blue bonnet vs mare's-tail

blue bonnet

noun
  • Any of several blue flowering plants, especially cornflower, Centaurea cyanus, and plants of the genus Scabiosa. 

  • A blue Australian parrot, especially Northiella haematogaster. 

  • A traditional flat Scottish hat made of blue wool; a blue tam-o'-shanter. 

  • Someone who wears such a hat; a Scotsman, especially a Scottish soldier. 

  • Any of several North American species of Lupinus common in the southwestern US, especially Lupinus texensis and Lupinus subcarnosus. 

mare's-tail

noun
  • Erigeron canadensis (syn. Conyza canadensis), an annual weed 

  • especially, common mare's tail (Hippuris vulgaris) 

  • Equisetum, a fern ally 

  • A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. 

  • Any aquatic plant of the genus Hippuris, having narrow leaves in whorls. 

How often have the words blue bonnet and mare's-tail occurred in a corpus of books? (source: Google Ngram Viewer )