Kurrajong/Bottle Tree

Brachychiton species.

Illawarra flame tree in full summer splendour! Chiswick. © JPM, 2023.

Ah, the kurrajong! One of Australia's internationally acclaimed, and strange, ornamentals is also top bush tucker!

Names

There are 30 species of these iconic Australian trees (one species exists in Papua New Guinea), hailing from the genus Brachychiton (B.), so-named for the hairy 'short tunic' coating each seed. The English word 'kurrajong' derives from the Sydney-region's Dharug language garrajung, meaning fishing line (tree), although there are as many native names for the tree as there are tribes. In older literature the trees are named under their previous classification, Sterculia, before the botanists decided to separate Brachychiton completely from that much larger genus of stubby trees (Guymer, 1988).

Of our 30-odd native species, twelve are common and easily encountered upon the majority of the Australian continent: the Illawarra flame tree (B. acerifolius); the common kurrajong (B. populneus); the brush kurrajong or lacebark tree (B. discolor); the northern kurrajong (B. diversifolius); the desert kurrajong (B. gregorii); red-flowered kurrajong (B. paradoxus); large-leaved kurrajong (B. megaphyllus); the Kimberley kurrajong (B. viridiflorus); the sticky kurrajong (B. viscidulus); Fitzgerald's kurrajong (B. fitzgeraldianus); the little kurrajong (B. bidwillii); and, the most iconic of all, Queensland's broad- and thin-leaved bottle trees (B. australis and B. rupestris respectively).

Some rarer kurrajong species are threatened or endangered, such as B. tridentatus, B. tuberculatus and B. incanus in the Kimberley region of WA; B. obtusilobus in the Exmouth region of WA; and B. velutinosus, B. garrawayae, B. compactus and B. vitifolius in the Cape York region of northern Queensland. Foragers should be wary of harvesting from or interfering with these rare species in these parts of the country.

Habitat and Range

Kurrajongs occupy diverse habitats and ranges, being very capable survivors in both wet and dry, tropical, subtropical and temperate climes alike. Despite Australia's expansive colonial clearing campaigns to make way for grassland pasture and cropland, kurrajongs and bottle trees were widely left alone and often dot Australia's farming regions with regularity, particularly in central Queensland. Young kurrajongs grow from a fat, water-storing taproot, thus they can take to virtually any soil type or condition, although most of them prefer full sun, open locations with little competition from surrounding trees. Kurrajongs of various species, particularly the flame tree (B. acerifolius), common kurrajong (B. populneus) and their hybrids, are widely used as ornamental street trees in Australian towns and internationally.

The common kurrajong (B. populneus) has the most expansive range, being found both east and west of the Great Dividing Range from Mackay on the central Queensland coast, inland to Roma and most of SE Queensland, along the eastern part of NSW, inland as far as Cobar and Griffith (this species can be found in the eastern 50% of NSW state), and well south into Dandenong country in Victoria. The common kurrajong can also be found in all Australian state capital cities and around the world as an ornamental street tree, particularly in the Mediterranean region of Europe and North Africa, South Africa, South America, and California, Arizona, Texas and other southern states of the USA (Nyerges, 2016).

The well-forested eastern coastline, mostly east of the Great Dividing Range, is home to the showy flame tree (B. acerifolius). This species is well adapted to rich soil, shade, altitude and rainfall. This species is also a very common street ornamental in Australian cities and around the world due to their spectacular show of flowers in midsummer; the tree will drop all, or most, of its foliage just prior to flowering, at which point the trees stand aflame with their stunning whole-crown fiery red floral display. They flower just after jacarandas do, usually in December-January.

The deciduous northern varieties (particularly B. diversifolius, B. paradoxus and B. megaphyllus) grow equally well in their bipartite wet-dry season regions, being found on a variety of terrain across Arnhem land in the NT and Cape York in far north QLD. The Kimberley region of northern WA is also very well represented, being home to the Kimberley kurrajong (B. viridiflorus), Fitzgerald's kurrajong (B. fitzgeraldianus), the sticky kurrajong (B. viscidulus), and many of the rarer, protected species mentioned above may also found in the northern parts of Australia.

