Scientific name

Acacia podalyriifolia A.Cunn. ex Don

Synonyms

Racosperma podalyriifolium (A.Cunn. ex Don) Pedley; Acacia fraseri Hook.; Acacia podalyriifolia var. viridis; Acacia podalyriifolia var. typica

Common names

Queensland silver wattle, Mt Morgan wattle, golden wattle, pearl acacia, pearl wattle, Queensland wattle, silver wattle

Family

Fabaceae (Leguminosae): sub-family Mimosoideae

Origin

Australia

Naturalised distribution (global)

Locations in which Acacia podalyriifolia is naturalised include Australia beyond its native range, southern and eastern Africa, the Indian sub-continent and south-east Asia, on some Indian Ocean islands, in New Zealand, in South America, and in in California in south-western USA.

Introduced, naturalised or invasive in East Africa

Acacia podalyriifolia is naturalised in parts of Kenya, invasive in parts of Tanzania and has been introduced to Uganda (A.B.R. Witt pers. obs.).

Habitat

Roadsides, urban open spaces and in riparian zones (banks of watercourses).

Description

An upright (erect) and spreading shrub or small tree usually growing 2-6 m tall, but occasionally reaching 10 m in height. It is relatively short-lived, usually only having a lifespan of 10-15 years.

The bark on the main trunk is grey or greyish-brown in colour and either relatively smooth or finely fissured. Younger branches are round or almost round in cross-section (terete or sub-terete) and velvety in nature, with a dense covering of soft spreading hairs (pubescent). They are also conspicuously whitish or bluish-green (glaucous) in colour due to the presence of a powdery substance (they are pruinose).

The 'leaves' of this plant are actually flattened and widened leaf stalks (petioles), and not leaves in the true sense of the word. These modified leaf stalks are called phyllodes, but serve the same function as a regular leaf. On very young plants, partially formed phyllodes can be seen which bear twice-compound (bipinnate) leaves at their tips, each with a single pair of branchlets (pinnae) bearing numerous leaflets (pinnules). As the seedling grows, each new 'leaf' has phyllodes that are more fully formed and the leaves at their tips eventually vanish altogether.

The phyllodes are alternately arranged along the stems, relatively thin, and broad (elliptic, oblong or ovate - egg-shaped in outline with broad end at base), generally being only 1.4-2.5 times longer than they are wide. These phyllodes (20-50 mm long and 10-27 mm wide) are relatively straight, sometimes with slightly wavy margins, and usually silvery-grey to bluish-green (glaucous) in colour (particularly when they are young). They usually only have a sparse covering of velvety hairs on both surfaces, but these hairs are much more apparent along the margins. However, sometimes the phyllode surfaces are almost hairless (sub-glabrous), particularly as they become older. Older phyllodes are occasionally dull green in colour and are sometimes covered in a black sooty mould. Each phyllode has a single prominent vein (midrib) that is usually slightly closer to the upper margin. The tips of the phyllodes are rounded (obtuse) to pointed (acute) in nature, but usually also have a tiny pointy extension (they are mucronate). At the base of each phyllode there is a very short structure (1-2 mm long) that looks like a leaf stalk (pulvinus). On the upper edge of the phyllode, between 5 and 25 mm above its base, there is a tiny raised structure (gland).

The small bright yellow or golden-yellow flowers are fluffy in appearance due to the presence of numerous stamens. These flowers have five relatively inconspicuous petals and sepals and are densely arranged into small globular clusters (5-8 mm across) containing 15-30 flowers. The small globular flower clusters are borne on short slender stalks (peduncles) 3-10 mm long and are alternately arranged on a short branch (2-11 cm long) emanating from the forks (axils) of the upper leaves. These much larger elongated compound clusters usually contain 8-22 of the small globular flower clusters (they form inflorescences resembling racemes) and are usually significantly longer than the leaves. The flowering branches and flower stalks (peduncles) are silvery-grey in colour and velvety in nature, like the other branchlets.

The fruit is an oblong or somewhat elongated pod (50-120 mm long and 10-22 mm wide) that is borne on a short stalk and has prominent margins. These pods are flattened and relatively thin, straight but often somewhat twisted or with undulating margins. They are velvety hairy (pubescent), bluish-green or silvery-grey in colour, and covered in a whitish bloom (pruinose) when young. The pods become hairless (glabrous) and turn brown or reddish-brown in colour as they mature. When fully mature they split open on one side to release several seeds. The seeds are black in colour, oblong in shape (6-7.5 mm long and 3-4 mm wide), dull in appearance, and have a fleshy structure (aril) attached to them.

Reproduction and dispersal

Acacia podalyriifolia reproduces through its long-lived seed that have a prolonged dormancy. Germination of these seeds peaks after fires and other forms of disturbance. The seeds are most commonly dispersed by ants and birds, which are attracted by the fleshy arils attached to the seeds. However, they are probably also spread by wind and water.

Economic and other uses

A widely cultivated and fast growing species which is highly decorative on account of its masses of bright golden, perfumed heads and normally silvery grey to glaucous phyllodes. However, this use cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Acacia podalyriifolia is regarded as an environmental weed in New South Wales, Victoria, South Australia and Western Australia. In South Africa, where it is relatively widespread, A. podalyriifolia competes with and has the potential to replace indigenous species and is therefore regarded as a 'potential transformer' of natural vegetation.

A. podalyriifolia has been listed as a Category 3 invader in South Africa (no further planting is allowed - except with special permission - nor is trade in propagative material. Existing plants must be prevented from spreading).

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Young plants can be pulled out by hand. Cut stump or basal bark methods (painting herbicide onto the bark) using a suitable herbicide can be used on larger plants and foliar sprays can be applied to small plants. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

The editors could find no information on the prospects for biological control of this species.

Legislation

Not listed as a noxious weed by any state or government authorities in Kenya, Tanzania or Uganda.

References

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Lazarides, M., Cowley, K. and Hohnen, P. (1997). CSIRO Handbook of Australian Weeds. CSIRO Publishing, Collingwood, Victoria.

Randall, R.P. (2002). Global Compendium of Weeds. R.G. and F.J. Richardson, Victoria, Australia.

Starr, F., Starr, K. and Loope, L. (2003). Acacia podalyriifolia. Queensland silver wattle. Fabaceae. Plants of Hawaii: Reports. www.hear.org/starr/hiplants/reports/index.html. United States Geological Survey, Biological Resources Division, Maui, Hawaii.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: [email protected]