| Golden wattle | |
|---|---|
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Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Rosids |
| Order: | Fabales |
| Family: | Fabaceae |
| Subfamily: | Caesalpinioideae |
| Clade: | Mimosoid clade |
| Genus: | Acacia |
| Species: | A. pycnantha |
| Binomial name | |
| Acacia pycnantha | |
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| Occurrence data fromAVH | |
| Synonyms[1] | |
Species synonymy
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Acacia pycnantha, most commonly known as thegolden wattle, is a tree of the familyFabaceae. It grows to a height of 8 metres (26 feet) and hasphyllodes (flattened leaf stalks) instead of true leaves. The profuse fragrant, golden flowers appear in late winter and spring, followed by long seed pods. ExplorerThomas Mitchell collected thetype specimen, from whichGeorge Bentham wrote thespecies description in 1842. The species is native to southeasternAustralia as anunderstorey plant ineucalyptus forest. Plants are cross-pollinated by several species ofhoneyeater andthornbill, which visitnectaries on the phyllodes and brush against flowers, transferring pollen between them.
A. pycnantha has become a weed in areas of Australia, as well as in Africa and Eurasia. Its bark produces moretannin than any other wattle species, resulting in its commercial cultivation for production of this compound. It has been widely grown as an ornamental garden plant and for cut flower production.A. pycnantha was made the officialfloral emblem of Australia in 1988, and has been featured on the country's postal stamps.
Acacia pycnantha generally grows as a small tree to between 3 and 8 metres (10 and 30 feet) in height,[2] though trees of up to 12 m (40 ft) high have been reported in Morocco.[3] The bark is generally dark brown to grey—smooth in younger plants though it can be furrowed and rough in older plants.[4] Branchlets may be bare and smooth orcovered with a white bloom.[2] The mature trees do not havetrue leaves but havephyllodes—flat and widened leaf stems—that hang down from the branches. Shiny and dark green, these are between 9 and 15 cm (3.5 and 5.9 in) long, 1–3.5 cm (0.4–1 in) wide and falcate (sickle-shaped) tooblanceolate in shape.[2] New growth has a bronze colouration.[5] Field observations atHale Conservation Park show the bulk of new growth occurs over spring and summer from October to January.[6]
Floral buds are produced year-round on the tips of new growth, but only those initiated between November and May go on to flower several months later. Flowering usually takes place from July to November (late winter to early summer) in the golden wattle's native range, and because the later buds develop faster, flowering peaks over July and August.[7][8] The bright yellowinflorescences occur in groups of 40 to 80 on 2.5–9 cm (1–4 in)-longracemes that arise fromaxillary buds.[2] Each inflorescence is a ball-like structure covered by 40 to 100 small flowers that have five tiny petals (pentamerous) and long erectstamens, which give the flower head a fluffy appearance.[4]
Developing after flowering has finished, the seed pods are flattish, straight or slightly curved, 5–14 cm (2–6 in) long and 5–8 mm wide.[8][9] They are initially bright green, maturing to dark brown and have slight constrictions between the seeds,[10] which are arranged in a line in the pod.[8] The oblong seeds themselves are 5.5 to 6 mm long, black and shiny, with aclavate (club-shaped)aril.[2] They are released in December and January, when the pods are fully ripe.[7]
Species similar in appearance include mountain hickory wattle (A. obliquinervia), coast golden wattle (A. leiophylla) and golden wreath wattle (A. saligna).[2]Acacia obliquinervia has grey-green phyllodes, fewer flowers in its flower heads, and broader (1.25–2.5 cm (0.5–1 in)-wide) seed pods.[11]A. leiophylla has paler phyllodes.[12]A. saligna has longer, narrower phyllodes.[4]
Acacia pycnantha was first formallydescribed by botanistGeorge Bentham in theLondon Journal of Botany in 1842.[13] Thetype specimen was collected by the explorerThomas Mitchell in present-day northernVictoria betweenPyramid Hill and theLoddon River.[14][1] Bentham thought it was related toA. leiophylla, which he described in the same paper.[13] The specific epithetpycnantha is derived from theGreek wordspyknos (dense) andanthos (flower), a reference to the dense cluster of flowers that make up the globular inflorescences.[15] Queensland botanistLes Pedley reclassified the species asRacosperma pycnanthum in 2003, when he proposed placing almost all Australian members of the genus into the new genusRacosperma.[16] However, this name is treated as asynonym of its original name.[1]
Johann Georg Christian Lehmann describedAcacia petiolaris in 1851 from a plant grown at Hamburg Botanic Gardens from seed said to be from theSwan River Colony (Perth).[14]Carl Meissner describedA. falcinella from material from Port Lincoln in 1855. Bentham classified both asA. pycnantha in his 1864Flora Australiensis, though he did categorise a possible subspeciesangustifolia based on material fromSpencer Gulf with narrower phyllodes and fewer inflorescences.[17] However, no subspecies are currently recognised, though an informal classification distinguishes wetland and dryland forms, the latter with narrower phyllodes.[18]
