About 170-200 species, including:
Melaleuca acuminata
Melaleuca alternifolia
Melaleuca armillaris
Melaleuca biconvexa
Melaleuca cajuputi
Melaleuca coccinea
Melaleuca cordata
Melaleuca cuticularis
Melaleuca decora
Melaleuca decussata
Melaleuca densa
Melaleuca diosmifolia
Melaleuca elliptica
Melaleuca ericifolia
Melaleuca fulgens
Melaleuca gibbosa
Melaleuca glaberrima
Melaleuca huegelii
Melaleuca hypericifolia
Melaleuca incana
Melaleuca lanceolata
Melaleuca lateritia
Melaleuca laxiflora
Melaleuca leucadendra
Melaleuca linariifolia
Melaleuca macronychia
Melaleuca microphylla
Melaleuca nesophila
Melaleuca nodosa
Melaleuca preissiana
Melaleuca quinquenervia
Melaleuca radula
Melaleuca spathulata
Melaleuca squamea
Melaleuca squarrosa
Melaleuca styphelioides
Melaleuca tenella
Melaleuca teretifolia
Melaleuca thymifolia
Melaleuca viridiflora
Melaleuca wilsonii

The plant genus Melaleuca is part of the myrtle family Myrtaceae and presently contains about 170 species. However most experts believe that the addition of presently unnamed or incorrectly named species will result in a total of more than 200 members. Most are endemic to Australia but example occur in the wild as far afield as Indonesia, New Guinea, New Caledonia and even Malaysia.

They are shrubs and trees growing (depending on species) to 2-30 m tall, with flaky, exfoliating bark. The leaves are evergreen, alternately arranged, ovate to lanceolate, 1-25 cm long and 0.5-7 cm broad, with an entire margin, dark green to grey-green in colour. The flowers are produced in dense clusters along the stems, each flower with fine small petals and a tight bundle of stamens; flower colour varies from white to pink, red, pale yellow or greenish. The fruit is a small capsule containing numerous minute seeds.

The melaleucas are closely related to callistemons: the main difference between the genera is that the stamens are generally free in Callistemon but grouped into bundles in Melaleuca.

In the wild, melaleucas are generally found in open forest, woodland or shrubland, particularly along watercourses and the edges of swamps.

Common names of many Australasian genera are inaccurate and unhelpful. In consequence, the best-accepted common name for Melaleuca is simply melaleuca; however most of the larger species are also known as paperbarks, and the smaller types as honey myrtles. Some melaleucas are used in the manufacture of an essential oil called tea tree oil and called "tea trees", which is confusing, as "tea tree" has also been used for several other plants, including Leptospermum, a related and superficially similar-looking genus.

Scientific studies have shown that tea tree oil made from Melaleuca alternifolia is a highly effective topical antibacterial and antifungal, although it may be toxic when ingested internally in large doses or by children. In rare cases, topical products can be absorbed by the skin and result in toxicity.

Melaleucas are popular garden plants, both in Australia and other tropical areas worldwide. In Hawaii and the Florida everglades, Melaleuca quinquenervia has become a serious invasive weed.

References

• Takarada K et al., 2004. A comparison of the antibacterial efficacies of essential oils against oral pathogens. Oral Microbiol. Immunol. 19 (1): 61-64.
• Hammer KA et al., 2003. Susceptibility of oral bacteria to Melaleuca alternifolia (tea tree) oil in vitro. Oral Microbiol. Immunol. 18 (6): 389-392.
• Hammer KA et al., 2003. Antifungal activity of the components of Melaleuca alternifolia (tea tree) oil. J. Appl. Microbiol. 95 (4): 853-860.
• Oliva B et al., 2003. Antimycotic activity of Melaleuca alternifolia essential oil and its major components. Lett. Appl. Microbiol. 37 (2): 185-187.
• Mondello F et al., 2003. In vitro and in vivo activity of tea tree oil against azole-susceptible and -resistant human pathogenic yeasts. J. Antimicrob. Chemother. 51 (5): 1223-1229. Epub 2003 Mar 28.