Abstract: Decolonization influenced the rise of environmental activism and thought in Australia and South Africa in ways that have been overlooked by national histories of environmentalism and imperial histories of decolonization. Australia and South Africa’s political and cultural movement away from Britain and the Commonwealth during the 1960s is one important factor explaining why people in both countries created more, and more important, public indigenous botanic gardens than anywhere else in the world during that decade. Effective decolonization from Britain also influenced the rise of indigenous gardening and the growing popularity of native gardens at a critical period in gardening history. Most facets of modern gardening—using plants indigenous to the site or region, planting drought tolerant species, and seeing gardens as sites to help conserve regional and national floras— can be dated to those two decades. The interpretation advanced here adds to historical research tracing how the former Commonwealth dominion settler colonies experienced effective decolonization in the same era. This article expands the focus of research on decolonization to include environmentalism. The interpretation of the article also augments national environmental histories that have hitherto downplayed the influence of decolonization on the rise of environmentalism. Putting decolonization into the history of the rise of environmental thought and action sheds light as to why people in contemporary Australia and South Africa are so passionate about protecting indigenous nature and worried about threats posed by non-native invasive species.