Societal Impact StatementPlants are fundamental to solving many of humanity's most important challenges: food insecurity, water scarcity, energy, health, and climate change. With more than 20% of the world's plant species currently threatened with extinction, the loss of plant diversity will result in reduced options for human innovation, adaptation, and resilience. The world's botanic gardens already conserve and manage around a third of all known plant species in their living collections as well as seed banks as an insurance policy against extinction and as a resource to support scientific research. This work needs to be expanded rapidly if we are to avoid further plant species extinctions.SummaryHistorically, botanic garden science has been dominated by the disciplines of economic botany and taxonomy. Today, with around 20% of plant species threatened with extinction, the author argues that unless botanic gardens shift their efforts toward the conservation, management and use of plants, the loss of plant diversity will stifle human innovation, adaptation, and resilience. The enormous body of taxonomic knowledge, skills, data, and collections built up over the past two centuries is fundamental to managing plant diversity. These resources need to be used to address challenges such as food insecurity, water scarcity, renewable energy, human health, biodiversity conservation, and climate change. At a time when botanic gardens are increasingly seen as visitor attractions, rather than scientific institutions, refocusing their efforts is in the best interests of botanic gardens as well as those of broader society. The author gives examples of how botanic gardens are already supplying crop wild relatives to plant breeders; using their living collections to assess resilience to climate change and vulnerability to pests and diseases; and conserving rare and threatened plant species for future use. However, in spite of these efforts, only a small fraction of the estimated 60,000 plant scientists and specialist horticulturists in the world's botanic gardens are engaged in scientific research that has demonstrable impact on how we conserve or manage plant diversity. The author argues that it is time for botanic gardens to develop a new contract with society—a contract that delivers outcomes for society that only botanic gardens, as custodians of the world's plant diversity, can deliver.