen The Netherlands Netherlands Centre of Expertise for Exotic Species Toernooiveld 1 6525 ED Nijmegen The Netherlands

Institute of Botany Department of Invasion Ecology The Czech Academy of Sciences CZ 252 43 Průhonice Czech Republic

Institute of Botany Department of Invasion Ecology The Czech Academy of Sciences CZ 252 43 Průhonice Czech Republic Department of Botany and Zoology Centre for Invasion Biology Stellenbosch University Matieland 7602 South Africa South African National Biodiversity Institute Kirstenbosch Research Centre Private Bag x7 Claremont 7735 South Africa

Institute of Botany Department of Invasion Ecology The Czech Academy of Sciences CZ 252 43 Průhonice Czech Republic Department of Ecology Faculty of Science Charles University Viničná 7 CZ 128 44 Prague Czech Republic

Institute of Evolution and Ecology University of Tübingen Auf der Morgenstelle 5 72076 Tübingen Germany

Institute of Integrative Biology ETH Zurich Universitätstrasse 16 8092 Zurich Switzerland Department of Botany and Zoology Centre for Invasion Biology Stellenbosch University Matieland 7602 South Africa

Senckenberg Biodiversity and Climate Research Centre Georg Voigt Straße 14 16 60325 Frankfurt Germany

Université Grenoble Alpes CNRS LECA Laboratoire d'Écologie Alpine F 38000 Grenoble France

Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Conservation Taizhou University Taizhou 318000 China Ecology Department of Biology University of Konstanz Universitätsstrasse 10 D 78457 Konstanz Germany

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