Metrosideros

Of all the roughly 2360 vascular plant species that are native to New Zealand, few command the attention of New Zealanders as much as the pohutukawa. The genus to which pohutukawa belongs, Metrosideros, also contains 11 other beautiful species that are native to this country, most of which are not embedded in the public consciousness as emphatically as pohutukawa.

The next best known members of the genus are the two tree rātā species; northern rātā (Metrosideros robusta) and southern rātā (Metrosideros umbellata). The former is one of the most impressive of our forest giants, whilst the latter is a notable component of forests in southern parts of New Zealand, stretching down towards the country’s southernmost limits in the subantarctic Auckland Islands.

Sometimes, a bit of detailed botanical information can be very useful in understanding garden plants better. Bearing this in mind, it is worth pointing out here that the genus, Metrosideros, is divided into three groups (called subgenera), of which two are represented in New Zealand. Put simply, this means that New Zealand species of Metrosideros fall within one of these two groups; one of which is confusingly also called Metrosideros (i.e., genus Metrosideros, subgenus Metrosideros), whilst the other subgenus is called Mearnsia. Generally speaking, the large tree species (like pohutukawa and the tree rātā) fall within subgenus, Metrosideros, and the climbing/shrubby species are part of the Mearnsia subgenus. This distinction helps one understand the relationship between an imposing tree like pohutukawa and a compact climbing shrub like Metrosideros colensoi.

The primary reason that pohutukawa and rātā are so beloved within New Zealand is the summer appearance of their brilliant red flowers. These are loaded with nectar, and consequently attract large numbers of birds and insects (from whom the plant receives the benefit of pollination). Previously, before their populations were seriously affected by the impact of humans, native bats and lizards also played a role in the pollination of the flowers. In addition to the flowers, the leaves of our rātā and pohutukawa are an important source of nutrition for many of our native stick insects.

The brilliant floral displays are mainly created by large clusters of the male part of the flower, which are called stamens. This is different from the way in which many garden flowers (such as a daffodil) announce their presence, via coloured petals (and other similar structures). The nectar that is so attractive to certain animals sits at the base of the flower (beneath the riotous display of stamens), in a cup-like part called a hypanthium.

All New Zealand Metrosideros species bear this same basic floral form, although their flowerheads vary widely in size and colour – occurring in white, light pink, orange-red, and even yellow (in certain natural variants, such as one form of pohutukawa).

The name, Metrosideros, is derived from the Greek terms, metra and sideron, which mean ‘core/heartwood’ and ‘iron’ respectively. This refers to the extremely hard heartwood of many members of the genus. The genus is part of the widespread myrtle family, named for the common myrtle (Myrtus communis) which is of such cultural significance to Mediterranean cultures. Other members of the family include feijoa, guava and Eucalyptus.

For in-depth information on the genus (including its cultural and mythological significance), an excellent book was published in 2005 by Te Papa Press. Entitled ‘Pohutukawa & Rata : New Zealand’s Iron-hearted Trees’, the author Philip Simpson presented an exhaustive and fascinating resource on the main tree species, as he did in his previous work on cabbage trees.

Although it is ostensibly the beautiful white flowerheads that mark this species out as something special, my most vivid impression upon first viewing it in the wild was of the foliage (more specifically, the new foliage). As M. albiflora sends out fresh growth, the slightly arching stems bear leaves that can possess a remarkable reddish colouration. It is unclear to me whether this was attributable to the high-altitude location in which I have viewed Metrosideros albiflora (in the upper reaches of Northland’s Waima Range), as I have seen no mention of this characteristic in botanical literature – or whether it is apparent in other populations.

The striking colour of the young leaves upon the red stems struck a particular chord with me, as we often plant a South American shrub (called Cavendishia bracteata) that exhibits remarkably similar growth on its new foliage. The comparisons between these plants don’t just apply to their appearance. C. bracteata also assumes a life amongst the canopy of trees (although as a perching epiphyte, rather than a root climber), and it grows in cloud forest in the wild within South America (M. albiflora grows in cloud forest at the Waima Range).

This does not suggest that the two species are in any way related – to the contrary, they are from completely different families. It is just interesting to note how unrelated plants from different parts of the world can develop similar characteristics within comparable conditions.

When in full bloom, Metrosideros albiflora is a breathtaking sight. Along with pohutukawa and scarlet rātā (M. fulgens), its flowerheads rank amongst the largest of out native rātā, with the key difference that they are a pure white colour. As a species of kauri forest and high-altitude habitats, it is unsurprising that it has acquired a reputation of being rather exacting in its cultivation requirements.