In 1921Joseph Maiden describedAcacia westonii from the northern and western slopes of Mount Jerrabomberra nearQueanbeyan inNew South Wales. He felt it was similar to, but distinct from,A. pycnantha and was uncertain whether it warranted species rank. His colleagueRichard Hind Cambage grew seedlings and reported they had much longerinternodes than those ofA. pycnantha, and that the phyllodes appeared to have threenectaries rather than the single one of the latter species.[19] It is now regarded as a synonym ofA. pycnantha.[1]
Common names recorded include golden wattle, green wattle, black wattle, and broad-leaved wattle.[1] AtEbenezer Mission in theWergaia country of north-western Victoria the Aboriginal people referred to it aswitch.[20][21]
Hybrids of the species are known in nature and cultivation. In theWhipstick forest nearBendigo in Victoria, putative hybrids with Whirrakee wattle (Acacia williamsonii) have been identified; these resemble hakea wattle (Acacia hakeoides).[2] Garden hybrids with Queensland silver wattle (Acacia podalyriifolia) raised in Europe have been given the namesAcacia x siebertiana andAcacia x deneufvillei.[1]
Golden wattle occurs in south-eastern Australia fromSouth Australia's southernEyre Peninsula andFlinders Ranges across Victoria and northwards into inland areas of southern New South Wales and theAustralian Capital Territory.[8][22] It is found in theunderstorey of openeucalypt forests on dry, shallow soils.[9]
The species has becomenaturalised beyond its original range in Australia. In New South Wales it is especially prevalent around Sydney and theCentral Coast region. In Tasmania it has spread in the east of the state and becomeweedy in bushland nearHobart. In Western Australia, it is found in theDarling Range and westernwheatbelt as well asEsperance andKalgoorlie.[4]
Outside Australia it has become naturalised in South Africa where it is considered an invasive alien plant and is uprooted to prevent water depletion and protect local flora, Tanzania, Italy, Portugal, Sardinia, India, Indonesia and New Zealand.[4] It is present in California as a garden escapee, but is not considered to be naturalised there.[23] In South Africa, where it had been introduced between 1858 and 1865 for dune stabilization andtannin production, it had spread along waterways into forest, mountain and lowlandfynbos, and borderline areas between fynbos andkaroo.[24] The gall-forming waspTrichilogaster signiventris has been introduced in South Africa forbiological control and has reduced the capacity of trees to reproduce throughout their range.[25] The eggs are laid by adult wasps into buds of flower heads in the summer, before hatching in May and June when the larvae induce the formation of the grape-like galls and prevent flower development. The galls can be so heavy that branches break under their weight.[26] In addition, the introduction in 2001 of the acacia seed weevilMelanterius compactus has also proved effective.[27]
Though plants are usually killed by a severe fire, mature specimens are able toresprout.[28][29] Seeds are able to persist in the soil for more than five years, germinating after fire.[29]
Like other wattles,Acacia pycnanthafixes nitrogen from the atmosphere.[30] It hosts bacteria known asrhizobia that form root nodules, where they make nitrogen available in organic form and thus help the plant grow in poor soils. A field study across Australia and South Africa found that the microbes are genetically diverse, belonging to various strains of the speciesBradyrhizobium japonicum and genusBurkholderia in both countries. It is unclear whether the golden wattle was accompanied by the bacteria to the African continent or encountered new populations there.[31]
Self-incompatible,A. pycnantha cannot fertilise itself and requires cross-pollination between plants to set seed.[32] Birds facilitate this and field experiments keeping birds away from flowers greatly reduces seed production. Nectaries are located on phyllodes; those near open flowers become active, producingnectar that birds feed upon just before or during flowering. While feeding, birds brush against the flower heads and dislodge pollen and often visit multiple trees.[6] Several species ofhoneyeater, including thewhite-naped,yellow-faced,[33]New Holland,[34] and occasionallywhite-plumed andcrescent honeyeaters,[33] andeastern spinebills have been observed foraging. Other bird species include thesilvereye,striated,buff-rumped andbrown thornbills. As well as eating nectar, birds often pick off insects on the foliage. Honeybees, native bees, ants and flies also visit nectaries, but generally do not come into contact with the flowers during this activity.[6] The presence ofA. pycnantha is positively correlated with numbers ofswift parrots overwintering inbox–ironbark forest in central Victoria, though it is not clear whether the parrots are feeding on them or some other factor is at play.[35]
