This is one of South Africa's most beautiful and useful trees.
It is integrally part of our country's history having been used
for everything from raft-making to sewing needles and fencing for
the houses of the royal Zulu women. The thorns were even used by
early naturalists to pin the insects they collected! It is very
widespread throughout southern Africa and there are different forms
in some places, which can be confusing. Acacia karroo may
be found from the Western Cape through to Zambia and Angola. In
tropical Africa it is replaced by Acacia seyal. The name
Acacia is derived from Greek "akis" a point or
barb. Karroo is one of the old spellings of karoo which cannot
be corrected because of the laws governing botanical nomenclature
(giving of names).
It is found in a variety of habitats from low lying areas to highveld,
although not usually found in mist belt and montane areas. It is
an indicator of sweet veld which is prized for the good grazing
and fertile soils. If an area is overgrazed the sweet thorn becomes
Acacia karroo has a rounded crown, branching fairly low
down on the trunk. It is variable in shape and size, reaching a
maximum of about 12m where there is good water. The bark is red
on young branches, darkening and becoming rough with age. Sometimes
an attractive reddish colour can be seen in the deep bark fissures
The leaves are finely textured and dark green. The flowers appear
in early summer in a mass of yellow pompons. Many insects visit
and pollinate these flowers. The seed pods are flat and crescent
shaped, sometimes with constrictions between the seeds. They are
green when young becoming brown and dry. The pods split open allowing
the seeds to fall to the ground. The thorns are paired, greyish
to white and are long and straight. On mature trees, the thorns
may be quite short. They may be held at 90° to the stem or raked
forward slightly. Technically the thorns are called "spines"
and are developed from modified stipules (small, leaf-like scales,
seen at the base of the leaf-stalk). In some other thorny acacia
species, the thorns are not stipular in origin and are called "prickles".
These originate in the epidermis ("skin") and are always
short and curved, a bit like rose thorns. Thorns on African acacias
are important for identification, they are divided into 5 main groups
according the size, shape and position of the thorns.
The sweet thorn gets its common name from the gum which is exuded
from wounds in the bark. This pleasant tasting gum is eaten by people
and animals, including the Lesser Bushbaby which feeds exclusively
on insects and gum from trees, particularly acacia trees. It also
had commercial value in the past when the gum was exported as "Cape
Gum" for making confectionary. This is apparently similar to
gum arabic which is used as a water soluble glue.
It is a particularly good fodder tree, stock and game feed on the
leaves, flowers and pods. Seed dispersal takes place this way. There
is no danger of hydrocyanic poisoning which is a self-protection
mechanism used by many trees. The bark contains tannin which is
used to tan leather to a reddish colour. (This unfortunately gives
the leather an unpleasant odour). The heartwood is heavy and hard
but susceptible to attack from borer. This apparently may be prevented
by seasoning the wood in water for six months before use. The "Dune
Forest" form found along the coast of Kwazulu-Natal northwards
of the Tugela river has soft wood so would not be suitable for woodworking.
A strong rope can be made from the inner bark which is pliable enough
for rope-making when it is wet. The flowers produce lots of nectar
and pollen for bee-farming and the honey has a pleasant flavour.
In arid areas the sweet thorn is an indicator of water, both underground
and surface. It was a very welcome sight to early travelers and
Acacia karroo has a life span of 30-40 years and is an adaptable
pioneer, able to establishing itself without shade, shelter or protection
from grass fires. Once over a year old, seedlings can resprout after
fire. Several fungi are associated with this tree and the crown
of mature trees may be parasitized by various mistletoes, leading
to the tree's decline. This tree has a long taproot which enables
it to use water and nutrients from deep underground, this and its
ability to fix nitrogen, lead to grasses and other plants thriving
in its shade.
The sweet thorn has many medicinal uses ranging from wound poultices
to eye treatments and cold remedies. The bark, leaves and gum are
usually used. It is also used to treat cattle which have tulp poisoning
(Homeria spp - bulbous plants which are poisonous to stock).
Growing Acacia karroo
The sweet thorn makes a beautiful garden specimen. The bright yellow
flowers look very striking against the dark green foliage. The rough,
dark brown bark is also most attractive. The flowers are sweetly
scented and are renowned for attracting insects which are essential
to any bird garden. Birds also like to make nests in thorn trees
as the thorns offer them some protection from predators. Caterpillars
of 10 species of butterflies are dependant on the tree for survival.
These include, the club-tailed charaxes (Charaxes zoolina zoolina)
and the topaz-spotted blue (Azanus jesous).
In cold and dry areas the tree will be deciduous. The roots are
invasive, so avoid planting near paving or buildings. It is a most
useful tree for small holdings and farms where it can be planted
for shade and as a windbreak. The sweet thorn is very adaptable
to soil types and is frost and drought hardy. However, for best
performance, water well and deeply (shallow, frequent sprinklings
only encourage shallow root growth) until established. Plant with
plenty of compost, bonemeal or superphosphates (commercial tree
tablets also work well). The growth rate is fast, up to 1m per year.
It may be grown from seed which should be soaked in hot water and
left overnight. You will see if this has been effective as the seed
will swell up. Sow the following morning. Seedling trays with seedling
mix can be used, or the seeds could be sown directly into black
bags. Cover lightly with sand and do not allow to dry out. Germination
usually takes 3 - 12 days. The seedling will transplant well in
spite of the long tap root. Wait until they unfurl their second
leaves before transplanting.
- Barnes, R D et al : 1996 Acacia karroo. Tropical Forestry
Papers 32. Oxford Forestry Institute. Oxford.
- Joffe, P. 2001. Creative Gardening with Indigenous Plants: A
South African Guide. Briza. Pretoria
- Palmer, E and Pitman, N. 1972. Trees of Southern Africa: Volume
2. A.A.Balkema. Cape Town.
- Pooley, E. 1993. The Complete Field Guide to Trees of Natal,
Zululand & Transkei Natal Flora Publications Trust. Durban
- Skinner, J.D, Smithers, R.H.N. 1990. The Mammals of the Southern
African Subregion. University of Pretoria.
- Thomas, V and Grant, R. 1998. Sappi Tree Spotting, Highveld
and the Drakensberg. Jacana Education. Johannesburg.
- Venter, F and J-A. 1996. Making the most of Indigenous Trees.
Briza Publications. Pretoria.
- Van Wyk, B and van Wyk, P. 1997. Field Guide to Trees of Southern
Africa. Struik Publishers. Cape Town.
Alice Aubrey with additions by Yvonne Reynolds
Witwatersrand National Botanical Garden