A large colony of duckweeds (Araceae: Lemnoideae) grows on the constantly wet, vertical wall of the rock shelter behind the Llanos de Cortés waterfall of the Potrero River, Guanacaste province, Costa Rica, the first such case ever documented. Its unique hygropetric environment is described. The colony covers several square meters distributed in several different patches, and consists almost entirely of Wolffiella oblonga (Phil.) Hegelm., with much fewer individuals of Lemna valdiviana Phil. and L. aequinoctialis Welw. This also constitutes the first case of the genus Wolffiella Hegelm. growing in a hygropetric manner (an extremely rare condition for duckweeds) and the first record of W. oblonga in Costa Rica.
Duckweeds, Araceae, Lemnoideae, Waterfall rock shelter, Hygropetric habitat, Costa Rica