Coprosma

There are few groups of plants that exhibit such a wide range of adaptations to New Zealand’s varied landscapes as the genus Coprosma. Creeping mats, upright small trees, large leaves, small leaves, berries of many different colours, shade-dwelling or inhabiting open ground; Coprosma really has an astounding variety of habits, characteristics and tolerances.

As might be expected, this makes Coprosma a very useful and versatile genus for gardeners and landscapers. In recent years, this genus has become doubly useful through the evolution of garden tastes – to embrace a wider range of native plants (especially those with small leaves and interlaced branching structures). Thankfully, this development means that there is now a greater focus on the merits of our many species of Coprosma (rather than man-made cultivars).

Despite the wonderful range of species on offer, the genus does suffer from a bit of a PR problem. For years, the main varieties to have been cultivated have been variegated abominations (of multifarious hues) and characterless hybrids (including many varieties of soul-numbing, creeping ’roundabout’ shrubs). Coprosma spp. are prone to such human tinkerings, for certain species hybridise readily, even within nature (a fact that was commented on by both Leonard Cockayne and W.R.B. Oliver¹).

Due to this unholy alliance of varieties (some garish, others featureless) the name ‘coprosma’ has become, to many people, a byword for the kind of native that you reluctantly plant because nothing else will work (similar to the oft-maligned Pittosporum).

However, as noted above, the tide has turned with increased interest in New Zealand’s small-leaved shrubs (known either as filiramulate or divaricate shrubs²). Most of our Coprosma spp. fit into this category. Several of these species (such as Coprosma virescens, C. rugosa and C. propinqua) form billowing clouds of foliage in a range of colours (which are often conferred by the stems rather than the leaves). Some (including C. cheesemanii and C. petriei) form creeping mats that adorn banks and open ground in our montane areas, whilst other species (like C. distantia, C. neglecta and C. spathulata ssp. hikuruana) tumble over our northern coastal cliffs.

A major part of the appeal of so many small-leaved species is that the reduced leaf size draws attention to the ‘architecture’ of these trees and shrubs. Now, the use of the word ‘architecture’ is not some lame attempt to make plants seem more hip; it is an actual term that is used by botanists to refer to a plant’s growth structures. Branches may be straight or arching; they may be densely packed or spaced wide apart; they may protrude in dense, voluminous masses or splay out in rigid fans; or they may be distinctly tiered.

One particularly appealing incentive for cultivating our species of Coprosma is the quantity of fruits that they produce, which constitutes a valuable food source for our native birds and lizards. These fruits come in an amazing array of colours, including white, orange, yellow, red, iridescent blues, shades approaching black, and translucent, milky hues. Aside from their wildlife value, they also add immensely to the aesthetic appeal of Coprosma spp., contributing small flashes of colour to a planting.

New Zealand has c. 53 species of Coprosma, which is almost half the worldwide number of species within the genus³. These occur throughout the New Zealand botanical region, from the sub-tropical Kermadec Islands to the Subantarctic Islands in the south. Its unfortunate (and misleading) name is derived from the Greek word for manure (kopros), and arose from the unpleasant odour of one of the first species to be collected (C. foetidissima).

If you set up camp in the path of shifting sands, you need to be able to dig your way out of trouble (so to speak). The sand coprosma (Coprosma acerosa) manages this by sending forth trailing stems to avoid becoming a victim of the constant accumulation of sand. Its relationship to the dunes is not, however, simply that of a passenger evading immersion – for its interlacing branching network also assists in the build-up and stabilisation of the dunes (by capturing sand and forming adventitious roots along the spreading stems).

The creeping habit that C. acerosa has assumed in response to its dynamic natural habitat is a significant factor in its popularity amongst gardeners and landscapers. Drought-tolerant, low-growing shrubs are particularly useful for public plantings and difficult garden situations. However, it is the orange colour conferred by the stems that has done most to establish it as one of our more popular native garden plants.

