Yarrow is the third of the blue oils, alongside German chamomile and blue tansy. The blue is chamazulene again, made during distillation from a colourless compound in the flower. How much you get varies a lot between plants, so yarrow oil ranges from deep blue to barely green. Camphor, borneol and sabinene sit underneath, giving it a sharper, more medicinal edge than chamomile. In lab tests it held back Candida taken from denture-sore patients,^[1] and it is toxic to some crop pest insects.^[2]