Tea tree smells like nothing else on the shelf. Fresh and green woody with a medicinal edge that reads clean rather than clinical, it is the oil most people meet first and the one that stays in the cupboard longest.

Plant Therapy takes it from the leaves of Melaleuca alternifolia grown in Australia and South Africa, and bottles it undiluted at 1/3 fl oz. The maker's own suggestions run past the diffuser: three to four drops per 100 mL of water to scent a room, 2 to 5 percent in a carrier oil for skin, or a few drops into a white vinegar spray or a homemade cleaner recipe for a steamy bathroom. The ingredient line is one item long. It carries Plant Therapy's KidSafe, Pregnancy Safe and Nursing Safe designations and keeps two to three years.