The desert kurrajong (B. gregorii) is at home in arid central regions of Western Australia, as well as north of Geraldton on the WA coast, and in the south-western part of the Northern Territory, particularly from Alice Springs towards Irrunytju in the north-western corner of SA. One protected species, B. obtusilobus, grows exclusively in the peninsulas surrounding Exmouth on the WA coast.

Queensland's iconic bottle trees (B. australis and B. rupestris) are found in the central-eastern parts of that state, mostly on the western side of the Great Dividing Range, particularly the North Brigalow Belt region from Townsville south to Rockhampton and inland to Emerald and Roma, but can be found as isolated specimens as far north as Cooktown and as far south as Narrabri in northern NSW. The little kurrajong (B. bidwillii) is also found in the same region as Queensland's bottle trees, down into south-east QLD and Brisbane. Bottle trees make splendid, quirky ornamentals in botanic gardens in Australia and around the world.

Figure 1. Distribution of Brachychiton (all species) across the continent. Atlas of Living Australia.

Identification

Key Identifying Features

• Tree grows up to 30 m (100 ft.) height, often in the 8-12 m (25-40 ft.) range, some northern and desert species only 3-7 m (10-23 ft.).

• Tapered trunk which is thick at the base and thin at the top and tapered foliage crown ('pachycaul' shape; Queensland bottle trees in particular have an iconic, squat, fat trunk similar to a baobab).

• Bark of younger trees usually smooth and green, with a distinct woody portion near the base.

• Bark of older trees develops fissures and may exhibit green patches.

• Large, broad leaves often 20-40 cm (9-15 in.) in length, usually with 3 or 5 lobes often giving them a "dino-foot" or "maple" shape; leaves of the same species may show significant variation (see images below).

• Thin-leaf bottle tree (B. rupestris) has small, 5-10 cm (2-4 in.) thin or broad spear-tip shaped leaves with no lobes; common kurrajong (B. populneus) also often lacks lobes.

• Northern and desert species are deciduous, losing all their leaves in the dry season (March - September).

• Southern species are semi-deciduous (particularly B. populneus, B. dicolor and B. acerifolius), losing all or most of their leaves just prior to flowering in summer (October - January).

• Flowers emerge on long, 30-50 cm (B. populneus, B. acerifolius, B. dicolor) or short (northern, desert and bottle trees) branched stems.

• Flowers are bell-shaped (perianth), usually cream outside with red streaks inside (B. populneus, B. gregorii) or wholly red (especially B. acerifolius, B. paradoxus and B. megaphyllus); some are pinkish with red or white stripes (B. discolor; B. bidwillii; B. viscidulus; B. australis) or plain white (B. rupestris).

• Flowers have 5 petals (rarely, four) which curl outwards from the main bell; petal tips may have fringed bristles (particularly B. rupestris).

• Flowers are either male or female; both genders occur on the same tree (monoecious).

• Bottle-tree (B. australis and B. rupestris) flowers are white and star-shaped with 5 petals.

• Pollinated female flowers grow into long or squat, green or brown, canoe- or boat-shaped pods with a pointy tip.

• Most species pods turn black (B. paradoxus, B. viridifolius, B. viscidulus are fuzzy brown) and split open when ripe, exposing the yellow seeds inside.

• Pods filled with 4-20 yellow-coated seeds individually housed inside a hairy, papery exotesta (outer seed sheath).

• The fine hairs of the exotesta are extremely irritating to skin, nose, mouth and eyes!

Kurrajongs are iconic, stately trees with distinctive, tapered trunks and crowns. The bottle trees in particular (B. australis and B. rupestris) have a bulbous, wine bottle-shaped appearance, often very similar to African or Australian baobab species. Their bark is usually green and smooth while trees are young, but will form fissured, fibrous lignin as they age past the 10 to 15 year mark, especially near the base. Species growing in Australia's tropical, subtropical and desert regions will be deciduous, losing all their leaves at the onset of the dry season (March-October). Southern species such as the common kurrajong (B. populneus) and flame tree (B. acerifolius) will dump some or all of their leaves just prior to flowering in the summer (November-January).

Figure 2. The overall profile of an Illawarra flame tree (B. acerifolius) in full flower. Chiswick, Sydney. © JPM, 2023.

Figure 3. The base of a flame tree (B acerifolius) is thick, with a fissured brown base which turns green up the trunk. Mt. Annan Botanical Gardens. © JPM, 2023.