However, given a position where its base is afforded shade and it is not permitted to dry out excessively, it is possible to successfully cultivate this beautiful species (as we have on the southern side of a coastal house).

When I was taken to see some of the best specimens of carmine rātā remaining in the Waitakere Ranges, I was not prepared for what confronted us as we rounded a bend in the stream. The behemoth that smothered the cliff above us ascended at least 15m from its base, and spread to a width of at least 20m. Here is a climbing rātā that dwarfs the scale of many pohutukawa.

It would be fascinating to know what the age of this giant is; its stout trunk snaking tortuously up the lower third of the rockface. It certainly forms a sharp contrast with the compact shrubby form in which Metrosideros carminea is normally seen within cultivation – a habit encouraged by propagation off adult cuttings, for the purpose of fast-tracking flowering in garden specimens.

We tend to plant specimens propagated from seed or juvenile cuttings, so that plants climb over walls or rocks as they do within nature. That said, we have specified M. carminea from adult material for containers, an application for which this species is well suited (and has been recommended by many authors). We normally place carmine rātā in positions with good air movement, to reduce the potential impact of thrips which, although not particularly harmful to plants’ wellbeing, mar the appearance of the foliage.

In common with other climbing rātā, there is a considerable distinction between the flattened, ordered juvenile foliage (pictured below) and the shrubby adult state (pictured above right, in flower). It flowers in spring (from August through to October, depending on season and location); and in good years, plants cover themselves in a blaze of carmine flowers.

Metrosideros carminea is native to northern parts of New Zealand, where it grows from the North Island’s northernmost tip to as far south as East Cape and northern Taranaki, particularly in forest and forest edges. It is a conspicuous feature of a number of limestone ecologies, including some highly disturbed farmland habitats near Port Waikato, from which Terry and Lindsay Hatch have shown me spectacular images of plants in full bloom.

Interestingly, M. carminea was present in Auckland’s lavafields prior to their degradation in the face of urban development. It is recorded as enduring at Mt Wellington into the early decades of the twentieth century, and is therefore a plant that should be considered by gardeners or landscapers planting lavafield sites in Auckland.

This climbing rātā is an elegant and distinctive plant, which forms curtains of hanging branches upon tree trunks or rock faces. It is especially associated with limestone areas, but is found in other habitats. The weeping habit is the most remarkable feature of this species; a characteristic that, furthermore, is unusual to find in any species (from New Zealand or otherwise).

Metrosideros colensoi has vibrant green foliage, similar to M. perforata but with more pointed leaves. The new season’s foliage comes out in a comparatively lighter shade of green, which contrasts with the darker colour of the existing foliage. The delicate, pinkish-white flowers appear in early summer, towards the end of the branches.

Within the garden, M. colensoi can be used to similar effect as its natural growth form (for climbing up trees or walls), or as a low shrub. If it is to be used for the latter application, it should be planted in a slightly raised position, so that its pendent habit can be best appreciated. Due in part to the comparatively small size of the flowers, M. colensoi has consistently been undervalued as a garden plant, in favour of its relatives that exhibit more spectacular flowering (it doesn’t feature, or barely receives mention, within most books on native trees and shrubs for gardens). However, it is in several ways superior to more commonly grown species, such as Metrosideros carminea, on account of the ordered appearance of its growth form, and the ‘movement’ conveyed by its downward arching branches.

The specific name commemorates William Colenso, an early missionary who was one of the great characters of New Zealand botany. On his travels through New Zealand (many conducted by foot), Colenso developed a familiarity with the New Zealand flora that few could emulate. Metrosideros colensoi grows within lowland forest as far south as Greymouth and Kaikoura.

In my opinion, Metrosideros fulgens is the noblest of our vine rātā. Its beautifully-formed heads of scarlet flowers are amongst the largest of our native species of rātā, and present themselves well at the tips of the branches. However, despite the fact that it is a common sight in our forest canopies, it is not as popular a garden plant as one might expect.

This is partially due to its preference for slightly more stable conditions than those that its relatives M. carminea and M. perforata will tolerate. Having said that, scarlet rātā is not a difficult species to grow; and as long as its roots are afforded some degree of shelter, it will grow in quite exposed sites. Indeed, the best flowering displays that I have seen of Metrosideros fulgens have been at the disturbed edge of forest areas (such as the large plant in the image below, from the roadside of Piha Rd in the Waitakeres).

