1. In a rapidly urbanising world, the ability of plant species to survive and build self-sustaining populations in urban environments is increasingly important for biodiversity conservation. Yet the contribution of cities to biodiversity conservation remains unclear because ecologists have studied biodiversity patterns, largely without considering the population establishment of plants and the ways in which different kinds of urban ecosystems harbour native and endangered plant species. These limitations can mislead conservation policies for cities. 2. To better-understand how urban ecosystems can contribute to biodiversity conservation, we propose a framework that links the population status (casual or established) of plant species with ecosystem novelty and highlights barriers to population establishment in different types of urban ecosystems, from natural remnants to novel ecosystems. 3. To quantify the relative importance of natural remnants vs. human-shaped ecosystems for the conservation of self-sustaining urban plant populations we re-analyse a unique dataset from a metropolitan region in Europe with information on the population status of 1199 plant species. 4. Results demonstrate that urban ecosystems harbour many established native and endangered species although a considerable share (37%) of species of conservation concern are confined to natural remnants. In hybrid and immature novel ecosystems, high species numbers reflect many species with only casual populations. The role of novel ecosystems as habitats for native and endangered plant species increases as novel ecosystems mature. 5. Synthesis and applications. General information about plant species richness in urban environments may mislead conservation policies as different kinds of urban ecosystems can play different roles in harbouring species of conservation concern. Moreover, presence-absence data can mask establishment failures of species. This proposed framework helps to distinguish between casual and established populations of plant species, and highlights barriers to population persistence in urban ecosystems; reflecting different land uses and land use histories over time. Revealing the role of natural remnants vs. hybrid vs. novel ecosystems as habitats for species of conservation concern illustrates opportunities for biodiversity conservation in all urban ecosystems and can support setting priorities for conservation.