Xanthorrhoeas; the wonderful grass-trees

Well actually they're not grasses, and only ambiguously trees, but Xanthorrhoea is never going to catch on as a common name, and the old name of 'blackboy' (for the supposed resemblance of the silhouette to an indigenous man holding a spear) has no place in today's world. I grew up in South Australia calling them Yaccas, and Balga is often used in the south-west of the country, but these are purely regional and so we seem to be left with grass-tree for now at least. It is evocative though.

Large (and very beautiful) X. glauca along the Mount Kiangarow Track,
Bunya Mountains NP, Queensland.
In general the species are not distinguished at common name level, though some are, at least locally.

So, what are these plants that must look pretty strange to eyes that didn't grow up with them? For a start they're monocots, and indeed fall within the broad 'lily' grouping. (For more on lilies, see here and work forward from there.) All 30-odd species are Australian, and all belong to the genus Xanthorrhoea. Traditionally they formed their own family (though if we go far enough back we'll find they've had a convoluted taxonomic history) but modern thinking would include the aloe family Asphodelicaceae and bigger family Hemerocallidaceae (in Australia that incorporates flax lilies, rush lilies, grass lilies etc) as sub-families within Xanthorrhoeaceae.

It is a common misconception that grass-trees are an ancient group but it seems in fact that they are a recent rapidly-evolving genus.

The apparently woody stems possessed by most species are in fact hollow, though contain a fibrous matrix; the meristem, which grows outwards as well as upwards, is surrounded by the woody bases of old leaves which have dropped off as they become shaded out by newer ones above - the characteristic hanging 'skirt' of dead leaves can be seen in the photo above.

These Desert Grass-trees X. thorntonii on the Mereeni Loop Road in central Australia
exhibit the typical rough grass-tree stem, formed by the ends of the leaf bases.
This is the only grass-tree that grows in the central deserts.

An impressive 'two-headed' X. glauca in the Warrumbungles NP, NSW.
This species seems particularly inclined towards multiple heads.

Cross-section of grass-tree stem.
Note that in this old stem the inner fibrous material has rotted away - the trunk isn't truly hollow.
See Gregg Muller's helpful comment below on this aspect.

The flower spike, comprising thousands of tiny lily-like flowers, may be several metres high. Flowering does not rely on fire, but is stimulated by it. However it doesn't flower immediately, but in the winter-spring after a summer fire; if the fire comes later in the year the mass flowering may be delayed until the second winter.

X. semiplana flowers Wanilla CP, Eyre Peninsula, South Australia.

  

X. resinifera flowers, Currarong, Jervis Bay, NSW.

Unburnt X. glauca flower spike growing by road in Goobang NP, New South Wales.

Unburnt X. fulva flower spikes, Wyrrabalong NP, Central Coast NSW.

X. johnsonii flowering on a steep volcanic hillside, Mount Kaputar NP, NSW.

X. platyphylla flowering in a burnt landscape, Fitzgerald River NP, Western Australia.
Each leaf base protects a growing bud, which starts growing immediately the living leaf is burnt off.

Xanthorrhoeas flowering in June, some five months after landscape-scale fire in 2020,
Jerrawangala NP near Nowra, NSW.

Flower spikes in October of the same year in Blackheath, Blue Mountains,
following huge conflagrations which burnt most of the Blue Mountains.

'Finished' flowering spikes at Currarong, Jervis Bay NSW, nearly two years after
a 'control' fire ceased to be controlled, immediately behind the village. On the morning
I took this I saw many flocks of Yellow-tailed Black-Cockatoos converging here
to feed on the seeds on the spikes.

These flower spikes provide a huge nectar resource when flowering and an unusually wide range of pollinators visit them, from gliders, bats and other mammals at night, to insects, honeyeaters and lorikeets during the day.

Hoverfly Family Syrphidae revelling in the abundant nectar of X. glauca, Goobang NP.

Only a few of the numerous flowers will be successfully pollinated, but this still produces a large crop of hard seeds when the flower spike dries out. 

Seed cases - many of them opened - on  X. semiplana, Wanilla CP, South Australia.

To Aboriginal people, grass-trees were of immense significance. The powdery resin was used for gluing tool heads to handles and as such was an important trade item. The same resin was regarded as an important medicine, and as a lacquer for smoothing and sealing surfaces. Pieces of the flower stalk were rubbed to make fire, and the resin was very flammable, so dried leaves made excellent kindling. The stalks were used as light spear shafts. Grass-trees were also a very diverse food source; edible grubs were found at the base of the plant, the base of the young inner leaves was eaten raw or cooked, the seeds crushed to make flour, and honey was extracted from the flower spike by drawing leaves up the stem, or by soaking it in water for a sweet drink. The fibrous leaves were woven into shelters.

X. glauca overlooking the dry plains of inland south-east Queensland, Bunya Mountains NP.

Europeans processed the resin to make medicines (including, I am intrigued to read, for both diarrhoea and constipation!), perfumes, varnish and explosives; it is said that Germany imported a lot for the latter purpose (based on the picric acid it contains) prior to the first World War. The fibrous trunks were used as brake blocks for steel wagon tyres.

Ancient X. glauca, Bunya Mountains NP.

In case it wasn't clear, I just love grass-trees. They are, to me, quintessentially Australian; however I am delighted, even anxious, to share them with you wherever you're reading this. I hope you can enjoy them too, though there's no substitute for meeting them on their home ground.

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