05 January 2012

Spinifex hirsutus - Hairy Spinifex

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Spinifex hirsutus - Hairy Spinifex

The Spinifex genus is a small but important group of grasses belonging to the Poaceae grass family. However when the name spinifex is mentioned, most think of the clumping spiky grasses from arid regions, but these generally inland grasses belong to the Triodia genus and spinifex is only their ‘common (unofficial) name.’ Whereas with these coastal grasses, Spinifex it is both their ‘common name’ and ‘genus’ (official botanical name).

Spinifex hirsutus is distributed from the Perth region, then along the south coast into SA, where scattered to Port Lincoln on the Eyre Peninsula, although odd populations are known further east, these have probably been introduced. Just across the Spencer Gulf to the Yorke Peninsula is a very similar looking grass, Spinifex sericeus that performs the same function of stabilising coastal dunes. This species begins its distribution from here and spreads further east around the coast to North Queensland and over Bass Strait to Tasmania. It is also found in New Caledonia and New Zealand. This species was until recently also known as Spinifex hirsutus, so differences between them are not obvious to casual observation, but relate to leaf and stem divergences. Spinifex sericeus is known from the odd collection in the Perth and SW region, but these are regarded as introduced, as this species does not naturally occur in WA.

Although male flowers can sometimes be found at the base of predominantly female flower heads, most heads are either male or female as shown above. In order to produce seed, pollen from the male flowers is blown by the wind to hopefully make contact with the often more numerous female flower heads (although there are fewer individual flowers). After the seed has matured, the flower heads die and dry to a straw colour, they then become detached from the stem at a joint below the head that has evolved for this purpose.

Each centrally placed female flower is subtended by a bract with a long stout bristle to 10 cm (4”) long, so each head resembles a ball of around 20 cm (8”) or more diameter. These are strong, but light and when propelled by coastal winds can travel at great speed and distance along the beach. If they end up in the water, they eventually get washed up somewhere else and possibly colonise there, but otherwise they are blown, trapped and buried by the sand around the high water mark, or on primary dunes. The seed can now germinate to permit a new spinifex generation to take hold. When you look at the very harsh sandblasted and salt-laden environments that these plants call home, you wonder how they survive, but these are just the conditions they like in preference to more sheltered and stable locations.

The very soft, silver-haired leaves (to 40 cm or 16” in length) and stems rely on being buried by the wind-blown sand, as they can then develop roots at their nodes that will not only help anchor them, but provide a new position for advancement. Flowering is recorded anytime from October to January and well worth a look.