|
Karoo Thorn and Giraffe Thorn: State prohibited weeds | LC0378 |
Ian Faithfull (DPI Frankston) and Kate Blood (DPI Beaufort)
February 2004 |  |
To view the Adobe Acrobat file, you will need the Adobe Acrobat reader. |  (PDF 420 kb) | |
This Landcare Note describes Karoo thorn, Acacia karroo and giraffe thorn, Acacia erioloba, State Prohibited Weeds in Victoria.
Common and scientific names
Acacia karroo Hayne, Karoo thorn. Also known as sweet thorn, Cape gum, gum arabic tree, mimosa thorn, sour thorn, whitethorn, umbrella thorn and mirros acacia. Other botanical names used include Acacia dekindtiana, A. hirtella, A. horrida, A. inconflagrabilis and A. natalitia
Acacia erioloba E. Mey., Giraffe thorn. Also known as camel thorn, kameeldoring, mimosa, and Transvaal camelthorn. Another botanical name is Acacia giraffae.
Family Mimosaceae.
Status
State Prohibited Weeds in Victoria, to be eradicated from the State. Serious weed risks, prohibited from entry to Australia by the Australian Quarantine and Inspection Service. Karoo thorn is on the National Environmental Weed Alert List. Cultivated in botanic gardens and zoos in rural Victoria and metropolitan areas, and similar situations in other States. Possibly present in other public and private gardens.
Origin
Southern Africa.
Description
Karoo thorn:
A shrub or tree to 25 m tall, with paired, straight, shining white thorns; typically with a somewhat rounded crown and often more than one main stem; very variable in its features; fast growing.
Roots – often very dense and robust, long and extensive on the surface or deep penetrating to ground water.
Stems – young branchlets bright green, not easily broken, epidermis flaking off to expose dark rusty-red inner layer; older stems covered with dark red-brown to blackish, rough bark or greyish smooth bark, and with numerous long, straight, sweet-tasting, shining white, dark-tipped spines (stipules) up to 25 cm long and 1 cm diameter, but usually about 7 cm long, in v-shaped pairs, most numerous on the lower branches and sometimes persisting on the trunk.
Figure 1. Karoo thorn tree.
Figure 2. Thorns, old pods and foliage of Karoo thorn.
Leaves – fern-like (bipinnate), with the main axis 1 to 5 cm long, usually bearing 2 to 7 pairs of main branches (pinnae), each having 5 to 15 pairs of oblong leaflets (pinnules), each leaflet 3.5-8 mm long and 1-2.5 mm wide. Glands present on the leaf axis at the base of each or some pinnae pairs.
Flowers – clusters of deep golden-yellow balls up to 12 mm diameter, each ball containing about 90 small flowers and on a stalk 0.7-2.4 cm long; the stalk of the flower cluster originating at the point where the leaf joins the stem; occurring on the current season's growth; sweetly scented. Plants may flower several times during the summer.
Fruit – slender, woody, sickle-shaped, smooth, resinous, brown or black pods, up to 16 (commonly 5-10) cm long and 10 (commonly 3-7) mm wide, slightly constricted between the seeds; ripen from autumn into winter and split open, exposing the seeds which often stay attached and hang by a thread-like stalk.
Seeds – 4.5-6.5 x 2-3.5 mm, elliptic or lens-shaped, compressed.
Fig. 3. Karoo thorn, Addo National Park, South Africa.
Fig. 4. Foliage and green pods of Karoo thorn.
Fig. 5. Flowers of Karoo thorn.
Fig. 6. Foliage and thorns of Karoo thorn.
Giraffe thorn:
2 m spiny shrub to 16 m tree with wide, rounded or umbrella-shaped crown and thick, hairy pods shaped like a half moon. Much slower growing than Karoo thorn.
Roots – very long taproot.
Stems – shiny reddish brown when young; branchlets grey, reddish brown or purplish; bark grey to blackish-brown, deeply furrowed; bearing pairs of almost straight whitish or brown spines (stipules) 0.5-6 cm long (rarely to 10 cm); the spines shorter and squatter on young parts where sometimes present at the base of each leaf, and larger and sharper elsewhere on the stems where often swollen and fused together at the base.
Leaves – bipinnate, with the main axis 1.5-5 cm long, and 2-5 pairs of main branches (pinnae) 1.3-4.2 cm long, each with 8 to 18 pairs of small leaflets (pinnules); each leaflet 4-13 mm long and 1-4 mm wide; a small gland at the junction of each pair of pinnae.
Flowers – in bright golden-yellow balls, each ball on a narrow stalk 1.8 to 4 cm long arising at the base of the leaf, 1-4 balls per leaf; sweetly scented; winter to spring.
Fruit – variable; small and almost cylindrical to typically large, flat, thick, semicircular or half-moon shaped pods, up to 13 cm long and 5 cm wide, covered in velvety grey hairs, semi-woody, but spongy inside; the pods do not open, even when ripe but fall to the ground in winter.
Seeds – 8-14 x 7-10 mm, thick, robust, lens shaped to elliptic.
Fig.7. Giraffe thorn in the Kalahari National Park, South Africa.