Metrosideros fulgens has the longest flowering period of any of our native rātā, with plants producing flowers from the end of summer well into winter (generally achieving their densest displays in autumn). The timing of its blooms is a matter worthy of special consideration, as they appear when few other conspicuous flowers adorn our forests. This makes akatawhiwhi a useful plant; not only for brightening up our own environments as the days become darker, but in providing a source of nectar to animals like the honeybee that is enthusiastically engaged in harvesting the nectar of M. fulgens in the photo below.

The foliage has a distinctive pale green colour that helps to distinguish M. fulgens from other vine rātā, whilst the slightly wavy form of the leaves also sets it apart from its relatives. As with other climbing rātā, it is frequently propagated from adult foliage – in order that it may be grown as a ground shrub, as well as promoting early flowering. It can therefore be grown as either a shrub or a climber. For the latter application, one should really plant seed-grown material; and it should be borne in mind that earlier flowering will be attained in an open aspect.

When akatawhiwhi has reached its desired position atop the forest canopy, the stem that previously clung to its support becomes suspended by its host tree’s trunk. These long cables (which can grow to be 15cm wide) had cultural uses for both early European bushmen and Māori that are worth recounting. In a practical usage that should not be repeated, bushmen used to drink a juice that they extracted by cutting a slit in the bark. One description states that a sample of this tasted like “dry cider”¹; but given the calibre of food and drink that was commonly available in those times, any enthusiasm towards its flavour should probably be taken with a grain of salt.

In traditional Māori palisades, the large stems of some species of ‘aka‘² were utilised as rails, due to their extremely hard timber and convenient dimensions (appropriate stems were like ready-made rails)³. Although the ethnographer Elsdon Best specifically mentioned akatea, akatorotoro and akatoki (all terms for white-flowered rātā) in relation to this purpose, Metrosideros fulgens would also surely have been utilised extensively for palisade rails (due to its abundance and the size of its stems).

Plants can go through many botanical names throughout history. These names are often quite interesting, as they not only give information about the characteristics of a plant, but also of people’s impressions of a plant. With regard to this, it is worth noting that one name formerly applied to this species (by the botanist, Allan Cunningham) is Metrosideros buxifolia. That is interesting because Cunningham made a comparison between this plant and the common box (Buxus spp.), which is ubiquitous as a hedging plant within New Zealand and abroad.

New Zealand has many species that should be turned towards common uses, such as hedging, for which we consistently utilise exotic species based on the tired (and often inappropriate) answers handed down by our European forebears, or from overseas trends. The photograph above shows a specimen of this plant (which is normally a root climber), growing in pasture as a compact shrub. If we take nature as our teacher, it is obvious that this is a plant that can be turned towards use as a compact hedge – in my opinion, best applied as a slightly informal hedge, so that one can enjoy the flowers.

As mentioned above, Metrosideros perforata usually grows as a climbing shrub (like other members of the subgenus, Mearnsia). Within the normal course of events, the species has two growth forms; the smaller-leaved juvenile state, which clings to tree trunks or other supports by specialised roots, and the adult state, which is exhibited in the plant shown above. In the absence of something to climb on, the plant assumes the adult growth form, and grows as a relatively compact shrub. This tendency is also aided by the propagation of plants from cuttings taken off adult plants.

Although it can grow taller than 1.5m as a ground shrub, M. perforata is very amenable to being trimmed to as low as 40cm high. Its bright, dark green foliage is highly attractive, and is arranged in a very ordered manner along the stems. In summer, it produces large quantities of pure white flowers, which are popular with bees. It also plays an important ecological role as a favoured host for several of our native stick insects (a combination of M. perforata and kanuka would make a particularly good combination for gardeners who wish to encourage these fascinating creatures).

Whilst it is able to grow within the damp interior of northern forests (albeit in juvenile form), M. perforata is also capable of growing in difficult, exposed situations, such as dry hillsides. It is predominantly a species of warmer areas, occurring in lowland forest as far south as the northern tip of Fiordland. Consequently, it is intolerant of heavy frosts.

Metrosideros perforata had a valuable practical purpose for Māori, as it was one of the primary species utilised for lashing (of weapons as well as palisades). The thin young stems were tied in a green state, when they were still pliable, and subsequently dried to become very hard and rigid³. The large cables that form on very old rātā vines were also used by Māori as a means of ascending cliffs – a practice that is adapted in the myth of Tawhaki, a warrior who ascended to the heavens on a giant ‘aka’ cable (to bring back his wife and child, in one telling).

Due to the beauty of its foliage, flowers and growth habit, and to the wide range of conditions in which it will thrive, Metrosideros perforata is a species that should be used much more extensively than it is, especially for the purpose of providing structure within plantings.