citronella vs lemon balm

citronella

noun
  • An essential oil obtained from this plant, often used as an insect repellent. 

  • A tropical Asian grass, Cymbopogon nardus, that has citrus-scented leaves. 

lemon balm

noun
  • A perennial herb, Melissa officinalis, of southern Europe; used as a flavouring in ice cream etc. 

How often have the words citronella and lemon balm occurred in a corpus of books? (source: Google Ngram Viewer )