Urban river walls represent some of the most common habitat available to riparian species within heavily engineered urban river corridors, but research into the characteristics and diversity of river walls is very rare. In this study, the flora of urban river walls was surveyed at 16 sites (92 walls) along 32 km of the river Thames through central London to determine the characteristics and habitat origins of species found on these walls, whether these species indicated ‘an urban cliff effect’ occurring on wall habitats and whether plant species richness varied between different wall types. A total of 90 species were found on the walls, with the majority of species associated with disturbed areas and riparian habitats, supporting the idea of a ‘mass effect’ occurring, whereby the flora is maintained by propagule pressure from remnant or introduced habitats within urban or peri-urban areas, rather than walls selecting for cliff/rock species. There was an increased incidence of plants from cliff/rock habitats found in the flora however, suggesting that an urban cliff effect does still occur. Species were organized along the river walls based on riparian/terrestrial gradients and stress tolerance, probably reflecting a gradient of wall height above river flows. Plant species richness was significantly higher on brick walls than that on sheet piling, indicating key differences between materials probably based on surface integrity. Large areas of connected river walls (‘wallscapes’) through heavily engineered urban rivers represent potential sites for the enactment of reconciliation ecology via habitat improvement in urban ecosystems. Copyright © 2011 John Wiley & Sons, Ltd.