Figure 4. The trunk of an older lacebark tree (B. discolor) tapers to a thin point. Older trees lose the green tint in the bark. The Domain, Sydney. © JPM, 2022.

Figure 5. The thin-leaved bottle tree (B. rupestris) has the most iconic trunk of all. Note the transition between brown, woody base and greener main trunk; this is a common feature of most kurrajongs and allows quick identification. This shape is the iconic 'pachycaul.' King's Park, Perth. © JPM, 2023.

Figure 6. An aged broad-leaved bottle tree (B. australis). Older trees lose the green tinge in the main trunk bark. Atlas of Living Australia. © B. Mclennan, 2019.

Figure 7. Close-up of the lacebark tree's trunk bark. (B. discolor). Mt. Annan Botanical Gardens. © JPM, 2023.

Kurrajong leaves are very variable, even among the same species. The most typical pattern is large, 20-40 cm (9-15 in.) long, five-lobed 'maple-leaf' shape (B. acerifolius, B. australis, B. gregorii, B. discolor; B. diversifolius; B. bidwillii), although the same species can easily exhibit 3-lobe 'dino-foot' shape leaves (particularly B. acerifolius and B. gregorii). The common kurrajong (B. populneus), thin-leaved bottle tree (B. rupestris), and northern larged-leaved kurrajong (B. megaphyllus) regularly lack lobes on the leaves, but can exhibit them.

Figure 8. Typical foliage of the flame tree (B. acerifolius), showing 3-lobes ('dino-foot'). Hornsby. © JPM, 2023.

Figure 9. This example is a 5-lobed ('maple') lacebark tree (B. discolor) leaf. The Domain, Sydney. © JPM, 2022.

Figure 10. The northern red-flowered kurrajong (B. paradoxus) has foliage similar to B. acerifolius above. King's Park, Perth. © JPM, 2023.

Figure 11. The foliage of the broad-leaf bottle tree (B. australis) very much resembles 5-lobed maple leaves. Atlas of Living Australia. © Coenobita, 2010.

Figure 12. The desert kurrajong (B. gregorii) has thin, 3- or 5-lobed leaves well adapted to its dry climate. Atlas of Living Australia. © N. Lambert, 2022.

Figure 13. Typical foliage of the common kurrajong (B. populneus). Note that this species has no or barely obvious (usually 3) lobes. Atlas of Living Australia. © R. Richter, 2019.

Figure 14. The thin-leaved bottle tree (B. rupestris) has the smallest foliage of all the kurrajongs, and is almost never lobed. Mt. Annan Botanical Gardens. © JPM, 2023. 

Kurrajong trees have showy flowers, especially the Illawarra flame tree which will drop all of its leaves prior to becoming wholly red, covered in its bell-shaped flowers from late November through late January (see figure 2 above). Most of the deciduous northern species flower at the onset of the spring-summer wet season (October-December); their flowers can be red or pinkish with white or yellow streaks and splotches. Common kurrajong (B. populneus), northern kurrajong (B. diversifolius) and desert kurrajong (B. gregorii) have greenish yellow-cream flowers with red streaks and splotches. The thin-leaved bottle tree (B. rupestris) has red-white streaked flowers; broad-leaved bottle trees (B. australis) are white only. Flower colour and shape in combination with the leaves is often a good indicator of the individual species. All kurrajongs and bottle trees are monoecious, presenting separate male and female flowers on the same plant. Male flowers have a wad of pollen bearing anthers in the flower core, whereas female flowers have a protruding, star-shaped pistil.

Figure 15. The midsummer floral display of the flame tree (B. acerifolius) is spectacular and the reason they are ornamental street trees the world over. Chiswick, Sydney. © JPM 2023.

Figure 16. Close-up of a male flame tree (B. acerifolius) flower, also showing its yellow flecks. Newington. © JPM 2023.

Figure 17. Male flowers of the brush kurrajong (B. discolor sub. 'Griffith Pink'). Atlas of Living Australia. © M. Fagg, 2012.

Figure 18. Female flowers of the red-flowered kurrajong (B. paradoxus). Atlas of Living Australia. © M. Fagg, 2007.

Figure 19. The large-leaved northern kurrajong also has beautiful red flowers that appear before the leaves at the onset of the wet season. Atlas of Living Australia. © A. Jonker, 2007.

