Pittosporum

The genus Pittosporum is, in my opinion, one of New Zealand’s most poorly represented genera in horticulture. Its members include some of the finest shrubs and small trees of the New Zealand flora, but the majority of species/varieties that have traditionally been offered by garden centres or nurseries are amongst the most inferior of the genus (synonymous with NZ gardens of the 1970s and 1980s). This regrettable situation has lain mostly unaddressed, and Pittosporum is still perceived by many as a genus of scrappy, variegated, uninspiring small trees (a situation that needs to be remedied).

The genus Pittosporum predominantly occupies tropical and sub-tropical regions of the Southern Hemisphere. Its botanical name refers to the sticky seeds, and is derived from the Greek words for tar (pitta) and seed (spora). There are around 200 species in the genus, of which c. 22 occur in New Zealand. All of our species are confined to our shores.

Pittosporum cornifolium is an elegant epiphytic species which is frequently found high up in the canopy of large forest trees, yet (despite its usual home) will thrive in a well-drained soil. It often grows on puriri (Vitex lucens) or rata (Metrosideros robusta) trees, amongst entire communities of epiphytes. These epiphytic communities are remarkable gardens suspended on the branches of our large forest trees, and their constituents can be surprising in their size and diversity.

P. cornifolium has been commonly sold in two forms by local nurseries. The true species has smaller leaves of a light green, and is variable in its flower colour. It eventually forms a tidy, dense shrub up to 2.5m high (although more commonly 2m).

The other form (from the Poor Knights Islands, and which has recently been described as a distinct species, Pittosporum roimata) has darker, larger leaves that are lusher in appearance, and resembles a vireya rhododendron whilst young. It eventually forms a shrub of a similar size and habit as the mainland form. It is a common phenomenon that the leaves and flowers of plants from many offshore islands are larger than those of their relatives of the mainland (a fascinating occurrence that requires further investigation). Both plants are fine garden subjects for well-drained soil, although my experience is that true P. cornifolium tends to retain a better form in the long run.

It may be that many commercially available forms of P. cornifolium sold in recent decades derive from a variety selected by Graeme Platt from the West Coast of Auckland (Maunganui Bluff), which was terrestrial, growing in full sun, and exhibited a thicker, more compact form than the species type.

As a naturally variable species, superior forms of the plant have come into cultivation from different sources; including the large-leaved Waitakere form that we normally specify within our work. P.cornifolium ranges from North Cape to the Marlborough Sounds.

This beautiful species has a very restricted distribution in nature; occurring only in North-west Nelson, an area which is somewhat of a biodiversity hotspot within New Zealand. It is readily distinguishable from all other Pittosporum, on the basis of its dark stems, intensely green, serrated foliage, and the large sprays of cream flowers that are produced in midsummer during good years. The form of the flowerheads bears a resemblance to the inflorescences of another of our most beautiful flowering plants, Pimelea longifolia (native daphne or taranga).

It has been pointed out by several authors that this species flowers sporadically in cultivation. It is, however, worth cultivating for its foliage and growth form alone. In his seminal work, ‘The Cultivation of New Zealand Plants’ (1923), Leonard Cockayne wrote of P. dallii as “foremost amongst desirable species for town planting”. At the time it was not in cultivation in New Zealand, and it is still little-known within New Zealand gardens – where it is mainly found in the collections of enthusiasts.

Pittosporum dallii is a threatened species (classified as Nationally Vulnerable), which primarily suffers from the depredations of introduced pest animals (deer and possums). It is often found on steep ground (a position in which I have observed this species in the wild, at Cobb Valley). That fact is unsurprising, for one often finds threatened species in such habitats, as they are less accessible to browsing mammals. Lawrie Metcalf has noted¹ that it is capable of tolerating considerable drought. This observation has been borne out in my personal experience, of having planted P. dallii in the extremely dry climate of the Mackenzie Basin, where it has thrived.