Coprosma acerosa occurs slightly behind the coastal ‘front line’, on the more stable, secondary dunes and the inland side of foredunes. It is found over the full extent of the two main islands, as well as on Stewart and the Chatham Islands. Like other native dune plants, it is has become much less common in the wild over the last century – due to the destruction and mismanagement of dune habitats throughout the country. Subsequently, it has been classified with a threat ranking of ‘Declining’.

Within gardens, C. acerosa is best employed in situations similar to its natural growing station – ideally, sloping ground with high sunlight and a fair degree of exposure to wind. Better yet, it should be foremost in the minds of anyone wishing to plant on or adjacent to dunes. An additional attraction for cultivating this species arrives in the form of translucent white or blue fruits in summer or autumn (sometimes bearing intricate patterns reminiscent of Venetian glass).

Whether on islands or the larger mountain ranges of New Zealand, high peaks exert a magnetic hold upon clouds (and the water that clouds transport with them). This is a major factor in our weather; most significantly, in the east-west pattern of the Southern Alps, where the rain mostly dumps on the western side and the eastern side sits within a rain shadow.

It is also expressed on a much more local scale, such as on Great Barrier Island’s Mt Hobson, where the higher parts receive significantly more moisture (both rain and general atmospheric moisture) than the remainder of the island, forming associations akin to cloud forest. It is in these interesting habitats that Coprosma dodonaeifolia (pictured below, centre) is found, on both Great Barrier Island and high points of the adjacent Coromandel Peninsula (where a similar effect occurs).

This understated shrub bears a similar character to the closely-related Coprosma lucida, in that it is a conventional foil against which other plant forms may be contrasted (or simply to be appreciated as a background of green). We frequently specify this shrub for this purpose; often in preference to Coprosma lucida, due to the more refined, narrower leaf and compact growth habit of C. dodonaeifolia.

Although rarely seen in gardens, it has been cultivated for many years by Oratia Native Plant Nursery, who introduced it to cultivation (and advised us to trial it within gardens). Despite its association with the distinctive upland habitats of Great Barrier and Coromandel, it is an easy plant to grow within gardens, and plays a valuable role as a compact, structural shrub (growing up to 2m).

Within the wild, C. dodonaeifolia is quite variable in appearance, especially with respect to how it looks in shade and open habitats. One of the most striking variations that I observed was on plants positioned on banks, or on the margins of scrub, lower down on the mountain – where shrubs of C. dodonaeifolia exhibited mottled red and olive tones upon their leaves, similar to the colouration of tawheowheo (Quintinia serrata), a native tree that grows in infertile upland habitats.

Coprosma dodonaeifolia was recognised as being a distinct species by W.R.B. Oliver, a former director of the Dominion Museum (in the 1930s and 1940s) who was the main authority on the genus Coprosma⁴. He identified it on the basis of trips to Mt Hobson in 1912 and 1929. Amongst other differences with C. lucida, Oliver separated it from its relative by the thin leaves, which do not possess the same shining upper surface as C. lucida. The specific epithet refers to the similarity of the leaves (which often have a wavy margin) to those of akeake (Dodonaea viscosa).

Within design, the ‘ordinary’ or ‘conventional’ can be easily undervalued, in favour of the radical or grand gesture. This relatively large-leaved species of Coprosma is a particularly good example of a plant that might well be termed plain, yet which bears itself with considerable elegance.

One of the most important attributes of this shrub (from an aesthetic point of view) is referred to in its specific name, ‘lucida‘, which means ‘light-reflecting’ or ‘shining’ – with regard to the lustre of the dark green leaves. This gives the foliage a vibrant hue that projects forth from more subdued shades of green (or the range of muted colours that are exhibited by many of our filiramulate shrubs²).

