ground ivy vs ground-ivy

ground ivy

noun

  • Glechoma hederacea, an aromatic, perennial, evergreen creeper of the mint family Lamiaceae. 

ground-ivy

noun

  • Glechoma hederacea, an aromatic, perennial, evergreen creeper of the mint family Lamiaceae 

How often have the words ground ivy and ground-ivy occurred in a corpus of books? (source: Google Ngram Viewer )