Alien ornamental plants on sale in Botswana, Namibia and Zimbabwe: potential sources for plant invasions

Ornamental gardening has historically been, and continues to be, a primary pathway for plant introductions into southern Africa. Knowing which alien plants are being sold and used as ornamentals in the subregion is important to assess the potential for future biological invasions and conflicts between stakeholders. We compiled a list of all plant taxa on sale across eleven nurseries in Botswana (3), Namibia (2) and Zimbabwe (6) by accessing the plants in stock either on the nurseries’ websites, on social media or via stock lists shared by the nursery owners. The compilation was matched with the Global Register of Introduced and Invasive Species to identify invasive taxa. Of 1 193 species that were found to be on sale, 784 are alien to the continental Southern African Development Community (SADC) subregion, 289 have established alien populations in at least one continental SADC country, and 82 are considered invasive in at least one continental SADC country. Many of the established and invasive plants are popular trees and shrubs in the subregion and worldwide. Most of them have a wide native range usually covering more than one continent. This paper provides a quantification of invasion debt due to the ornamental sector in southern Africa and a baseline to identify priorities for risk analysis and potential regulation or control at a subregional level.