© 2017 British Ecological Society. Human settlements and transport networks are growing rapidly worldwide. Since the early 20th century their expansion has been accompanied by increasing illumination of the environment at night, a trend that is likely to continue over the decades to come. Consequently, a growing proportion of the world's ecosystems are exposed to artificial light at night, profoundly altering natural cycles of light and darkness. While in recent years there have been advances in our understanding of the effects of artificial light at night on the behaviour and physiology of animals in the wild, much less is known about the impacts on wild plants and natural or semi-natural vegetation composition. This is surprising, as effects of low-intensity light at night on flowering, phenology and growth form are well known in laboratory and greenhouse studies. In a long-term experimental field study we exposed a semi-natural grassland to artificial light at intensities and wavelengths typical of those experienced by roadside vegetation under street lighting. We found that lighting affected the trajectory of vegetation change, leading to significant differences in biomass and plant cover in the dominant species. Changes in flowering phenology were variable between years, with grass species flowering between 4 days earlier and 12 days later under artificial light. Policy implications. Our results demonstrate that artificial light, at levels equivalent to those in street-lit environments, can affect species composition in semi-natural vegetation. This highlights the importance of considering artificial light as a driver of vegetation change in urban, suburban and semi-natural ecosystems, and where possible, of minimising or excluding artificial light from habitats of conservation importance.