Figure 20. The common kurrajong (B. populneus) has greenish, yellow-cream flowers with red streaks and splotches. Atlas of Living Australia. © Jiggy, 2019.

Figure 21. The northern kurrajong (B. diversifolius) has flowers like the common kurrajong above; they can be distinguished by their leaves. Atlas of Living Australia. © C. Pearce, 2019.

Figure 22. The desert kurrajong (B. gregorii) is similar, with greenish cream-white flowers. Atlas of Living Australia. © A. N. Schmidt-Lebuhn, 2010.

Figure 23. The thin-leaved bottle tree (B. rupestris) flowers are white with red streaks and star-shaped. © A & N. Kapitany, n.d.

Figure 24. The broad-leaved bottle tree (B. australis) has white, star-shaped flowers. Atlas of Living Australia. © S. Dew, 2021.

Figure 25. A clear example of a female flower on a sticky kurrajong (B. viscidulus). See figure 16 above for a male flower. Atlas of Living Australia. © Liana, 2022.

After pollination, female flowers grow into green, canoe- or boat-shaped pods (some northern species have fuzzy brown coatings on the pods), usually dangling down on long stems up to half a metre in length. The stems of the pods turn yellow as ripening approaches; when fully ripe the pods dry out, turn black and split open to reveal their densely packed and well-protected payload of yellow seeds. Each seed is coated in a hairy exotesta (which is where the name Brachychiton comes from) covered in miniscule, irritating spikes. Some species’ hairs can be as sharp and penetrating as glass; others are like prickly velvet. The overall shape and hairiness of the pods can be an indicator for individual species.

Figure 26. Unripe flame tree (B. acerifolius) pods are long, canoe-shaped, smooth and green, dangling in bunches of 1-5. Newington. © JPM, 2023.

Figure 27. Ripe flame tree pods start turning yellow at the stem when ripe, blackening and splitting open to reveal their payload of well-protected seeds. © JPM, 2023.

Figure 28. Common kurrajong (B. populneus) ripe pods displaying seeds (centre left). This species is one of the easier ones to harvest. Atlas of Living Australia. © Mike, 2017.

Figure 29. Old pods of a desert kurrajong (B. gregorii) are stubby and boat-shaped, having already discarded their precious cargo. Atlas of Living Australia. L. Fedec, 2021.

Figure 30. The thin-leaved bottle tree (B. rupestris) has small pods with a light payload. The irritating hairs (exotesta) are very evident. Atlas of Living Australia. © A. Kapitany, n.d.

Figure 31. The red-flowered kurrajong (B. paradoxus) has velvety, brown pods with obnoxious exotesta seed coats; also home to a menagerie of arachnids! Several other northern species (e.g. B. viscidulus, B. viridifolius) also have furry brown pods like this. King's Park, Perth. © JPM, 2023.

Culinary Uses

A favourite of Australia's aboriginal tribes across the country for millennia, kurrajong seeds are an excellent source of nutrition and, once roasted appropriately, very tasty. Testing of B. populneus seeds yielded 18.1 g protein, 24.7 g fat and 14.6 g carbohydrates per 100 g; they are an excellent source of fibre at 33.5 g/ 100 g, as well as potassium, 567 mg/100 g, and the highest known native Australian food source of magnesium, 288 mg/100 g (Low, 1989: 220-221). A more recent study had lower mineral numbers for B. populneus seeds, with 200.9 mg potassium, 41 mg sodium, 170.5 mg calcium, 56.6 mg magnesium, 5 mg iron, 0.75 mg zinc and 1.4 mg manganese present per 100 g (Rjeibi et al., 2020: Table 1). Hiddins' testing of two northern species (B. paradoxum and B. australis) yielded much lower results of 0.8-1% fat and 4 to 5.88% protein, and negligible vitamin C (0.8 to 8 mg) (Hiddins, 2002a; 2002d); I suspect Hiddins' testing methodology was flawed as other phyochemists have reported in excess of 41% fat in kurrajong seeds (Dahwood et al., 2021).

Figure 32. A roasted flame tree pod (B. acerifolius), cracked open to display the seeds inside, with some hairy outer coats torn away. Yes, it looks like corn! © JPM, 2023.

