Vachellia karroo (Sweet Thorn)
Plant image

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.

Name & classification

Botanical name:
Vachellia karroo

Common names:
Sweet Thorn (E)
Soetdoring (A)
Mookana (NSo)
Mooka (Tsw)
umuNga (Z)
Leoka (SSo)
Singa (Tso)
Muunga (V)
Umnga (X)

Synonyms:
Acacia karroo

Plant family:
Mimosoideae
-

Plant categories:
Trees

SA tree no:
172
Zimbabwe tree no:
189

Name derivation & history:
Vachellia is in honour of George Harvey Vachell (1789 to 1839), chaplain to the British East India Company in Macoa (1825 to 1836) who collected plants in China.

Karroo is one of the old spellings of karoo which cannot be corrected because of the laws governing botanical nomenclature.

The sweet thorn gets its common name from the pleasant tasting gum which is exuded from wounds in the bark.

Pictures

Features

Leaf habit:
Deciduous

Height:
Up to 12 metres

Width:
8 metres

Has thorns

Plant shape:
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 12 metres where there is good water.

Leaf description:
The bipinnate leaves are finely textured and dark green.

Spine or thorn description:
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).

Flower description:
The flowers appear in early summer in a mass of bright yellow, 1½ cm pompons in clusters at the ends of branchlets.

Flower colour:
Yellow

Flowering months:
November to February

Flower scent:
Sweetly scented

Fruit description:
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. They remain on the tree long after flowering.

Bark or stem description:
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.

Other distinctive features:
A sweet-tasting gum is exuded from wounds in the bark.

Habitat

Natural distribution:
Vachellia karroo is found from the Western Cape through to Zambia and Angola.

Habitat:
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 invasive.

Water requirements:
Waterwise (little water required)

Frost tolerance:
Hardy

Light conditions:
Sun

Other Characteristics

Drought resistant
Wind resistant
Has aggressive roots
Low maintenance
Suitable for bonsai

Edibility & Toxicity

We have no confirmation that any part of this plant is edible
We have no confirmation that any part of this plant is toxic, however we urge caution as this information may be incorrect.

Ecology

Interaction with physical surroundings:
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.

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), Topaz-spotted Blue (Azanus jesous), Thorn-tree Blue, Velvet-spotted Blue, Black Heart, Common Hairtail, Mashuna Hairtail, Talbot’s Hairtail, Black-striped Hairtail and Common Scarlet.

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.

In arid areas the sweet thorn is an indicator of water, both underground and surface.

Acacia karroo has a lifespan of 30 to 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.

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.
Attracts birds
Attracts insects
Attracts mammals

Other information

Uses & Cultural aspects:
This pleasant tasting gum is eaten by people. 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. 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.

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 nomads.

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).

Cultivation:
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.

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 to 12 days. The seedling will transplant well in spite of the long tap root. Wait until they unfurl their second leaves before transplanting.

Pests & diseases:
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.

Notes:
Technically the Vachellia karroo 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.

References:
South African National Biodiversity Institute, South Africa.

Your notes & lists

You need to be a registered user in order to create plant lists and notes. If you have already registered, then please log in now, otherwise, register here. It will only take a moment and we won't pass your information on to anyone else.

You can then use this section to create lists of your own plants in your garden, or a list of favourite plants or simply a wish list