Tree rātā belong to the Myrtle family (Myrtaceae), a large family of flowering plants that includes pōhutukawa, gum trees, feijoa, guava, clove and allspice. New Zealand has 19 species in this family:
- trees: rātā species, pōhutukawa, kānuka, swamp maire
- shrubs: mānuka, rōhutu, ramarama
- vines: six species of climbing rātā.
Rātā and pōhutukawa are species of Metrosideros. This is the most widespread genus of trees in New Zealand, and is found from the subtropical Kermadec Islands to the subantarctic Auckland islands, 500 kilometres south of the mainland.
Of New Zealand’s Metrosideros species, six grow into trees and six are vines. Their flowers are a mass of brightly coloured stamens around a cup containing nectar, which attracts insects, birds, lizards and bats. The fruit is a dry capsule containing hundreds of tiny seeds. The seeds need light, and germinate in open sites. All species can produce roots from their stems.
The giant northern rātā (Metrosideros robusta) can grow over 40 metres tall. It was once widespread in wetter areas of the North Island, and the north-west South Island.
Although it can begin life on the ground, it usually starts out as an epiphyte (perching plant), high in the canopy of a host tree. As it grows it sends roots down to the ground. These aerial roots become thicker and woody, and may join to form one or several trunks. These support the northern rātā’s spreading crown when the host tree dies.
Northern rātā’s clusters of attractive flowers look like those of southern rātā and pōhutukawa, but are a darker red-brown. Its leaves have a notch at their tip. The very dense wood is dark red-brown. It makes excellent fuel, and will burn when green.
Northern rātā is vulnerable to browsing by possums. Many trees died in the Ruahine and Aorangi ranges as possum numbers increased in their forests.
Bartlett's rātā (Metrosideros bartlettii) is one of New Zealand’s rarest plants. It was only discovered in 1975 near Te Paki, Northland, where there are 34 adult trees. Like northern rātā, it usually starts life perching high in the canopy of a host tree and grows into a tall tree. Unlike other tree rātā, it has white flowers and white, papery bark.
Southern rātā (Metrosideros umbellata) does not grow as tall as northern rātā, usually only reaching 15 metres. It is widespread in the western South Island, on Stewart Island, and on the coastal edge of the Auckland Islands. In the North Island, it grows on the high peaks of Northland and Coromandel. Southern rātā tolerates most soil types, including the limestone and toxic soils around Nelson.
It is easily distinguished from the other tree rātā by its shiny leaves, which have many oil glands and a pointed tip.