Metrosideros

Metrosideros is a genus of approximately 50 trees, shrubs, and vines native to the islands of the Pacific Ocean, from the Philippines to New Zealand and including the Bonin Islands, Polynesia, and Melanesia, with an anomalous outlier in South Africa. Most of the tree forms are small, but some are exceptionally large, the New Zealand species in particular. The name derives from the Ancient Greek metra or "heartwood" and sideron or "iron". Perhaps the best-known species are the pōhutukawa, (M. excelsa), northern rātā (M. robusta), and southern rātā (M. umbellata) of New Zealand, and ōhia lehua, (M. polymorpha), from the Hawaiian Islands.

Distribution
New Caledonia has 21 species of Metrosideros, New Zealand has twelve, Hawaii has five, and Papua has four. The remainder are scattered across small islands of the Pacific, with one outlier described from South Africa. Metrosideros seeds can disperse on the wind, which accounts for their wide distribution from a presumed origin in a greater New Zealand continent, which at the time of the breakup of Gondwana in the late Cretaceous, included New Caledonia. How the genus reached Hawaii appears puzzling because the prevailing trade winds blow from the east. However high altitude wind patterns may have brought seeds north from the Marquesas Islands, which molecular evidence suggests as the origin of the Hawaiian species from a single colonising event (the Hawaiian M. polymorpha is similar to the widespread M. collina found in the Marquesas Islands, and was long classified as a subspecies of it). Considering that the group likely spread north and east from New Zealand, counter to prevailing ground-level winds, this is not surprising.

Cultivation
Metrosideros are often cultivated for their showy flowers, as street trees or in home gardens. The flowers are generally red, but some cultivars have orange, yellow or white flowers. Some names listed in horticultural catalogs and other publications, such as M. villosa and M. vitiensis, are actually the names of varieties or cultivars (usually of M. collina) rather than valid scientific species. The pōhutukawa of New Zealand has several cultivars grown in Australia, Hawaii and California and it has been planted successfully in the north of Spain and on the Scilly Isles off the south-west coast of Britain, but the species is considered an invasive pest in parts of South Africa. Metrosideros kermadecensis is recently naturalised in Hawaii, and has the potential to become a pest. In turn, various cultivars of M. collina and M. polymorpha are widely grown in New Zealand under various names. Metrosideros umbellata occurs naturally south of mainland New Zealand in the Auckland Islands at 50° South latitude, and is the hardiest member of the genus, and a few cultivated specimens are growing in Scotland.

Metrosideros species
There are approximately 50 species of Metrosideros, in three subgenera: Mearnsia, 24 or 25 species, trees, shrubs (some epiphytic) and vines, with red, pink or white flowers; Metrosideros, 26 species, trees and shrubs, flowers mostly red, but some species have yellow or white flowers; and Carpolepis, 3 species of hemi-epiphytic rainforest trees from New Caledonia, all with bright yellow flowers.


 * subgenus Metrosideros
 * M. bartlettii J.W.Dawson - Bartlett's rātā (New Zealand)
 * M. boninensis (Bonin Islands)
 * M. cherrieri (New Caledonia)
 * M. collina (from Vanuatu in the southwest to French Polynesia in the east)
 * M. engleriana (New Caledonia)
 * M. excelsa Gaertn. - Pōhutukawa (New Zealand)
 * M. gregoryi (Samoa)
 * M. humboldtiana (New Caledonia)
 * M. kermadecensis - Kermadec pōhutukawa (Kermadec Islands)
 * M. macropus A.Gray - Lehua mamo (Oahu in Hawaii)
 * M. microphylla (New Caledonia)
 * M. nervulosa - Mountain Rose (Lord Howe Island)
 * M. nitida (New Caledonia)
 * M. ochrantha (Fiji)
 * M. oreomyrtus (New Caledonia)
 * M. polymorpha Gaudich. - Ōhia lehua (Hawaii)
 * M. punctata (New Caledonia)
 * M. robusta A.Cunn. - Northern rātā (New Zealand)
 * M. rugosa A.Gray - Lehua papa (Oahu in Hawaii)
 * M. salomonensis (Solomon Islands; this species has been variously placed in both subgenera)
 * M. sclerocarpa (Lord Howe Island)
 * M. tetrasticha (New Caledonia)
 * M. tremuloides - Lehua āhihi (Oahu in Hawaii)
 * M. umbellata Cav. - Southern rātā (New Zealand)
 * M. waialealae (Kauai, Molokai, and Maui in Hawaii)


 * subgenus Mearnsia
 * M. albiflora (New Zealand)
 * M. angustifolia (South Africa)
 * M. brevistylis (New Caledonia)
 * M. cacuminum (New Caledonia)
 * M. carminea W.R.B.Oliv. - Carmine rātā (New Zealand)
 * M. colensoi (New Zealand)
 * M. cordata (New Guinea)
 * M. diffusa (New Zealand)
 * M. dolichandra (New Caledonia)
 * M. fulgens Sol. ex Gaertn. - Scarlet rātā (New Zealand)
 * M. halconensis (Philippines)
 * M. longipetiolata (New Caledonia)
 * M. operculata (New Caledonia)
 * M. ovata (New Guinea)
 * M. paniensis (New Caledonia)
 * M. parkinsonii (New Zealand)
 * M. patens (New Caledonia)
 * M. perforata (New Zealand)
 * M. porphyrea (New Caledonia)
 * M. ramiflora (New Guinea)
 * M. rotundifolia (New Caledonia)
 * M. scandens (New Guinea)
 * M. whitakeri (New Caledonia)
 * M. whiteana (New Guinea)
 * M. n. sp. (unnamed) (Solomon Islands)


 * subgenus Carpolepis
 * M. elegans (New Caledonia)
 * M. laurifolia (New Caledonia)
 * M. tardiflora (New Caledonia)