Plant profiles  -  Collospermum

Family : Asteliaceae

Although common names should not be paid as much heed as their scientific counterparts, they are sometimes especially evocative. My favourite example is the widow maker, Collospermum hastatum - a name that takes us inside the world of the men that worked our forests in earlier times, and carries information about both the physical character of these plants and their position within the forest.

Before anybody gets spooked into continually looking upwards during bush walks, lest they get squashed by a nest of kahakaha, I should qualify the above description. The main reason that tree fellers were justifiably wary of these plants is that they were shaking and eventually destroying the trees in which Collospermum hastatum perches. For this reason, clumps of kahakaha could become dislodged (or break the branches on which they rest), and fall from a great height to kill a man. So unless you're planning on chopping down a forest giant, or taking a leisurely stroll in the forest during a hurricane, there's nothing to worry about.

The genus Collospermum comprises four species - two of which occur in (and are endemic to) New Zealand. The other two species are found naturally in the Pacific, one in Fiji and the other in Samoa. Prior to the 1930s, the species of Collospermum were understandably included within Astelia, as their appearance and growth habit is very similar.

Kahakaha are more specifically adapted to a life off the ground than their Astelia cousins, the most notable distinction being the rounded leaf bases of Collospermum - that effectively form tanks for the storage of water (similar to the way in which bromeliads store water in their leaf bases). Roots subsequently grow up into these water reservoirs, thereby providing them with an alternative supply of moisture from the usual sources that plants rely upon1.

Collospermum hastatum
Kahakaha; Widow maker

In her seminal book on gardening with native plants2, Muriel Fisher notes the way in which asteliads3 "give our bush such a tropical look". This observation is borne out particularly well in the case of Collospermum hastatum, which forms majestic nests of its wide, sword-shaped leaves in the canopies of our forest trees.

Although predominantly epiphytic, kahakaha is sometimes found growing on the ground - especially in rocky sites like Auckland's lavafields, the best extant example being Rangitoto Island. Such habitats provide similar conditions to the branches of trees; and as a result, epiphytes like Collospermum hastatum find opportunities within them.

C. hastatum is not commonly thought of as a garden plant, but it makes a striking species for both the forks of trees and extremely well-drained situations - such as at the base of trees and within crib walls. We have seen it used in the latter application, to great effect, at the Hortresearch grounds in Mt Albert, Auckland (in combination with another epiphytic/rupestral species, Xeronema callistemon). If one is to plant kahakaha in the ground, it is best to surround the base with stones that are dug into the ground, and place the plant in a slightly raised position.

The above photos show a kahakaha in flower, during autumn. I noted that the flowers were the subject of much interest by visiting honeybees, who were deriving nectar from the extended flower-spikes. This piqued my curiosity about which animals find sustenance from these large inflorescences and the subsequent fruits.

One fascinating creature for whom Collospermum is an important source of food (in the form of nectar, pollen and fruits) is the lesser short-tailed bat - an increasingly rare inhabitant of our forests that is only found within New Zealand. As may be expected, the fruits (which vary in colour from yellow through to red, depending on their stage of development) are also visited by native birds, including tuis and wood pigeons4.

Collospermum hastatum grows from the Three Kings Islands (off the northern tip of the North Island) down to the northern end of the South Island5. The specific epithet, 'hastatum', means spear-shaped; in reference to the anthers, I assume (not the leaves, as I had previously surmised - they are in fact sword-shaped)6. It generally grows to between 1 and 1.5m tall, and can form substantial clumps that are several square metres in area, over a long period of time.

 


Footnotes

  1. A very good account of the kahakaha's adaptation to epiphytic life is provided within John Dawson and Rob Lucas' book, 'The Nature of Plants : Habitats, Challenges and Adaptations' (2005. Nelson : Craig Potton Publishing).
  2. In 'Gardening with New Zealand Plants, Shrubs and Trees'. (Fisher, M.; Satchell, E.; & Watkins, J. 1975. Auckland : William Collins Publishers). Lawrie Metcalf makes a similar comment in 'The Cultivation of New Zealand Plants' (1993. Auckland : Godwit Press)..
  3. This term refers to plants similar to Astelia, and is a useful way of discussing our so-called 'perching lilies' collectively.
  4. I am indebted to an account by the Taranaki ornithologist, David Medway - on the website of Pukekura Park (www.pukekura.org.nz) - for the actual observation of birds feeding upon the fruits of kahakaha. He states that these are, in fact, a preferred food for these species, when they are available.
  5. As far south as Greymouth in the west, and near Kaikoura in the east.
  6. In Colenso's original description of the species (as Astelia hastata), he made special mention of the hastate anthers. The name is made doubly suitable by the often spear-shaped bracts, the whitish-silver leaf-like parts that accompany the flowerheads.