Research has shown that spending time in nature is beneficial for children's mental wellbeing, but how long must they spend to gain some benefit? Here we present the results from two studies: the Schools' Biodiversity Project, which took place over a year, with children spending an hour a week exploring biodiversity in their school grounds; and a one-off BioBlitz activity, where children spent just one hour looking for species in their school grounds. Both studies showed significant improvement in the children's mood after taking part in nature activities, as well as an increased awareness of the species present in their grounds.