Knowledge about the impacts of urban heat islands (UHI) and associated thermal gradients on vegetation seasonality (i.e. phenology) is vital for understanding spatial patterns in vegetation ecosystem functions. However, in contrast to temperate cites, there is little evidence to show how UHI influences landscape phenological processes in the tropics. In this study, we examined vegetation phenological responses to urban form, distance from the city centre and surface temperatures, in the tropical city of Kampala, Uganda. Estimates of vegetation growing season length and land surface temperature were derived from MODIS satellite imagery for multiple years (2013–2015) and urban form was characterised using the Local Climate Zone (LCZ) classification. We showed that growing season length increased along the urban–rural gradient (p < 0.001) and was longest in the least built-up LCZ class (p < 0.001). Growing season length was significantly reduced as land surface temperature increased (p < 0.001). These findings contrast with results reported for temperate cities, where higher temperatures are often associated with longer vegetation growing seasons. Our findings suggest that enhanced surface temperatures associated with UHI are a limiting factor to season length in the urban tropics. Urban planners in tropical cities should therefore account for vegetation sensitivity to UHI when developing targeted management strategies aiming to optimise the benefits accrued from vegetation.