The wood serves as food for larvae of thejewel beetle speciesAgrilus assimilis,A. australasiae andA. hypoleucus.[36] The larvae of a number of butterfly species feed on the foliage including thefiery jewel,icilius blue,lithocroa blue andwattle blue.[37]Trichilogaster wasps form galls in the flowerheads, disrupting seed set[38] andAcizzia acaciaepycnanthae, apsyllid, sucks sap from the leaves.[39]Acacia pycnantha is a host to rust fungus species in the genusUromycladium that affect the phyllodes and branches. These includeUromycladium simplex that forms pustules andU. tepperianum that causes large swollen brown to black galls, which eventually lead to the death of the host plant.[40][41] Two fungal species have been isolated fromleaf spots onA. pycnantha:Seimatosporium arbuti, which is found on a wide range of plant hosts, andMonochaetia lutea.[42]
Golden wattle is cultivated in Australia and was introduced to the northern hemisphere in the mid-1800s. Although it has a relatively short lifespan of 15 to 30 years, it is widely grown for its bright yellow, fragrant flowers.[15][38] As well as being an ornamental plant, it has been used as a windbreak or in controlling erosion. Trees are sometimes planted with the taller sugar gum (Eucalyptus cladocalyx) to make a two-layered windbreak.[3] One form widely cultivated was originally collected onMount Arapiles in western Victoria. It isfloriferous, with fragrant flowers appearing from April to July.[43] The species has a degree of frost tolerance and is adaptable to a wide range of soil conditions, but it prefers good drainage.[43] It tolerates heavy soils in dry climates,[44] as well as mildsoil salinity.[45] It can suffer yellowing (chlorosis) in limestone-based (alkaline) soils.[3] Highly drought-tolerant, it needs 370–550 mm (10–20 in) winter rainfall for cultivation.[3] It is vulnerable to gall attack in cultivation.[46] Propagation is from seed which has been pre-soaked in hot water to soften the hard seed coating.[15]
Golden wattle has been grown in temperate regions around the world for the tannin in its bark, as it provides the highest yield of allwattles.[15] Trees can be harvested for tannin from seven to ten years of age.[3] Commercial use of its timber is limited by the small size of trees, but it has high value as a fuel wood.[47] The scented flowers have been used forperfume making,[15] and honey production in humid areas. However, the pollen is too dry to be collected by bees in dry climates.[3] In southern Europe, it is one of several species grown for the cut-flower trade and sold as "mimosa".[48] Like many other species of wattle,Acacia pycnantha exudes gum when stressed.[49] Eaten byindigenous Australians, the gum has been investigated as a possible alternative togum arabic, commonly used in the food industry.[2][49]
Wattles, and in particular the golden wattle, have been an informal floral emblem of Australia for many years (for instance, it represented Australia on theCoronation gown of Queen Elizabeth II in 1953). While some advocates forcefully argued for the adoption of thewaratah, during Australia's bicentenary in 1988 the golden wattle was formally adopted as thefloral emblem of Australia.[50] This was proclaimed by governor-general SirNinian Stephen (on the advice of theHawke government) in theCommonwealth gazette published on 1 September.[51] The day was marked by a ceremony at theAustralian National Botanic Gardens, which included the planting of a golden wattle byHazel Hawke, theprime minister's wife. In 1992, 1 September was formally declared "National Wattle Day".[15]
TheAustralian Coat of Arms includes a wreath of wattle; this does not, however, accurately represent a golden wattle. Similarly, thegreen and gold colours used by Australian international sporting teams were inspired by the colours of wattles in general, rather than the golden wattle specifically.[15]
The species was depicted on a stamp captioned "wattle" as part of a 1959–60Australian stamp set featuring Australian native flowers. In 1970, a 5c stamp labelled "Golden Wattle" was issued to complement an earlier set depicting the floral emblems of Australia. To markAustralia Day in 1990, a 41c stamp labelled "Acacia pycnantha" was issued.[15] Another stamp labelled "Golden Wattle", with a value of 70c, was issued in 2014.[52]
Clare Waight Keller included golden wattles to represent Australia inMeghan Markle'swedding veil, which included the distinctive flora of eachCommonwealth country.[53]
The 1970Monty Python's Flying CircusBruces sketch includes a reference, by one of the stereotyped Australian characters, to "the wattle" as being "the emblem of our land", with suggested methods of display, including "stick[ing] it in a bottle or hold[ing] it in your hand" — despite the wattle prop itself being a large, forked branch with sparse patches of leaves and generic yellow flowers.
