noun
Japanese bindweed; Calystegia japonica
A flowering plant native to Japan, often found in fields and roadsides. The name literally means 'daytime face' because the flowers bloom during the day.
道端にヒルガオが咲いていた。
Japanese bindweed was blooming by the roadside.
Compound of 昼 (hiru, 'daytime') and 顔 (kao, 'face'), from the appearance of the flowers opening during the day. The exact historical derivation is uncertain.