Acacia robusta Burchell, Fabaceae

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Present on Pacific Islands?  yes

Primarily a threat at high elevations?  no

Risk assessment results:  High risk, score: 7 (Go to the risk assessment).

Common name(s): [more details]

English: splendid acacia, splendid thorn

Habit:  tree

Description:  "Shrub or small to medium-sized tree up to 20 (-25) m tall; root system moderately deep, with strongly spreading lateral roots; bole up to 70 cm in diameter; bark smooth to fissured, grey-brown to dark brown or black; crown spreading; young branches smooth, with paired, straight stipular spines up to 6 (-11) cm long. Leaves alternate, bipinnately compound, with 2-6 (-10) pairs of pinnae; rachis 2.5-7 cm long, pubescent or glabrous; leaflets in 9-27 pairs per pinna, obliquely oblong, (2-) 3.5-13 (-16) mm x 1-5 (-7) mm, glabrous or ciliate on the margins. Inflorescence an axillary globose head 9-12 mm in diameter, usually in clusters; peduncle 2-2.5 cm long, with a pair of small bracts in basal half. Flowers bisexual, regular, usually 5-merous, small, creamy white, fragrant, sessile; calyx cup-shaped, with short lobes; corolla shortly lobed, glabrous; stamens numerous, free, up to 5 mm long; ovary superior, 1-celled, style slender. Fruit a linear pod 7-19 cm x 0.5-3 cm, straight or curved, more or less woody, glabrous, brown, longitudinally veined, dehiscent, up to 15-seeded. Seeds quadrangular to ellipsoid, compressed, 8-15 mm x 5-9 mm, smooth. Acacia robusta is variable, and is subdivided into 3 subspecies: subsp. clavigera (E.Mey.) Brenan (synonym: Acacia clavigera E.Mey.), which may develop into a medium-sized tree and is characterized by its pubescent leaf rachis and curved, comparatively thin pods; subsp. robusta, which remains a small tree (up to 8 m tall) and is characterized by its glabrous leaf rachis and straight, comparatively wide pods; and subsp. usambarensis (Taub.) Brenan (synonym: Acacia usambarensis Taub.), which is characterized by its glabrous leaf rachis and very narrow pods. Intermediates between the subspecies are rather common" (Plant Resources of Tropical Africa).

subsp. clavigera (E. Mey.) Brenan:  "Tall near-evergreen tree. Thorns straight and short, c. 1 cm, in pairs at the nodes. Leaves with 3-7 pairs of pinnae; leaflets in 10-25 pairs per pinna, small, dark green, paler below. Flowers in axillary clusters of white spherical heads. Pods slightly sickle-shaped, thickened but not woody, dehiscent"

subsp. robusta:  This subspecies is distinguished by the hairless leaf rhachis; 2-4 pairs of pinnae with 10-15 pairs of leaflets; often wider leaflets, up to 5(7) mm wide; pods are usually straight or nearly so and 2-3 cm wide. (Flora of Zimbabwe online).

Habitat/ecology:  "Occurs in woodland and wooded grassland, often near rivers, where large specimens can be found, up to 1800 m altitude. The tree is drought and frost resistant" (Plant Resources of Tropical Africa). In Zimbabwe, "in low altitude riverine fringes and other areas near water. Up to 1000 m"  (Flora of Zimbabwe online).

Propagation:  Seed.  "Dispersal of the seeds is probably by browsing animals"  (Plant Resources of Tropical Africa).

Native range:  "Ethiopia and Somalia south to Namibia and northern and eastern South Africa. It has been introduced elsewhere, e.g. in South Asia"  (Plant Resources of Tropical Africa).

Presence:

Pacific
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
State of Hawaii Hawaiian Islands	O‘ahu Island	introduced invasive cultivated	Lau, Alex/Frohlich, Danielle (2013) (p. 14) subsp. clavigera (F. Mey.) Brenan. Voucher cited: D. Frohlich & A. Lau 2012031205 (BISH) Spreading from cultivation in a botanical garden.

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia Australia (continental)	Australia (continental)	introduced cultivated	Randall, R. P. (2007) (p. 15)

Control:  If you know of control methods for Acacia robusta, please let us know.