Spinifex and spinifex – Background briefing for travellers in Australia

Spinifex and Spinifex

P1020087 (2)
Hummocks of Triodia sp. on the sand dune above the tidal lake, East Vansittart Bay, Kimberley, W.A.
Most Travellers in Australia encounter a new range of plants once they move out of their own familiar surroundings. This article has drawn together bits and pieces of information about Spinifex and Spinifex plant groups in the hope that some of the misunderstandings and complexity about Australian plant diversity is simplified.

Spinifex and Spinifex

Spinifex may be a confusing name since Spinifex is the generic name for example of the hairy and the long leafed spinifex, a grass that trails over the dunes behind the beach. Spinifex (not including italics) is the common name for a group of porcupine grasses that often grow as hummocks.

Spinifex-the genus

In southern Australia hairy spinifex, Spinifex hirsutus predominates and in the north a long-leafed species, S. longifolius is common in sandy beach environments.

Interestingly these Spinifex species have both male and female plants. They look to be the same except at flowering when the male inflorescence is smaller than that of the female. At maturity the female plants produce a ball-like structure about the size of a baseball that once mature breaks away from the parent and may be rolled around by the wind so spreading the seeds held with it. Spinifex produces long stoloniferous shoots that spread over the sand with roots developing from some of the nodes (swelling along the stem). In total these sand-binding plants tend to stabilise the windblown sand of the dunes.

Spinifex longifolius from hind beach on mainland, west of Winyalkan Island, Kimberley WA.
Note the many shoots arising from a horizontal stolon/rhizome that spreads the plant
and sends down roots that help stabilise sand movement.

Male flowers of Spinifex longifolius
Female flowers head of Spinifex longifolius

Spinifex -the common name

Spinifex is used as the common name for arid and semi-arid plants mostly in the genus Triodia along with a few in the closely related genus Plectrachne. In central Australia there are ten Triodia species and four in Plectrachne but when we include all Australian habitats we find there are 35 species of Triodia and 10 Plectrachne for this group of endemic species.

As a moisture conserving adaptation, several species of Triodia roll up along the midline of their upright leaves. These become stiff and the folding protects the surface of the leaf bearing the leaf pores, the stomates. The ends of these leaves may be quite pointed and sharp and were complained about by many of the inland explorers who were not ready for this assault on their legs. It is this characteristic that has led to suggestions that ‘porcupine grass’ may be a more apt as a common name than ‘spinifex’. With some exceptions this type of leaf is not attractive to most herbivores and their spiny nature offers protection to small mammals that live under the plant. Much of the leaf stiffness is due to the presence of silica either incorporated in the walls of some of the epidermal (surface) cells or as cells with microscopic concentrations of silica referred to as silica bodies. It is possible to find small plants amongst the larger plants that are constantly grazed by animals such as the short-eared rock wallaby. It would appear that these animals are nibbling on young leaves and not the older (tougher?) leaves.

(L) Hummock of T. microstachya and (R) close up of their flower stems
and the short pointy green leaf base. Wary Bay, Bigge Island, Kimberley WA.

A well grazed hummock of Triodia sp. Note the many tell-tale scats
of the short-eared rock wallaby. Mainland west of Winyalkan Island, Kimberley WA.

Many of these species form hummocks (i.e. little rounded hills) as they grow rather than tussocks, or runners, a more common growth pattern in other grass species. In the arid zone the hummock grasses (mainly Triodia spp.) make up 31% of the area. Of course they are not limited to arid zone and as seen in the examples used in this article Triodia species seem well adapted to parts of the wet and dry tropics in northern Australia especially growing in well drained sandy and rocky habitats.

As the plants age growth continues outward from the hummock base and eventually the centre of the plants dies. It is this phenomenon that leads to the production of circular plants with growth at their perimeter but the centre of the ring has the dead remnants of earlier growth. The rings often with complete perimeters may reach 10s of metres across.

Hummocks of Triodia sp. Gawler Ranges National Park

Rings of Triodia sp. hummocks, (L) roadside north of Karoonda, SA.
(R) Sand plain Wary Bay, Bigge Island, Kimberley WA.

Central Australian Aboriginal quartz scraper with a Triodia sp. resin ‘handle’.
(Scale Aus$1 coin.)