One study noted that kurrajong seed oil (B. acerifolius) can contain cyclopropene fatty acids (specifically, sterculic acid and malvalic acid), up to 0.4% (Farag et al., 2014). Cyclopropene fatty acids, also present in cottonseed and baobab oil (malvalic acid), are generally toxic to humans in large concentrations. However, the concentration of 0.4% in the sample tested earned only a "might still present a health hazard" in Farag's estimation; Farag et al. recommended targeted breeding of kurrajong to reduce its cyclopropene fatty acid seed oil content. No mention has been made on the effect of roasting the seeds and its effectiveness in potentially denaturing cyclopropene fatty acids to render the seeds and seed oil suitable for human consumption. In any case, they have been roasted and eaten by Australia's First Nation tribes for millennia prior with seemingly minimal ill effect. The seeds of every species were eaten.

Figure 33. Close-up inside the contents of a kurrajong (flame tree, B. acerifolius) seed. Each seed has 4 visible layers - the hairy exotesta (not present), the yellow mesophyll, a woody brown exotegma, and the white endosperm (for cellular details see Guymer, 1988: 202). © JPM, 2023.

Kurrajong pods which have split open are best roasted on slow coals to incinerate the fine, irritating hairs, after which pods may be split open and the seeds extracted by rolling the contents in the hands. I still have not attempted coal-roasting but can affirm it is very likely the most superior method for preparing seeds for removal from the pods and human consumption. Seeds can be ground into an oily brown meal and added to breads, biscuits and pastries, but I prefer them as is, toasted and crunchy!

I have harvested approximately 2.3 kg of flame tree (B. acerifolius) pods (57 pods in total, approximately 41 grams per pod) as a trial for preparation methods; harvesting the pods took approximately 10 minutes and exhausted an estimated 15-20% of the pods on a single tree. Note that most pods will be well out of reach unless you bring a ladder, telescopic secaturs or skillful tree climber! After giving them all a good scrub to remove dust, spiders and gum, I roasted the 2,369 grams (83 oz.) of pods in the oven at 180 C for 45 minutes. I then cracked each pod open with a pair of scissors and spent an excruciating 6 hours to extract a total of 380 grams (13 oz.) of seeds from their irritating, hairy coatings with a pair of tweezers, a 16% w/w yield or 6.7 grams (1/4 oz.) of seeds per pod. The extracted 380 grams of seeds were roasted a second time (approx. 20 minutes at 180 C) until the yellow coatings began to turn golden and the inner, brown seed shells became crispy. Concurrent with the persistent observation that "They look like corn," they also taste like roasted corn and peanut at the same time.

Figure 34. A bountiful harvest! This was approximately 15-20% of the pods from a single flame tree (B. acerifolius), weighing 2,369 grams. Sourced from Newington. © JPM, 2023.

Figure 35. The end result of my hard work: 380 grams of precious treasure, or a 16% w/w yield from the pods. Sourced from Newington. © JPM, 2023.

Figure 36. Fresh pods have yellow-coated seeds (left); in old pods the seeds shrivel and turn white (right). Sourced from Newington. © JPM, 2023.

Young kurrajong trees have bulbous, edible taproots much like large parsnips, usually much larger and thicker than the trunk of the young tree above. Virtually all of the common species, especially the common kurrajong (B. populneus), the northern kurrajong (B. diversifolius), and both species of bottle tree (B. australis and B. rupestris), were targeted for this use (Lim, 2015 IX: 17). During Australia's colonial period, Maiden (1920) noted that common kurrajongs (B. populneus) were well known:

as ‘Yam-tree,’ on account of the large yam- like root the tree possesses, at all events in the young state, which root is locally called yam, and it is stated that these were formerly much sought after by the aborigines for food. In the case of some small trees, less than 1 inch in diameter, which were dug up for planting, they had yams from 8 to 12 inches long, and 2 or 3 in diameter, weighing several pounds. A correspondent tells me they have been got 8 to 10 lb in weight, and are not despised by Europeans. The outside skin or bark of these yams can be easily removed, and looks like the skin of a radish. The inside is beautifully white, a little sweetish in taste, but otherwise rather insipid.

Kurrajong yams can be eaten raw or roasted; other species than those mentioned here can also be eaten after the removal of the stringy outer root bark but this was typically a famine food (Williams, 2010 I:142). I have not yet harvested a kurrajong yam because all the young trees in my area are ornamental council plantings.

