This vegetation type is dominated by Azolla filiculoides, a thermophilous water fern of North American origin. It forms both open stands containing some lemnid species and dense stands, which entirely cover the water surface and may include several overlapping layers of the dominant species. In autumn these stands are remarkable owing to their purple to reddish colour. In the Czech Republic, vegetation with A. filiculoides has been so far only recorded in three small ponds inside villages.