Pomaderris

The members of this genus bear some of the most conspicuous flowering displays in the New Zealand flora. These range from the early spring appearance of kumarahou’s large, albeit short-lived, yellow flowerheads, to the abundantly-borne frothy cream inflorescences of species like tauhinu (Pomaderris amoena) and P. rugosa. They are plants of open ground, often growing in impoverished soil – such as is found in scrub conditions or cliff habitats. It is, therefore, easy to understand one of their requirements for successful cultivation – Pomaderris spp. do not respond well to the application of fertiliser.

Pomaderris is an exclusively Australasian genus of around forty species, most of which are confined to Australia. Of the seven New Zealand species, four are endemic to our country – including the tainui (P. apetala ssp. maritima), which is pictured below.

Pomaderris is part of the widespread buckthorn family (Rhamnaceae). It is interesting to note that the only other New Zealand plant from this family is the matagouri (Discaria toumatou), a shrub of drier areas of New Zealand (especially colder regions). This thorny character bears little resemblance to the species of Pomaderris, but shares a preference for open habitats like grassland and scrub.

Many of these types of open ecologies fit into the description of seral communities – that is, communities of plants that are ‘intermediate’ in nature, occupying a place between bare ground and the final vegetation types (like forest) that will cover a given area. Although essentially transitional, seral ecologies are not always short-lived, as nature works on cycles that don’t necessarily relate to our own lifecycles. Two examples are the kinds of tussockland or shrubland that are found in the dry east of the South Island. Neither are ‘climax’ communities (on the broad scale that they occur), but it would take hundreds of years (or longer) for many of these areas to return to their climax vegetation type¹.

Even without human disturbance (which opens up landscapes in unnatural ways), seral ecologies are part of the matrix of habitats that form natural landscapes. They occur in permanently open habitats (like rocky ground and cliffs), and in places where storm events have opened up the landscape. When one considers the place of Pomaderris in these habitats, one can understand their potential role in gardens better.

This attractive little shrub adorns many of our roadside banks, where its slightly cascading mounds of fine foliage are covered in sprays of creamy flowers as spring turns to summer. They are well adapted to the difficult conditions of these places, including the regular trimming applied by tractors to keep vegetation back from the road. One of the best situations in which I have viewed this species was on the open road to Port Waikato, where the unthinking path of the tractor trimmer only served to encourage dense growth of P. amoena and fellow native opportunists.

P. amoena is also found in more natural associations, such as dry shrubland of the kind found on the east coast of the North Island. In addition to the cream flowers, the leaves are often a highly attractive bright green colour. Due to its tolerance of harsh conditions, it is a useful species for plantings in difficult positions, such as steep clay banks or infertile ground.

As stated above in the main description of Pomaderris, this plant does not respond well to fertiliser (although it shows a greater tolerance to it than other species like P. kumeraho and P. rugosa). This preference for infertile conditions can, in fact, be turned to good advantage in the weed-prone environments of northern New Zealand. By eliminating the need for fertiliser in a given area, one decreases its suitability for a number of weed species (thereby reducing the range of weeds that may occupy it).

Tauhinu (its common name in Māori) is a somewhat confusing title, as it is also applied to members of the genus, Ozothamnus, that are commonly found in coastal and shrubland habitats. This has evolved out of the similarities between the species concerned (despite their botanical distinctness). Another example is the use of the name mingimingi for a considerable number of filiramulate shrubs that bear a likeness to each other.

The tainui is a botanical and cultural enigma. Critically endangered in its two remaining wild habitats (on the border between the King Country and Taranaki), it has naturalised in other parts of the country (thereby taking on the rank of a native ‘weed’ within some contexts). Despite its success in other areas, it is extremely important to maintain its survival in its true home, on the coastal fringe from Waikato to north Taranaki.

Meanwhile, in cultural terms, it has elevated status within Tainui tradition – as legend states that it was brought to New Zealand as canoe flooring/skids on the Tainui canoe. The veracity of this is highly doubtful, due to the fact that the genus Pomaderris is endemic to New Zealand and Australia. However, that does not decrease the cultural importance of the plant; as it may well have played a role as canoe skids along the western coastline, when early Maori arrived (and recognised its suitability to this purpose).

When I visited one of the extant populations (near Mohakatino), I was fascinated by the growth habit exhibited by many of the large specimens that clothed the slope. The trees formed an ecology that looked like a coppiced woodland (as pictured above); a cultural vegetation type that has been grown for centuries within Europe to produce large amounts of readily available timber for different purposes (such as firewood).

Coppicing involves chopping trees back to their base, from which many new stems arise – creating an aesthetic in which one looks through a mass of many smaller trunks. Tainui achieves this in a natural manner, as the larger trunks periodically fall over and many smaller suckering trunks emerge from the base. The overall impression is intriguingly ambiguous; a natural plant community that looks like a managed woodland.

