Nurseries and garden centres as hubs of alien plant invasions

The growing global horticultural trade is having a steadily increasing impact on the rate at which alien species are introduced into new areas, partly because horticultural trade also entails the unintentional dispersal of many contaminant species. Although there are reports about noteworthy occurrences of alien plant species in garden centres, this phenomenon has hardly been studied systematically. To bridge this knowledge gap, we conducted systematic field surveys in 12 garden centres in Hungary to assess their alien flora. We hypothesised that (i) the number of alien species inhabiting a garden centre is positively correlated with its size, (ii) relative to their size, garden centres host a disproportionately large proportion of the local alien flora, and (iii) alien species inhabiting garden centres differ from the regional alien flora in their traits. We recorded altogether 93,788 individuals of 67 introduced species, seven of which have not yet been reported from the country. There was considerable variability in the number of species and individuals found in each garden centre, but there was no correlation between the size of the garden centres and the number of species they host. Despite their relatively small size, the studied garden centres hosted a considerable proportion of the local alien flora, indicating that they strongly accumulate alien species and that they can act as invasion hubs for several alien species. Alien species inhabiting garden centres differed from the regional alien flora in some of their trait values, indicating that the species that are most successful at establishing populations inside garden centres are both good dispersers and possess an effective resource-acquisitive strategy. We conclude that established alien plant populations in garden centres may induce local invasions, and in the meantime, individuals and seeds inside the containers of ornamental plants are regularly transported to distant areas by the customers. Therefore, plant species dispersed as contaminants of horticultural stock need to be better considered in invasion biology to reduce the threat they may present.