Hummock grasses in the genus Triodia dominate the vegetation over more than 20% of Australia, and occur in all states except Tasmania.. They are found on low-nutrient soils of sand plains and rocky low mountain ranges in the arid inland, but also occur on rocky outcrops along the coasts. Triodia species (which now include the species formerly classifed in Plectrachne) are also called "porcupine grass" or "spinifex" , but they should not be confused with grasses in the genus Spinifex which is found only in sandy habitats along coastal beaches.

There are presently 64 species described, all perennials. Of these, about 20 species are distributed widely across the continent (e.g. T. pungens, T. basedowii), or have very large local populations (e.g. T. schinzii). The remaining species are less common.

Hummock grasses have a number of striking characteristics: they typically grow as an expanding dome or dense hummock, with the green leaves on the outer surface. The inside of the hummock consists of densely matted stems (stolons) and dead leaves. When the leaves are young they are flat and relatively soft, but as they age the edges roll under and the leaf becomes very stiff and pointed.

Some growth forms are more open and resemble large tussock grasses, but nevertheless retain the rigid leaf form.   Generally the hummocks are less than a metre tall, but occasionally  reach sizes over 2 metres tall and 2 - 3 metres wide.  As the plants age the centre of the hummock may collapse, and rings are formed with dead material surrounded by live leaves.  In places where  it has been 25 or more years since the last fire spinifex rings are common.

Although they occur primarily in the semi-arid zones of Australia, spinifex species are well adapted to fire. Wildfires are started by lightning strikes and Aboriginal people have traditionally used fire in spinifex landscapes for a number of purposes, as do other contemporary rangeland and natural resource managers.   Many species of spinifex are extremely resinous, to the extent that resin may drip down  the stems and leaves on hot days,  and large residual lumps of resin often may be seen at the bases of hummocks which have burned.   The image on the right shows a resprouting Triodia pungens plant which was burned in a fire about 1 year previously.  The black lump of resin, against which the ruler is leaning, is about 15 cm high.

Following a fire, populations can regenerate either as seedlings arising from a soil seed bank, or from resprouts at the bases of burned hummocks, as seen in the previous image. Depending on the timing, frequency and intensity of any given fire, many species are able to use both fire-response strategies. Fires in the hummock grasslands tend to be patchy, and fire scars of different ages add to the subtle patterning of  inland arid landscapes.

For example, in many areas of central Australia, the species Triodia longiceps and T. pungens both grow on the slopes of low rocky hills. The patterns in the photo below reflect species distributions which could have been caused by variation in fire intensity or timing, possibly in conjunction with different post-fire growing conditions.
 

Here the gray-green vegetation on the lower slopes of the 2 hills on the left is Triodia longiceps and scattered Eucalyptus trees. At the centre, on the right-hand hill,  is a reddish-brown scar from a recent fire; there is no regrowth of hummock grasses as yet.  The brighter green vegetation on the lower-right slope of this hill is T. pungens. The flats in the foreground are dominated by T. pungens and Acacia shrubs. A third spinifex species, T. spicata, is found on the flat rocky hill tops.

The photo below was taken  about 18 months after a control burn on a research plot in the Flinders Ranges of South Australia.   The pair of large flowering stalks in the foreground belong to  Xanthorrhoea quadrangulata.   Xanthorrhoea species (Grass Trees) often flower very profusely  1 - 2 years after fire, although they may flower in inter-fire intervals as well.  Immediately behind the Grass Trees, Triodia bunicola has resprouted, but has not yet flowered and appears deceptively fluffy.  In the middle background are the lighter flowering tops of spinifex hummocks which were not burned.