I imagine that this growth habit is the reason that tainui’s Australian relative, Pomaderris aspera, received its common name Hazel Pomaderris (and that this species is sometimes referred to as New Zealand Hazel) – for hazel is a typical coppicing species in Europe. The similar P. aspera has previously been confused with tainui, to the extent that P. aspera has often been mistakenly sold as tainui in New Zealand (a situation that still sadly occurs at times).

Tainui is a co-dominant member of the canopy in the windshorn scrub at both of the natural populations. The trees achieve a large size (exceeding 5m high and 8m wide), and many of the specimens on upper parts of the slope exhibited flattened tops where the action of blasting salt winds places a natural check on plant growth.

Upon my visit, I was disappointed to see a group of goats freely moving around the edges of this remnant, which is not fenced. Goats have been implicated in tainui’s disappearance from Kawhia (where it naturally grew), and one would hope that a species now confined to just 2 sites (and of great cultural significance) should receive protection from such a destructive animal. I noticed scratching on the trunks of some of the trees on the lower part of the slope, which I strongly suspect to be from goats.

Few native trees withstand exposed coastal conditions as well as tainui. Due to its rapid growth rate and bushy appearance, it is well suited as a hedge for difficult sites – a purpose for which we have specified it. The leaves and stems bear an attractive brown tinge, whose effect is amplified when it is planted as a hedge. It is also particularly appropriate as a shelter species (for the western Waikato and north Taranaki), an application in which its attractive cream flowers (which appear over summer) are much more likely to be appreciated.

Due to the floral exertions of certain native plants and some weedy species (notably gorse), early spring is a very yellow affair in the north of New Zealand. While most attention is directed towards kowhai’s golden blooms at this time of year, another spectacular native flower, kumarahou, does its best to brighten some of our most unpromising habitats.

Kumarahou is most closely associated with scrub and exposed sites on infertile clay soils, from the western Waikato and Bay of Plenty north (up to the tip of the North Island). One of kumarahou’s best flowering displays within Auckland happens at Waikumete Cemetery, where considerable tracts of remnant scrub contain a significant populations of many native plants (including a wide array of orchids, Dianella haematica² and two regionally rare grasses).

It received the common name, ‘Gumdigger’s Soap’, from early European settlers, who sometimes used its flowers (mixed with water) as soap. It has also traditionally been employed (by Māori, and subsequently by European New Zealanders) as a remedy for various ailments, notably chest complaints and coughs³.

In cultivation, kumarahou is a relatively short-lived shrub (between four and seven years is a normal lifespan) that typically grows to a height of 1.8 to 3m. It is especially useful for providing early cover on barren clay sites (where most other plant species struggle). It is by no means dependent on clay soils, and is able to be cultivated in ordinary garden soils, provided they are well-drained and not too rich. With regard to this point, it is worth reiterating that, like other Pomaderris spp., kumarahou does not respond well to fertiliser.

Rusty brown is an enigmatic colour on foliage, as it seems to belong more to the mineral world. Visually, it can serve to ‘ground’ plants back into the soil and rocks that they occupy, especially where a plant emerges from denuded or harsh habitats (as Pomaderris rugosa does).

As such, P. rugosa seems to truly belong to the clay and weathered rock on which it grows at Kawakawa Bay and eastern Coromandel, where I have encountered this plant in the wild. The rusty colouration is most apparent on the underside of the leaves⁴, whilst the upper leaf surface bears a dull, deep green hue. As a result, flashes of both colours alternate on the shifting canopy of the shrubs, when they are shaken by the wind.

Another distinctive feature of Pomaderris rugosa is its tiered growth habit, which gives well-grown specimens a rather ordered look. The spaces between each tier of branches also lend this species a lightweight appearance, in contrast with species with a denser structure.

P. rugosa is easy to cultivate, on condition that it is planted in well-drained soil and does not receive any fertiliser (as is the case with all of our species of Pomaderris). It is especially useful on impoverished clay (where the range of species that will succeed is limited) and where extreme drought is experienced. Within cultivation, P. rugosa normally reaches between 1.8 and 2.4m in height (and around 1.6m wide), although it is capable of growing to 3m tall in the wild.

It is naturally found in the Coromandel Peninsula, the Firth of Thames (including Pakihi, Rotoroa and Ponui Islands), and the western Waikato (Aotea and Kawhia Harbours). There are also a couple of isolated occurrences north of Auckland (near Silverdale and at Herekino), although some botanists suspect that these may have originated from deliberate plantings. Pomaderris rugosa is an interesting example of a native species that is well adapted to growing within highly modified environments, such as pine forests and roadside cuttings.