Essays  >  Cultivating Northland's Threatened Plants

By David Green

One in thirteen of New Zealand's native plants are considered acutely threatened¹, and a significant number of these plants are found in the northern part of the North Island. Factors contributing to this include Northland's almost subtropical climate; many of the plants found in the warmth of Northland are not found in the wild elsewhere in New Zealand. The warm climate is also particularly suitable to the growth of weeds, which out-compete the native flora. The Northland region was also one of the very earliest areas to be colonised by Māori and by Europeans. I have a particular interest in the endangered plants of Northland because I am from Opua, a small town in the Bay Of Islands.

There are 2300 species of vascular plants in New Zealand and only about 60 are commonly used in gardens (with a considerable proportion of the landscape industry using less), so who looks after the rest of them - we do! At O2 Landscapes we try and explore the possibilities of the full range of New Zealand plants, and if an endangered plant is suitable for a job, then all the better. Not only do endangered plants fulfil the role of a usual garden plant, they provide the added excitement of breaking new ground and give the satisfaction of helping preserve New Zealand's native ecologies.

We use many rare and threatened plants from the Northland region in our gardens including Carmichaelia williamsii, Nestegis apetala (Coastal Maire), Pimelea tomentosa, Metrosideros bartlettii (Bartlett's Rata), Pseudopanax gilliesii, Hebe perbella (Bartlett's Hebe), Pittosporum kirkii, Hibiscus diversifolius, Pittosporum pimeleoides, Pittosporum michiei, Pratia physaloides (syn. Colensoa physaloides; commonly known to Maori as Koru), Pouteria costata (Tawapou), Streblus banksii (Ewekuri), Christella dentata and Pittosporum obcordatum (Heart-leaved Kohuhu). Some are described below.

Carmichaelia wiliiamsii

We use this as a structural shrub in gardens. It grows to 2m high and has an interesting weeping form. An extremely small number of these plants remain in the wild on the mainland. This number is on the decline with introduced animals such as sheep and cattle destroying Carmichaelia williamsii's natural habitat. As with many northern New Zealand natives, the stronghold of this species is the offshore islands off the northern coastline.

We find C. williamsii particularly useful for growing climbers through, such as Parsonsia heterophylla (scented native 'jasmine'). It is an important plant in the Wolfe garden at Hotwater Beach, where it is covered in flowers throughout spring. The cream flowers of C. wiliiamsii are the largest of any New Zealand broom.

Christella dentata

This attractive fern has a fine creeping form and is only found in 3 extremely confined locations around Kaitaia. It comes from a tropical family and is suited to native woodland and subtropical style gardens. C. dentata grows to 1m high and tolerates a variety of conditions. Creeping ferns are particularly useful as connective gap fillers within planting schemes. There are only about 50 plants known in the wild; yet surprisingly, this is one of the most robust fern species for use in northern New Zealand gardens.

Hebe perbella

This species, which is endemic to western parts of Northland (notably gumland areas of Ahipara), is our favourite species of Hebe, because of its particularly graceful form and racemes of white to lilac flowers. Traditionally, the species and varieties of Hebe that have been grown in northern New Zealand lack the more natural form that Hebe perbella possesses.

Many hebes that are commonly seen in New Zealand gardens are extensively bred and, in our opinion, have the appearance of a somewhat artificial generic-looking plant. Several little-known species, such as Hebe perbella (other notable species/varieties that we have trialled are H. 'Bald Knob Ridge' and H. stenophylla var. hesperia) exhibit the natural character that we prefer to see in garden plants.

H. perbella has only been described in the last 15 years which makes it a comparatively newly discovered species. Interestingly, it was initially found by J K Bartlett, who also discovered the critically endangered white-flowered tree rata, Metrosideros bartlettii. In the cultivation of H. perbella, it is important to provide good air movement and a well-drained position, such as the location of planted species in O2 landscapes gardens at Hotwater Beach and Mt Eden.