Its appeal is further reinforced by the neat, rounded shape of the leaves, and C. lucida‘s compact, overall form – both of which set it apart from its rangy (albeit ecologically important) cousin, Coprosma robusta. Shining karamu is a useful and handsome species for establishing structure within urban gardens. Its unassuming nature makes it a good foil, against which more idiosyncratic plant forms may be read. Its size and growth habit also weigh in its favour, as it is very easily maintained at 2m or smaller, and retains thick growth on lower parts of the shrub.

C. lucida is widespread throughout New Zealand’s main islands, and occurs in a wide range of habitats, from coastal to montane forest (up to 1000m). It performs best where it can derive sufficient moisture and a modicum of shade, although it is tolerant of challenging habitats – such as the coastal shrubland on the Matapouri Bay headland shown in the photo above.

It exhibits a variety of characteristics (including leaf sizes), depending on the site in which it grows. For example, in exposed conditions, the leaf is typically smaller and plants are especially compact; whilst on the coastline, plants often bear particularly thick leaves. In my opinion, native horticulture would benefit from selection of forms from different locations, such as the coastal form pictured above – to assess whether characters such as the fleshy leaves and attractively-coloured stems remain consistent in cultivation (and whether such forms are more resilient in dry or windy conditions).

C. lucida was first collected by Joseph Banks and Daniel Solander, in the early part of Cook’s first voyage to New Zealand, in 1769. Like its near relative, Coprosma robusta, shining karamu was an important plant within Maori spiritual traditions, and was used by tohunga within rites.

Moss-like plants constitute a plant type that is on many people’s wishlist – frequently inspired by serene visions of Japanese gardens. However, this is a difficult effect to achieve in gardens, as the kinds of conditions that promote verdant moss growth (like those seen in Japanese temple and palace grounds) are not replicated in most gardens.

In New Zealand, a moss-like effect is most frequently attempted through the use of one of our species of Scleranthus. However, the members of that genus are regularly specified in situations in which their lifespan is limited – due to factors such as high rainfall or humidity, or a lack of air movement. Scleranthus is by no means the only genus that is suited to this role, and one of the other leading candidates is the diminutive Coprosma petriei.

This is an extremely rugged species that survives in some of the most unlikely places, such as the rabbit-ravaged short grasslands of the Mackenzie Basin, or on exposed, rocky ground in montane areas. Although plants often exhibit a brownish-green colour (due to the tough conditions that they frequently endure in places like the Crown Range or Mt John, near Tekapo), certain forms of Coprosma petriei can develop dazzling emerald-green foliage within garden conditions.

C. petriei is one of those surprising mountain dwellers that will grow in many different parts of the country, even in the warmer north. We are growing it very successfully at a garden in the inner suburbs of Auckland, where it wends its way around the base of other native shrubs. Terry Hatch has also grown extensive carpets of it in his own Pukekohe garden for a long period, in both open and slightly shaded conditions.

Some experience in growing this species has shown that it tends to get a better start where it can be afforded a small amount of shelter at the beginning (from an adjacent small shrub or rock), and then creep out towards the open conditions that it prefers. This seems to decrease drought stress on young plants which, although they are extraordinarily resilient to arid conditions in nature, can suffer from drying out in their first season. As with so many plants, establishment is the critical phase.

Like other members of the genus, C. petriei has attractive berries – which in the case of this species are pale blue in colour. These sit atop the foliage like beads on a garment, and are sometimes preceded by massed flowering displays (as pictured above, left), in which the opaque flowers reflect light in a remarkable manner. Coprosma petriei commemorates Donald Petrie, one of New Zealand’s early botanical figures. Petrie was a highly respected botanist, who searched for plants in many parts of the South Island (in particular), and was renowned for having a careful eye (regarding the observation of plants).

This obliging species can lay claim to the dubious honour of being one of the most promiscuous plants within the New Zealand flora. Coprosma propinqua, it seems, will sleep with just about any tall, dark stranger whose pollen happens to blow into town⁵.