Similar species
The presence of both bipinnate foliage and sharp pointed spiny stipules over 15 mm long enables the two species to be distinguished from all Australian native acacias. However two other acacia species found in Australia have these characteristics. Acacia nilotica subsp. indica, prickly acacia, a native of India, is naturalised in tropical Queensland and parts of the Northern Territory and South Australia and is a Weed of National Significance. It has pods that are usually markedly constricted between the seeds, like a necklace. Acacia farnesiana, dead finish, is widespread in northern Australia and is believed to have been introduced from Central America via the Philippines before European settlement. The leaves have a gland (the petiolar gland) on the leaf stalk before the first side branches of the leaf.
In Africa A. karroo hybridises with A. rehamanniana and possibly with A. tenuispina. A. erioloba hybridises with A. haematoxylon but has no close relatives of similar appearance.
Properties
Karoo thorn is the most widespread tree in Southern Africa and is found in a diverse range of climates and soils. It has become a weed in Europe, South America (Bolivia and Chile) and in its country of origin, where it is the most important woody invader of grasslands. It has the potential to invade and form monocultures across southern Australia, particularly in open grasslands and rangelands in the south-east. The spines injure and impede grazing animals, render thickets impenetrable and make the plant difficult to handle.
The seeds either fall or are eaten by animals including birds and are then carried internally and deposited in excreta. They are long-lived, with buried seeds remaining viable for up to 7 years and stored seeds surviving over 57 years. Large trees may produce up to 19,000 seeds per year. Plants have been recorded to live for 40 years.
Karoo thorn is very adaptable and can tolerate a range of conditions, including frost, drought, fire, salinity, wind and salt spray. In its natural range, it grows from sea level to 1800 m on soils ranging from sand to heavy clays, in areas with an annual rainfall from 1500 mm, to less than 200 mm where ground water is available along drainage lines.
Karoo thorn has been promoted and grown for various uses including fodder, pollen and nectar, tan bark, rope, gum, medicine, timber, firewood, shade, fencing, hedging and ornament. The pods are sometimes used as stock fodder but may on occasion be poisonous. It reshoots after cutting or if burnt by fire.
Giraffe thorn is a relict of the parental stock of African Acacia species and is one of the major trees, and frequently the only sizeable tree, of the deserts of southern Africa. It is a long-lived plant that grows on sands in areas with annual rainfall from less than 40 mm to 900 mm, and tolerates mean daily temperatures from at least 15C to over 45C and severe frosts. In very dry areas giraffe thorn occurs along watercourses or where underground water is present. The taproot can descend to 60 m, providing access to deep ground water.
The thorns place it amongst the cruelest of all the thorny acacias. The pods are useful fodder for cattle in Africa, the timber is strong and makes good firewood, and the plant has various other uses. The seeds germinate under suitable conditions after passing through the digestive tract of large grazing mammals, which disperse the seed in their dung. Trees start to flower when about 10 years old.
Giraffe thorn is a competitive species that can displace preferred vegetation. It has been assessed as potentially very highly invasive in Australia. Climate predictions indicate that it could occupy large areas of inland and northern Australia if allowed to spread.
Management
The Departments of Sustainability and Environment (DSE) and Primary Industries (DPI) are responsible for the control of State Prohibited Weeds. Please provide details of any occurrences of these species to a Catchment Management Officer at a local DSE/DPI office.
References
- Barnes, R. D., Fagg, C.W. and Milton, S. J. (1997) Acacia erioloba Monograph and Annotated Bibliography. Tropical Forestry Papers 35, Department of Plant Sciences, University of Oxford.
- Barnes, R. D., Filer, D. L. and Milton, S. J. (1996) Acacia karroo Monograph and Annotated Bibliography. Tropical Forestry Papers 32, Department of Plant Sciences, University of Oxford.
- Coates Palgrave, K. (2000) Trees of Southern Africa. Cape Town, C. Struik Publishers.
- Csurhes, S. and Edwards, R. (1998) Potential Environmental Weeds in Australia. Candidate Species for Preventative Control. Canberra, Environment Australia.
- Fabian, A. and Germishuizen, G. (1982) Transvaal Wild Flowers. Johannesburg, MacMillan South Africa.
- Groves, R. H. (convenor) (1997) Recent Incursions of Weeds to Australia 1971-1995. Adelaide, Cooperative Research Centre for Weed Management Systems.
- Randall, R. P. (2002) A Global Compendium of Weeds. Melbourne, R. G. and F. J. Richardson.
- Ross, J.H. (1975) Fabaceae. In J.H. Ross (Ed.), Flora of Southern Africa 16 (1), 159 pp.
- Scott, J. K. (1991) Acacia karroo Hayne (Mimosaceae), a potentially serious weed in Australia. Plant Protection Quarterly 6(1), 16-18.
- van Wyk, B. and van Wyk, P. (1997) Field Guide to Trees of Southern Africa. Cape Town, Struik Publishers.
Acknowledgements
Fig. 1 by Franz Mahr; Fig. 2 by Kate Blood; Figs. 4 and 6 by Sarah Keel; Figs. 3, 5 and 7 by Robin Adair. Linda Iaconis and John Weiss provided reference materials.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
|