Hibiscus diversifolius

We use this plant primarily for its attractive flowers, which measure 5-9cm and are a pale to yellow lemon colour, with a dark heart. Hibiscus diversifolius is particularly useful in a sub-tropical style garden, where its sprawling form can be used to advantage, growing through other species. With only a dozen wild populations known, this Hibiscus is a rare but beautiful plant. Also of interest, when researching this plant I learnt that Hibiscus is the old Latin name for marshmallow.

Pittosporum pimeleoides ssp. pimeleoides (syn. P. pimeleoides)

This dwarf shrub is one of the most exciting plants that we have been trialling in recent projects. In the wild, populations continue to be destroyed, as their sites are cleared for farming. In late winter, Pittosporum pimeleoides is covered in scented flowers. P. pimeleoides has historically occurred in reserves in the Bay Of Islands.

P. pimeleoides succeeds in a wide variety of conditions (including exceptionally dry ground), and transforms from a slightly straggly small shrub (in its first 2 years) to a compact 1m high ball of delicate foliage. We refer to this variety and the following variety by their previous names (as distinct species). Although that is technically incorrect, they are still referred to by the old species names in the trade, and we prefer to perpetuate the usage of Ross Michie's name for the following species. Ross Michie was a Northland personality, with a well-known garden (particularly for its Kiwiana style) in Kaitaia, who was also a talented plantsperson (interested in conservation in a period when that barely registered in the public interest).

Pittosporum pimeleoides ssp. maius (syn. P. michiei)

Closely related to Pittosporum pimeleoides, this variety is also easy to grow and flowers well. Pittosporum michiei is known to exist in the wild only in the North Cape area and less than 1000 plants are known to survive. The geology of the area of the North Cape (particularly the Surville Cliffs) is unusual, as it consists of serpentine rock, an extremely toxic substrate. Unique plants have evolved on this serpentine area which can cope with the toxicity of the soil and the consistent battering from the coastal weather conditions. The plants of the Surville Cliffs are often dwarf in form, compared to related species in other areas. This is also true with P. michiei, which is like a lower growing, sprawling Pittosporum pimeleoides. These species also differ in that P. michiei has wider rounder leaves. Pukekohe plantsman, Terry Hatch, informs us that one significant population (containing some of the largest plants he has ever seen of the form) was destroyed by roadside maintenance, a reminder of the commonplace risks that some plants face in the wild.

Pittosporum obcordatum

P. obcordatum suffered almost complete depletion in its Northland habitat, which was destroyed by the clearance and farming of land and the draining of swamps. The juvenile form of P. obcordatum differs from its adult form. It is a heteroblastic tree, which means it starts its life as a non-divaricating seedling and once it reaches 50cm tall, P. obcordatum changes into a slender divaricating tree. The largest existing population within Northland is found in degraded swampland west of Whangarei. The small patch of riverine scrub is a remnant of the once vast Hikurangi swamp, an area that has been almost entirely converted to farmland. This type of forest/scrub is characterised by a large proportion of divaricate shrubs and trees, several of which grow in the columnar form typical of P. obcordatum. They form shadowy spires which we find useful as an 'architectural' plant form. P. obcordatum bears massive numbers of flowers that are strongly fragrant in the evening.

Note: For more information on the cultivation of little-known and gardenworthy species of Pittosporum, have a look at the article by Philip in the NZ Garden Journal ('Pittosporum; the misunderstood genus') - in the "Publications" section of the website - , in which the three species given above are also desribed.

Pseudopanax gilliesii

We have used this shrub in several garden designs and it has shown excellent versatility. This is a dwarf Pseudopanax which makes a perfect sized shrub for urban gardens. Horticulturists are constantly searching for shrubs in this size range and often experiment with numerous exotics, which to us seems more than a little crazy when they have Pseudopanax gilliesii on their doorstep (and could be using it to help protect one of our native endangered species). Plants of this species can be sourced from Oratia Native Plant Nursery in Auckland.

Footnotes

1. C. 190 of New Zealand's plant species are ranked as acutely threatened, whilst a further c. 650 species are considered to be 'At Risk'. This means that more than a third of our native plants are either acutely threatened or 'At Risk'.