Figure 37. A herbarium specimen of B. viridifolius showing the exceedingly fat taproots on this young plant. Atlas of Living Australia. © Centre for National Biodiversity Research, 1989.

Figure 38. A herbarium specimen of broad-leaved bottle tree (B. australis) showing the exceedingly fat taproot on this young plant. Atlas of Living Australia. © Centre for National Biodiversity Research, 1988.

A clear, mucilaginous gum much resembling tragacanth (which is sourced from Astragalus legume species) can be extracted from all species of kurrajong (Williams, 2010 I: 142; Low, 1989: 187; Maiden, 1889: 59-61). I noted this clear jelly forming naturally from the sap which exuded from the baked, green pods I had harvested from a flame tree (B. acerifolius), the heat of the oven forcing the sap out of the cut end. I accidentally mixed the sap with water while washing the cooked pods a second time, and this became a clear lump of jelly that got stuck underneath my sink plug! I harvested this sap when I collected my second crop of pods a week later. I suspect it may be possible to flavour this kurrajong jelly by soaking it in flavourful substances like fruit juice, wine, coffee or various fragrant teas. The clear jelly can also be obtained by pouring boiling water over shavings of the whitish inner bark or young, woody shoots, particularly from B. rupestris (Maiden, 1889: 60 & 221).

Figure 39. The clear, jelly-like gum of all species of kurrajong can be eaten to alleviate thirst; it can absorb water and other liquids and swell considerably in size. Gum can be harvested from damaged branches, pods or inner wood shavings soaked in water. © JPM, 2023.

Young spring shoots of the thin-leaved bottle tree (B. rupestris) have been eaten to save many a lost explorer; the same may be done by chewing shavings of this species's soft, juicy inner bark.

Figure 40. Young spring shoots of the deciduous thin-leaved bottle tree (B. rupestris) are refreshing to chew upon. Mt. Annan Botanical Garden. © JPM, 2023.

Medicinal Uses

First nation tribes of the Top End have long utilised various kurrajongs medicinally. An infusion (tea) of northern kurrajong (B. diversifolius) leaves was used as a whole body wash for fevers, and the thick, jelly-like gum of the tree was applied to wounds, sores and ulcers to hasten healing and reduce inflammation (Lassak and McCarthy, 2011: 130). It is likely that other species of Brachychiton were utilised in a similar way elsewhere around Australia, but little written record of this remains.

Kurrajong species possesses a complex phytochemical profile with many potential medicinal properties that have, sadly, been little clinically studied; an excellent phytochemical and ethnobotanical overview of the Brachychiton (and closely related Sterculia) genus is available (Thabet et al. 2018a). Of the available studies, we find that common kurrajong (B. populneus) leaves and flowers have a wide range of phytochemical constituents useful mostly as free-radical antioxidant scavengers. Water extraction of dried, powdered B. populneus resulted in varying concentrations of alkaloids, flavonoids, phenols, terpenoids, triterpenoids, quinones, oils and resins, phlobatannins, vitamin C, proteins, glycosides, coumarins, saponins, antraquinones, betacyanins and sterols (Batool et al. 2019, esp. Table 1). Methanol extraction of B. populneus resulted, in addition to the above, varying concentrations of anthocyanins, steroids, and phytosteroids (Batool et al., 2019), although methanol extracts themselves are not suitable for internal herbal medicine use. Overall, various extracts and fractions of B. populneus show significant promise as powerful antioxidants, protect red blood cells (erthrocytes), inhibit brain cholinesterase enzyme activity (potentially useful for Alzheimers treatment) and have impressive anti-inflammatory effects (Rjeibi et al., 2020). A combined ethanol/water extract would likely contain the most active herbal compounds from kurrajong leaves and flowers in a form safe for human use, although precise dosages remain uncertain.

All parts of the flame tree (B. acerifolius) have been tested and shown to possess 56 different secondary metabolites of interest; flame tree leaves showed significant antioxidant effects far exceeding vitamin C. (Farag et al., 2014, esp. p. 121). Another study determined that the foliage of this species also has potent anti-hypoglycemic effects, which may be of interest for diabetes treatment (Zeid et al., 2017); dosages to achieve this effect in humans remains unknown and clinically untested. The showy red flowers of this species also have antibiotic properties (Williams, IV: 133).

