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

Background: Ornamental gardening is one of the main historical and current pathways for plant introductions into southern Africa. A few introduced ornamental plants have become invasive, sometimes triggering conflicts between stakeholders. Knowing which alien plants are being sold and used as ornamentals in the sub-region is important to assess the potential for future biological invasions and conflicts between stakeholders. Objectives: This paper identifies and characterises the ornamental plants on sale in three southern African countries, and compared these to lists of plants considered invasive in the area of the continental Southern African Development Community (SADC). Methods: We compiled all plant taxa sold across eleven nurseries in Botswana (3), Namibia (2) and Zimbabwe (6) by accessing the plants in stock either on the nurseries’ website, on social media, and 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. This sub-group was characterised by growth form and biogeographical origin. Results: 1,193 species were found to be sold, of which 784 are alien to the continental SADC sub-region, 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 sub-region and worldwide. Most of them have a wide native range usually covering more than one continent. Conclusions: This paper provides a quantification of the 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 sub-regional level. Approaches to reduce the sale of invasive species, however, need to be considered carefully, since their current popularity across the sub-region could make them a source of conflict between the ornamental horticulture sector and those tasked with managing biological invasions.