The aquatic duckweed Spirodela polyrhiza propagates itself vegetatively by forming turions – bud-like perennation organs – in the autumn, which spend the winter on the bottom of ponds and then germinate in the following spring and proliferate on the water surface. Newly formed turions usually require a period of cold after-ripening and light to germinate effectively, but an ample supply of exogenous sugar can lead to germination even in the dark and independent of after-ripening. The results of the present study indicate that the availability of readily metabolised carbohydrates is a determining factor for turion germination. Freshly harvested turions do not contain soluble, low-molecular weight carbohydrates at a level sufficient to allow germination to take place, but after-ripened turions do. Augmentation of the soluble carbohydrate content during after-ripening derives from gradual breakdown of reserve starch of the turions. The long time required for any germination to be observed in turions incubated in darkness and the limited frequency of germination in the dark (about 50% of turion population), even with an ample external sugar, supply emphasise that both after-ripening and light are essential for ensuring rapid germination and subsequent frond proliferation at an ecologically appropriate time. The carbohydrate supply required for rapid proliferation of the fronds produced at germination is provided by the rapid light-induced breakdown of turion reserve starch.