Pittosporum kirkii is, like its relative P.cornifolium, epiphytic (occurring on rocks or trees) and occurs commonly on the trunk and branches of rata and other larger forest trees. However, unlike P.cornifolium, it is often terrestrial as well. It usually occurs in hilly forest from Mangonui (Northland) to Mt Egmont, from 240m to 900m, and is most plentiful at about 650m.

It forms a shrub from 1.2m to 3.6m high (but seldom exceeds 2m), and has an open habit. A notable feature of P. kirkii is the presence of outsize, flat seed-capsules that adorn the plant following flowering; these are highly conspicuous and very unusual in appearance. It is named after its discoverer, Thomas Kirk, who first collected it on Great Barrier Island.

Due to its place in nature (as a perching plant), Pittosporum kirkii should be given a well-drained position in relatively poor soil, if grown in the ground. However, it is particularly well suited as a pot plant, an application in the garden that recreates many of the conditions in which it grows within nature.

P. kirkii is rare in the wild (with an official threat ranking of ‘Declining’), a status that is borne out in my personal experience – for, although I have seen this species in a number of parts of the country, the only place that I have viewed reasonable numbers of it is on Great Barrier Island (where the specimen in fruit below was situated).

In botanical ‘geek-speak’, the phenomenon by which trees and shrubs bear distinct juvenile and adult stages is termed heteroblasty. This character is most spectacularly demonstrated in the New Zealand flora by the toothed lancewood, Pseudopanax ferox. However, it is also well represented in other genera, such as in the case of one of our most beautiful native trees, Pittosporum patulum.

There are clear parallels between the toothed lancewood and Pittosporum patulum, including the toothed margin that both trees exhibit in their juvenile forms, the ‘lollipop’ form that both assume in their adult phases, and the extraordinary colour of both species’ juvenile foliage. The leaves on young plants of P. patulum differ from juvenile P. ferox, in their exquisite bronze hue (as opposed to the usually brown leaves of P. ferox) and their more delicate, narrow form.

This small tree, which is restricted to scattered inland localities of South Island, is nationally endangered – suffering predominantly from the depredations of introduced mammals (most noticeably deer and possums).

P. patulum is mostly a plant of open habitats, such as subalpine scrub and openings within beech forest (as occur following storms or earthquakes, or the longer-term habitats provided by bluffs). It is found naturally in Nelson, western Marlborough and central areas of Canterbury. One of the major strongholds for the species is the Temple catchment (near Lake Ohau), where most of the photos in this profile were taken.

In this location, P. patulum emerges from subalpine scrub on sloping ground, at the headwaters of one branch of the Temple Stream. The communities in which it occurred included typical montane scrub species such as mountain toatoa (Phyllocladus alpinus), mountain wineberry (Aristotelia fruticosa), Coprosma propinqua and snow totara (Podocarpus nivalis). In the photograph above, it is easily recognised as the vivid green, upright tree in the centre of the shot – in this case, the specimen in question was a tree on the turn towards its adult stage (at which point it assumes the vibrant green hue displayed below).

Pittosporum patulum is still relatively unknown in cultivation, outside the gardens of collectors. However, it has great potential as a landscaping plant for South Island gardens, on account of its outstanding appearance and agreeable size (like Pseudopanax ferox, it is ideal for establishing a sense of scale within confined spaces). In addition to these characteristics, the dark red flowers on P. patulum (which are said to be the most fragrant within the genus) are a major incentive for growing this small tree. These emerge in early summer.

It is becoming increasingly available for sale, primarily in Nelson and Christchurch – two cities where it should be included in public plantings, to assist in advocacy for its plight in the wild. P. patulum is ideally planted with some degree of shade to the roots, preferably from small shrubs that it can eventually overtop (such as Aristotelia fruticosa or Melicytus alpinus). It is not a particularly difficult plant to grow in the South Island, especially when grown within communities of plants, rather than as isolated specimens.