It certainly has a great number of opportunities to come into contact with other species, for C. propinqua grows in a huge range of habitats throughout New Zealand (from dry mountain slopes to lowland swamps). In line with this adaptability, it takes on a diverse range of forms, from low-growing hummocks on the coastline to 3m tall, rounded shrubs (or even higher).

Within older literature, authors often make the comment that New Zealand’s small-leaved shrubs (or ‘filiramulates’²) are sufficiently similar that they can be hard to discern. This is, of course, rubbish in a great number of cases. Several species with coloured stems (like Muehlenbeckia astonii and Coprosma rugosa) are very easy to distinguish, whilst the growth forms of others (like Coprosma rhamnoides and C. acerosa) make them unmistakable.

However, in the case of Coprosma propinqua, there is a fair amount of truth to this statement, for some other species (notably C. rigida, C. tayloriae and C. crassifolia) approach it very closely in appearance. This is reflected in its specific epithet, which can be translated as ‘closely resembling’ – in reference to its similarity to other species.

Falling into this trap, I had always confused plants that I was viewing in the Mackenzie Basin, and grey scrub from nearby areas, with Coprosma rigida (especially because northern forms of C. propinqua look so different from their stiffer southern namesakes) – until I was corrected by the Nelson botanist, Shannel Courtney.

We have planted this dryland form of Coprosma propinqua in our Tekapo garden, propagated off local stock (from a scrub remnant just 60m from the house). A major reason for specifying it is that it is one of the last native plants to endure in degraded ecologies, and is therefore an excellent garden plant for that demanding climate. Another reason is that its contained form makes it a fine structural shrub for gardens, as it will reach around 2m and requires almost no pruning.

The plants that are pictured above, growing at Titahi Bay, are examples of the low-growing forms that hug rocks on the coastline. Examples of this form that I have seen bear brighter green leaves than montane types (which assume a browner hue, due to the smaller leaf size and duller leaf colour). These coastal forms are useful garden plants that make an attractive counterpoint to their bolder-leaved neighbours. In their case, it should be remembered thatthey will not remain as prostrate in garden conditions as they do under the constant battering of coastal winds.

There is a particularly leafy Chatham Island variant, C. propinqua var. martinii, that grows in two forms; one that develops into small trees, the other growing as a creeping shrub. The latter is sold on mainland New Zealand as Coprosma ‘Taiko’. In northern parts of New Zealand, C. propinqua is frequently associated with swampy sites (as shown in the photo below, right, from a wetland on the shores of the Kaipara), and has comparatively flexuous branches and long, narrow leaves (combining to give it a looser appearance).

In recent years, Coprosma virescens has become increasingly popular within gardens, due largely to the attractive orange colour of its stems. These are softened by the pale green foliage, to give it a more delicate colouration than its lower-growing relative, C. rugosa (which exhibits quite vivid orange hues). C. virescens tends to grow as a relatively upright tree, with ‘clouds’ of foliage tumbling over each other. The smooth bark, which is patterned in grey and green, is an additional attraction that rewards closer observation. Like other small-leaved trees and shrubs, it is a fine plant for providing a sense of depth within gardens.

It is primarily a plant of cold localities on the drier eastern sides of both main islands (occurring from the centre of the North Island south), where it grows in shrubland (like the habitat pictured below in northern Southland) and on forest edges.

Despite the fact that it is native to cold, dry parts of the country, Coprosma virescens can be cultivated throughout much of New Zealand (including as far north as Auckland). However, it is less suited to planting in warmer, humid regions than some other small-leaved trees of similar dimensions (such as Pittosporum obcordatum, Melicytus micranthus and northern forms of Myrsine divaricata).

Coprosma virescens is one of the few native trees to be either wholly or partially deciduous, although it remains evergreen in warmer locales. The specific epithet, ‘virescens‘, means ‘becoming green’⁶ – possibly in reference to the greenish bark⁷.