Definition of granny's nightcap in English:

granny's nightcap

noun

Regional
  • Any of several herbaceous plants whose flowers were thought to resemble traditional women's nightcaps, especially a wood anemone (Anemone nemorosa), a bindweed (genus Convolvulus), and the common columbine (Aquilegia vulgaris).

Origin

Mid 19th century; earliest use found in Phytologist.