In the north of the country, it is a challenging species to grow (as might be expected of a plant of high-altitude areas from the South Island). Despite this, it is such an interesting and attractive species that it is hard to resist. If one is to try P. patulum in the north, it should be planted in situations that are not subjected to the worst of our northern humidity (such as slopes with good air movement, or adjacent to running water), and which receive a considerable degree of shade at the hottest times of the day (for example, south-facing banks).

Its specific epithet, patulum, means ‘wide-spreading’, or ‘open’. This presumably refers to the arrangement of leaves on the stems, as Hooker (who described the species) only had a single branch specimen to examine (and the tree’s growth habit couldn’t be described as ‘spreading’). Pittosporum patulum was discovered by Dr Andrew Sinclair, an important figure in the early botanical exploration of New Zealand, high up in mountains of Marlborough², in 1860. Sinclair’s contributions to science are commemorated in the naming of several species, including Meryta sinclairii and Jovellana sinclairii.

As our experience of wild places and their inhabitants has developed over the last two decades, we have become increasingly conscious of the poetry that often accompanies evolutionary processes. One of the best examples of this is the vast multitude of guises (like botanical sheet music) assumed by Alseuosmia – the poster child for Batesian mimicry³.

In the case of that genus, a single species can demonstrate remarkable variation in leaf form and growth habit, with the most well-known examples mimicking the unpalatable Pseudowintera colorata. In this way, an ‘undefended’ or ‘unprotected’ species signals to prospective predators (herbivores, if you’re a plant) that it’s probably not a great dietary option.

The same thought occurred to me as I stood in the midst of a bewildering array of forms of Pittosporum pimeleoides subsp. pimeleoides on a coastal headland in the Bay of Islands – the tones of which melded seamlessly with other species of dry northern woodlands.

In this case, the ‘model’ (the species that is being mimicked) seems to mostly flit between Kunzea robusta (kanuka), Leucopogon fasciculatus (mingimingi) and Leptecophylla juniperina subsp. juniperina (prickly mingimingi) – although variation within this population occurs along a continuous, subtly shifting spectrum. In the context of Batesian mimicry, none of these ‘model’ species are particularly prominent on the menus of predominating herbivores⁴.

Our interest in P. pimeleoides (of which there are 2 subspecies) was instigated by Oratia Native Plant Nursery and the former Auckland Council nursery at Kari St (where my friend, Paul Duffy, propagated it during his time there). In horticultural terms, it belongs to a particularly valuable category for the establishment of informal structure within plantings –  that of shrubs that maintain a compact, dense habit over time.

When one adds night-scented flowers, the fine texture of its foliage, and tolerance of drought & shade to this picture, its value becomes even more apparent. Over the last two decades, we have trialled four main forms of this subspecies, in addition to the lianoid subspecies from Hikurua/Surville Cliffs, P. pimeleoides subsp. majus.

In stark contrast to the wild variation exhibited within some populations of P. pimeleoides subsp. pimeleoides, plants that we have observed next to Whangaroa Harbour (which Terry Hatch advised us of) are relatively uniform in exhibiting a wide leaf and rounded habit.

The images above show another seemingly consistent form, from a population at the southern end of the species’ range (west of Whangārei). This is the closest wild variant to the form that we now favour within designs; on the basis that it retains a dense, rounded form over many years, as well as possessing a recessive tone to its foliage (similar to Leucopogon fasciculatus).

P. pimeleoides subsp. pimeleoides has a somewhat disjunct distribution throughout Northland, with the largest populations occurring near Bay of Islands. It primarily grows in coastal woodlands and kauri forest, particularly along ridgelines and other well-drained habitats. As one might expect, it is better suited to well-drained conditions in cultivation, whilst it is adaptable to both high light and shaded conditions.

Although it mainly flowers in winter, it can produce flowers at unexpected times of year. In common with other night-scented plants, P. pimeleoides subsp. pimeleoides is probably adapted largely to attract night-flying moths for its pollination.