Several of the porcupine grass species produce a gummy resin on their leaves which may be felt and smelt. In these species the gum accumulates as a solid mass at the base of the leaves over time and becomes particularly obvious after the plants have been burnt. This material is a mixture if the gum and pieces of the stem and is collected by Aboriginal people and used the attach tips to spears, to form the smooth section of a stone knife and scrapers and for example to patch holes in a coolamon, the multipurpose carrying vessel.

The value of this material is it has thermosetting properties and is soft and pliable when heated up but hard at ambient temperatures. The resinous leaves make clumps of this species quite flammable and when burnt it issues a black smoke and is reported as an Aboriginal signalling device.

An interesting find

While most of the porcupine grass on Bigge Island on the Kimberley coast WA grows on the sandy sections, I have found one area between the sandstone rocks where the Triodia sp. plants are not rounded hummocks of porcupine grass but chewed off plants with most green shoots less than 10cm high. The tips of fresh growth are likely to be grazed by Monjon (Petrogale burbidgei) a small rock wallaby that lives on the island. Over the rock and bare sand there are also tunnels constructed by spinifex ants (Ochetellus flavipes) that use the resin produced by the grass to stick the sand together. The tunnel runways also extend to the plants many of which have cylinders of sand grains, also constructed by the ants, attached to their stems. Within these cylinders that the ants tend sap-sucking mealybugs that produce honey dew that is then consumed by the ants. This mutualistic relationship between the ant and the bug is limited to the fresh shoots of the stunted Triodia. But there are other interesting relationships to consider including the herbivorous consumer Monjon and the producer Triodia, the constructing and consuming spinifex ants and the mealybugs that parasitise the plant!

Five grazed Triodia sp. plants and the spinifex ant tunnel runways seen to the left
of the sandal. Rock platform north side of Wary Bay, Bigge Island, Kimberley WA
Grazed off tips of the Triodia sp.with distinguisable mealybug sand-cylinders mid-top. Rock platform north side of Wary Bay, Bigge Island, Kimberley WA.

Acacia – Background briefing for travellers in Australia


Golden Wattle, Australia's National Floral Symbol
(Source: Wikipedia Commons)

Most Travellers in Australia encounter a new range of plants once they move out of their own familiar surroundings. This article has drawn together bits and pieces of information about Acacia/Wattle plant groups in the hope that some of the misunderstandings and complexity about Australian plant diversity is simplified.

Acacia is Australia’s largest plant genus and the 1000-odd Acacia species are referred to as wattles. The golden wattle (A. pycnantha) is the Australian National Flower and Australia wide, Acacia’s run second to Eucalypts as having the most forest and woodland trees. In the arid zone Acacia shrublands make up 33% of the area.

When not flowering many Acacia species are distinguished by their leaf shape. Some species such as Cootamundra wattle, Acacia baileyana, retain the true leaf shape that is evident in the seedling stage. These species do not have a single entire leaf blade (the lamina) like an apple tree, but a compound bipinnate one where the lamina is twice divided into small leaflets call pinnules. This unit is attached to the branches with a stalk called the petiole


The bipinnate leaf of Cootamundra wattle, Acacia baileyana.

The bipinnate leaf of Cootamundra wattle, Acacia baileyana.
However, for the majority of Acacia species it is the leaf stalk (the petiole) that develops without the formation of a leaf blade and this serves as the main photosynthesising part of the plant. The flattened petioles are called phyllodes, they can be of many shapes and sizes and the majority have what are referred to as nerves (not veins) running along them. Most species have between one and five prominent nerves on their phyllodes but mulga, A. anura as suggested by the Latinised specific epithet anura (’a’ not; ’neura’ nerve), has thin phyllodes without a prominent nerves.

Well chewed large phyllodes of candelabra wattle (Acacia holosericea), east Kimberley, WA. Note the four prominent nerves and the blue grey colour typical for this species.

In their seeding stage most phyllode bearing wattles develop small bipinnate leaves at the tip of a broad stalk, the petiole. However in subsequent development only of the flattened petioles, then known as phyllodes are evident without a bipinnate lamina on all future growth of the foliage

Occasionally leaves and phyllodes may have spines on the stem located below leaves or phyllodes. In fact, the generic name Acacia is Latin is the Greek word for ‘thorn’. This arose given it was a characteristic of the Type plant, Acacia niloctic, the prickly acacia from Africa was the first species to be scientifically described. This is not a characteristic feature of most Australian endemic species, but A. victoriae, the bramble acacia, common in arid and semi-arid habitats, is one endemic thorny species and is often used by birds, lizards and small mammals for protection from predators.

