Interactions between humans and backyard gardens are commonplace, however, their resulting impacts on local biodiversity especially in Afrotropic cities are still poorly understood. We used local expert knowledge to understand such interactions and to explore plant biodiversity and modes of species arrival in home gardens along a gradient of urbanization in Morogoro city, Tanzania. Plant biodiversity and owner-backyard garden interactions data were collected using sampling plot and questionnaire methods respectively, and analysed using multivariate methods in R software. There were significantly more interactions between humans and backyard gardens influenced mostly by the livelihoods accrued from the gardens. Backyard garden-user interaction resulted, mostly, in more species being assembled in the area, thus risking arrival of non-native species. Species movement across urbanization zones exhibited multidirectional patterns perpetuated by human and nonhuman factors and these were higher between outside and sub-urban localities with the later acting as sink and source of the species movements to other localities. Species introduction by city residents were driven by the ecological and socio-economic services potentially offered by the urban plant biodiversity thereby increasing risk of species homogenization of the urban landscapes. These data are important for planning sustainable biodiversity conservation strategies and to avoid impoverishing the native species, and the services they offer to the urban ecosystem as whole.