The northern kurrajong (B. diversifolius) fared similarly, its leaves being a very potent source of antioxidants; however this species' branch wood extracts also has such potent antibiotic properties the researchers discussed its potential as an antimicrobial agent for agriculture and internal use (Abdel-Megeed, 2013).

Bottle trees (both B. rupestris and B. australis) have similar antioxidant properties in the leaves (Kassem et al. 2002; Thabet et al. 2017). Studies on the leaf mucilage of B. australis and B. rupestris show it has significant anti-diabetic effects (Kassem et al. 2002; Thabet et al. 2018c). Methanol leaf extracts from B. rupestris were shown to be effective against Gram-negative bacteria, as well as against Candida fungi; it is suspected the high levels of flavonoids in the leaves, especially quercetin, kaempferol, isorhamnetin and their glycosides, are responsible for much of this plant's medicinal properties. (Thabet et al. 2017). B. rupestris (and B. discolor) was also showed to exhibit anti-inflammatory and anti-allergic properties because of its phytochemical ability to reduce elastase production by neutrophils in the blood (Thabet et al. 2018b). Exact dosages for human use remains clinically untested.

Gum exudates from Brachychiton show promise as excipients for drug delivery, being capable natural hydrogels and thus suitable for use as stabilisers, emulsifiers and thickeners (Thabet et al., 2018a: 461-466, although this study concentrated mostly on genus Sterculia).

Other Uses

Kurrajongs and bottle trees are famed for their water storage in their roots and trunks. Aborigines burnt the top and trunk of the tree to drive water down into the roots. Then they would unearth lateral roots and cut 50 cm sections, standing them on end into a wallaby leather storage container. Litres of clean water can be obtained in this way, an important desert survival tip (Williams, 2010 I: 141-142). Excessive root harvest can easily kill the trees, however. Another method involved puncturing a root on one side of the tree, then putting a fire to a root on the other side (Nyerges, 2016). The heat of the fire drives the water to the cut root, causing it to gush copious quantities of potable water. A third method involved drilling a hole in the trunk of trees known to contain internal water reservoirs.

Kurrajongs and bottle trees have a fibrous, stringy inner bark which is suitable for fashioning fishing lines, nets and rope and was long used for this purpose (Turner, 1891: 11). The lacebark tree (B. discolor) was especially esteemed for this purpose.

Leaves of all species of Brachychiton make excellent emergency stock fodder during drought periods (Turner, 1891: 11-12; Maiden 1889: 140). Colonial farmers would lop the branches off the tree, from which their cattle and sheep would gleefully feast, and the tree recovers amicably with new growth shortly thereafter.

The slippery yellow arils of the seeds can be used as a pigment for dye or painting, and a dye of uncertain colour (probably yellow) can be obtained from the seed pods themselves (Maiden, 1889: 300).

The high-fat seed oil of B. populneus has been experimented with as a source of biodiesel using novel nickel oxide catalysts made from Ficus elastica latex (Dahwood et al, 2021). If there was another Top End industry that could promote independence for our First Nation tribes, this would be one of them.

Caution!

The hairy seed coats (exotesta) are extremely irritating to skin, eyes, nose and mouth! Some species have very prominent glassy hairs that will stab into the skin as easily as shattered silicon shards, others are more like velvet and barely bothersome. Avoid direct contact with these irritants; burn the hairs off the pods with a naked flame or roast pods in warm coals before attempting to harvest seeds. Australian rural children are also well aware of the effects of suddenly stuffing several open pods down the back of someone's shirt.

References/Further Reading

Abdel-Megeed, Ahmed (2013). "Brachychiton diversifolius as a Source of Natural Products: Antibacterial and Antioxidant Evaluation of Extracts of Wood Branches." Journal of Pure and Applied Microbiology 7(3):1843-1850 [LINK]

Atlas of Living Australia, "Brachychiton acerifolius" [LINK]; "Brachychiton australis" [LINK]; "Brachychiton bidwillii" [LINK]; "Brachychiton discolor" [LINK]; "Brachychiton diversifolius" [LINK]; "Brachychiton fitzgeraldianus" [LINK]; "Brachychiton gregorii" [LINK]; "Brachychiton megaphyllus" [LINK]; "Brachychiton paradoxus" [LINK]; "Brachychiton populneus" [LINK]; "Brachychiton rupestris" [LINK]; "Brachychiton viridiflorus" [LINK]; "Brachychiton viscidulus" [LINK].

