Shoka (waka poem quoted as evidence of naming Koboku) is 'the water around Makomo (Manchurian wild rice) in a pond is increased by early-summer rain. Therefore, it is hard to tell which one is Ayame (iris)' (believed to be composed by MINAMOTO no Yorimasa).
証歌は「五月雨に池のまこもの水ましていつれあやめと引きそわつらふ」（源頼政か?）である。