milkweed vs sow thistle

milkweed

noun
  • Also of the genus Calotropis in order Gentianales, family Apocynaceae 

  • Principally of the genus Asclepias in order Gentianales, family Apocynaceae 

  • A monarch butterfly (Danaus spp). 

  • Euphorbia peplus and Euphorbia heterophylla, in order Malpighiales. 

sow thistle

noun
  • Any of the thistles in one of the genera Cicerbita and Sonchus. 

How often have the words milkweed and sow thistle occurred in a corpus of books? (source: Google Ngram Viewer )