Batool, Riffat et al. (2019). "Estimation of phytochemical constituents and in vitro antioxidant potencies of Brachychiton populneus (Schott & Endl.) R.Br." BMC Chemistry 13(32) [LINK]

Dawood, Sumreen et al. (2021). "Synthesis of biodiesel from non-edible (Brachychiton populneus) oil in the presence of nickel oxide nanocatalyst: Parametric and optimisation studies." Chemosphere 278 (130469). [LINK]

Farag, Mohamed A. et al. (2014). "Metabolomic fingerprint classification of Brachychiton acerifolius organs via UPLC-qTOF-PDA-MS analysis and chemometrics." Natural Product Research 29(2): 116-124. [LINK]

Guymer, G. P. (1988). "A taxonomic revision of Brachychiton (Sterculiaceae)." Australian Systematic Botany 1(3): 199 - 323. [LINK]

Hansen & Horsfall (2019), Noongar Bush Tucker, pp. 130-131 ("Brachychiton gregorii").

Hiddins (2002a), "Big Leafed Kurrajong (Brachychiton australis)." [LINK]

Hiddins (2002b), "Bottle Tree (Brachychiton rupestris)." [LINK]

Hiddins (2002c), "Brachychiton (Brachychiton tuberculatus)." [LINK]

Hiddins (2002d), "Kurrajong (Brachychiton paradoxum)." [LINK]

Kassem, H. A. et al. (2002),"Phytochemical and Hypoglycemic Studies of the Leaves of Brachychiton australis (Schott & Endl.) A. Terrac. Grown in Egypt." Bulletin of the Faculty of Pharmacy Cairo University. 40(2): 85-91. [LINK]

Lassak and McCarthy (2011), Australian Medicinal Plants, p. 130 (Brachychiton diversifolius).

Low (1991), Wild Food Plants, p. 183.

Low (1989), Bush Tucker, pp. 27 (B. populneus), 87 (B. gregorii and B. populneus), 92 (B. acerifolius), 187 (Brachychiton sap), 220-221 (nutritional information).

Maiden, Joseph. (1920), "Brachychiton populneum (No. 232)." The Forest Flora of New South Wales, Vol. 7, LXIII. Sydney: NSW Government Printer.

Maiden (1889), Useful Plants, pp. 59-61, 140, 219-221, 300.

Nyerges, Christopher (2016). "The Kurrajong Tree from Down Under." A Voice in the Wilderness [LINK].

Rjeibi, Ilhem et al. (2020). "Brachychiton populneus as a novel source of bioactive ingredients with therapeutic effects: antioxidant, enzyme inhibitory, anti‐inflammatory properties and LC–ESI‐MS profile." Inflammopharmacology 28:563-574. [LINK]

Thabet, Amany, et al. (2017). "Anti-infective Properties of Brachychiton rupestris and Brachychiton luridum Leaves and their Qualitative Phytochemical Screening." Medicinal and Aromatic Plants 6(4): 1000299. [LINK]

Thabet, Amany et al. (2018a). "Sterculia and Brachychiton: a comprehensive overview on their ethnopharmacology, biological activities, phytochemistry and the role of their gummy exudates in drug delivery." Journal of Pharmacy and Pharmacology 70(4): 450-474. [LINK]

Thabet, Amany, et al. (2018b). "Study of the anti-allergic and anti- inflammatory activity of Brachychiton rupestris and Brachychiton discolor leaves (Malvaceae) using in vitro models." BMC Complementary and Alternative Medicine 18:299. [LINK]

Thabet, Amany, et al. (2018c). "Validation of the Antihyperglycaemic and Hepatoprotective Activity of the Flavonoid Rich Fraction of Brachychiton rupestris Using in Vivo experimental Models and Molecular Modelling." Food and Chemical Toxicology 114:302-310. [LINK]

Turner, F (1891). The Forage Plants of Australia. Sydney: G.S. Chapman.

Williams (2010, 2013), Medicinal Plants, Vol I pp. 141-142 and Vol IV. p. 133.

Zeid, Aisha H. A. et al. (2017), "Flavonoid chemical composition and antidiabetic potential of Brachychiton acerifolius leaves extract." Asian Pacific Journal of Tropical Biomedicine 7(5): 389-396. [LINK]