Susanne,'Tea-tree' [the Maquarie Dictionary gives a hyphen, but the word is often seen in print without one] is 'any species of Leptospermum or any of several species of the allied genus Melaleuca'. Found in Australia and NZ, it is often developed as an ornamental with white, pink or red flowers. The oil of the tree is extracted for use as a natural germicide and fungicide. The word is sometimes spelt 'ti-tree' - for example, a town in the Northern Territory is named after it with this spelling. Evidently, the name 'tea-tree' stems from its use as a tea substitute in early colonial days.
--Stewie.