It is believed that phyllodes are an adaptation to dry conditions where phyllodes function more efficiently than leaves with respect to moisture use via transpiration. The upright phyllodes of mulga, another wattle from arid and semi-arid habitats is an adaptation that reduces the midday radiation exposure and could therefore reduce heat stress and loss of water via transpiration. This upright arrangement of phyllodes also serves to channel rainwater from it canopy towards the trunk where it runs to the base of the plant. One calculation assessing the efficiency of this arrangement suggests this increases usable water to the roots three-fold.

Acacia seedlings and the change with age from bipinnate leaves
with thin phyllodes and their replacement to broad phyllodes

And finally, two other adaptations to arid conditions: mulga has a very deep root system and plants under stress from drought conditions will begin to drop their phyllodes.
Many areas of inland Australia once covered by mulga are now bare of the species with only a scattering of old dead trunks to indicate their past presence. One of the main contributors to this situation is related to their seedlings being vulnerable to the high grazing pressure often inflicted by sheep or rabbits. This factor along with a fairly precise sequence of climatic events necessary for seeding establishment has led to the general demise of mulga.
Wattles are found in most habitats of Australia from alpine heights, the edges of rainforests and across the broad reaches of semi-arid and arid country. Brigalow (A. harpophylla), grew in extensive communities in inland south eastern Queensland and northern New South Wales but due to the favourable soil and rainfall regimes in which it grew the area has been extensively cleared in favour of broad-acre farming cropping.
Many wattle species and varieties have been selected as suitable garden plants and while gardeners generally appreciate their rapid early growth they are disappointed that most species have lifespans less than two decades. This feature contrasts with mulga (A. aneura), a species that has been reported, in the absence of fire, as having a 200 to 300year lifespan. Another of the many drought tolerant Acacias is A. peuce, Birdsville wattle or waddi wood. A small copse of this endangered species found just north of Birdsville in arid southern western Queensland. Like mulga waddi wood may live up to 200 years.
As noted above mulga a very widespread with its densest distribution on nutrient poor soils in arid habitats. Through natural selection at least nine varieties of this species are recognised. Comparative studies on these varieties are likely to demonstrate adaptive differences that are related to the specific habitats conditions in which they naturally grow. A general characteristic of varieties within a species is that they are capable of successfully interbreeding. However, with A.aneura some taxonomists are suggesting that several varieties are so different from the typical mulga that these varieties should be considered as distinct (new) species in their own right.

A lone mulga shrub growing in the stony capping rock, at Kanki – Breakways Conservation Park,
north of Coober Pedy. One way to recognise Acacia aneura is the fact that
its phyllodes are erect (point upwards), a feature visible in the image.

We usually recognise wattle by their fluffy yellow blossoms. Unlike many other plants that gain their floral colour from their petals, wattle flowers have tiny petals and their colour is via their masses of stamens that are in most species yellow. The individual flowers are very small and masses of them are arranged either as spheres or as a cylinder. After successful pollination some of the flowers will develop pods containing seed, pods that hang from the matured flower before splitting open to release its seed.

(L)An Acacia species with cylindrical arrangement of the flowers. Wilson’s Headland, N. Wooli in Yuraygir National Park, NSW
(R) A. aneura flowering with many mostly unopened spherical bundles of buds. Coober Pedy, SA.

All wattles fall within a group that have legume like characteristics. Not only do they have pod that splits open when dry to disperse seeds, but they also develop rhizomes on their roots. Specific soil bacteria, Rhizobium, cause legumes to form these rhizome swellings on their roots that are capable of converting nitrogen in the soil air into chemically fixed nitrogen rich compounds within the rhizome. This nitrogen source benefits the plant given that the soils they often grow in are nitrogen poor, such as hind dune sand on beaches, and that these compounds are essential for protein production. This feature enables wattles to colonise nutrient depauperate habitats like the lighter textured sandy soils in the arid area. When the plant dies or the root ages, the rhizome and the protein rich plant parts break down releasing the nitrogen compounds into the soil and of course the seed of legumes including beans, peas and other lentils tend to have high protein levels. It is not surprising that the seed of several wattles are collected, and ground to a flour and prepared as one of the seasonal diet components of many Australian Indigenous clans.

Pod of wattle filled with seed. Pods average 8 cm long and the longest phyllode is 16 cm long.