The "Gaga-imo" (Metaplexis japonica) belongs to Asclepiadaceae (the Milkweed family). It is a perennial herb that is found throughout Japan, as well as on the Korean Peninsula, in China, and in the Russian Far East. This herb grows in fields and spreads by trailing rhizomes. The leaves are long-ovate-cordate and opposite. The racemes are borne on the long peduncles and bloom pale purple flowers in August. The pollens form the pollen mass. The fruits are follicles and the seeds are carried on the wind. The young shoots and fruits are edible and the leaves are used for tonic medicine. In Chinese, it is called "蘿藦" (luo mo).
In Korean, it's called "pak-ju-ga-ri" (박주가리). It is said to have been named because it resembles a "cracked gourd" (박 쪼가리).