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Plant Highlight: Xanthorrhoea quadrangulata
by Brian Kemble
About the Genus
Xanthorrhoea is an Australian genus of monocots, with 28 species recognized in Flora of Australia (1986). They are often referred to as grass trees, with the “grass” referring to their tufts of very narrow needle-like leaves, and the “tree” referring to the fact that some of them eventually develop impressive trunks, though not all of the species do this. Even the species that form trunks do so gradually, so that the tall plants seen in some habitats may be many decades old, or even centuries old. The genus is now placed in the subfamily Xanthorrhoeoideae within the Asphodelaceae family, though in older references it was the type genus of the family Xanthorrhoeaceae. The various species occur across Australia, with some coming from the winter-rainfall regions in the country’s south and west, while others come from the summer-rainfall regions in the east and north. One of those occurring in a winter-rainfall area is Xanthorrhoea quadrangulata, endemic to the state of South Australia, where it occurs in the vicinity of Adelaide and northward as far as the Flinders Ranges and the Gammon Ranges. This part of eastern South Australia is at the eastern end of the country’s Mediterranean climate zone.
About the Plant
Without an inflorescence present, a young plant of X. quadrangulata is a clump of radiating ultra-narrow blue-green leaves; these are consistently more glaucous-bluish than is usual in the genus (a trait shared with the aptly-named Xanthorrhoea glauca, though the latter is a larger-headed plant). The leaf length is listed as up to 3 feet (90 cm), and our plants at the Ruth Bancroft Garden have leaves measuring from 27 to 34 inches (69 to 86 cm). An important character distinguishing one Xanthorrhoea species from another is the shape of a leaf cross-section, and those of X. quadrangulata are diamond-shaped: four-sided but a little bit vertically compressed, so that the top-to-bottom measurement (the thickness) is less than the side-to-side measurement (the width). On average, the thickness is about 0.04 to 0.06 inches (1 to 1.5 mm), while the width is about 0.07 inches (1.8 mm). While the leaves radiate relatively stiffly, they are not as rigid as some similar-looking needle-leaved plants, such as Yucca linearifolia; they sometimes are slightly arching, and they do not poke you if you brush up against the plant. Our oldest plant at RBG first flowered 27 years ago, and in all this time it has not developed a noticeable trunk. It has divided to become 2-headed, and plants that branch are not uncommon in habitat, though others develop a considerable trunk without ever branching. Older plants can have a trunk up to 6½ feet tall (2 m).
About the Flowers
The flowering period for the species is listed in Flora of South Australia as March to August, which is fall to winter in the Southern Hemisphere (corresponding to September to February in the Northern Hemisphere). Our plants at the Ruth Bancroft Garden have generally been on the early end of this schedule, flowering mainly in August and September, and ending by mid-October or earlier, though one individual was later, coming into flower in November. The inflorescences are dramatic, rising like exclamation points from amongst the leaves. They have a basal peduncle, or shaft, and a slightly wider upper portion, where the flowers emerge. The upper portion is like a carpet of narrow dark-green bracts called packing bracts, and the flowers emerge from this carpet. The shaft is initially a very pale milky blue-green, contrasting with the dark color of the flowering portion above. Because the flowers are small and short, they are mostly hidden among the packing bracts, but when they open, their six radiating stamens elongate and are very visible. The effect is like little white asterisks sprouting from a sea of astro-turf. The white filaments are widest at the base, then narrowing toward the tip where the yellow anthers are perched. The pistil is initially quite short and inconspicuous, but later it elongates and is ready to receive pollen from visiting insects. The flower length is about 0.375 inches, or 9.5 mm. At RBG, the flowers are consistently visited by bees, with an occasional hummingbird stopping in to sample the nectar. Both Flora of Australia and Flora of South Australia state that the packing bracts are dark brown at the time of flowering, but this is clearly not a good generalization. At RBG, the dozens of inflorescences produced by multiple plants over several decades have consistently had dark green bracts at flowering, and this can be seen also in habitat photos posted on iNaturalist. Eventually, though, the packing bracts turn brown, as does the shaft, which by the second year looks gray. The height of an inflorescence is quite variable, and measuring 8 of them at RBG yielded lengths from 56 to 88 inches (1.42 to 2.24 m). The ratio of the shaft length to the length of the flowering portion is also very variable; often the shaft length is a little greater, but there are many exceptions to this.
About the Fruits
As is usual in the Asphodelaceae, a flower of X. quadrangulata has a 3-chambered ovary, and when a flower is pollinated, this begins to enlarge and turn into a 3-chambered seed capsule. The capsule is at first hidden among the packing bracts, but as it expands, it protrudes and is readily visible. A mature capsule is dark brown and 0.6 to 0.8 inches long (15 to 21 mm), with a rounded base and a beak-like tip that emerges from the packing bracts. When ripe, it splits open from the tip downward, exposing the light tannish-yellow interior of the chambers, as well as the black seeds within. Only one seed is present in a chamber, and only one of the three chambers produces a seed.
The black seeds resemble shards of charcoal, with a shape like a miniature arrowhead and a length of up to 0.47 inches long (12 mm). On our plants at the Ruth Bancroft Garden, they measure from 5 mm to 8.5 mm long. At the widest point, they measure from 0.12 to 0.24 inches across (3 to 6 mm). They are 3-faceted, with a wide side that has a trough running down its length and 2 narrower sides that are slightly concave. At the base of the ridge between the 2 narrow sides is the white hilum, marking the attachment point when the seed was developing. The hilum is oval, teardrop-shaped or bar-shaped and 1 to 2 mm long.
Plants in Cultivation
Plants of Xanthorrhoea quadrangulata are not often available in nurseries, and no doubt the fact that they are so slow-growing contributes to this. They are also fussy about being transplanted, and not all of those that germinate make it to adulthood. However, once established in the garden, they are tough and reliable, able to withstand winter lows down to at least 20° F (-7° C). Although they get most of their rainfall during the winter months in their habitat, they do not mind getting some summer water as well.
