Knob Thorn [Acacia nigrescens]
It grows 5-18 m in height and is fire-resistant. The common names in English and Afrikaans refer to the very characteristic thorns, which are knobbed. The knobs, which are borne on the trunks and branches, are occasionally lacking in individual specimens. The leaves are double pinnately compound. The plants are deciduous and the trees are usually leafless for the duration of the winter and early spring. The leaf form is characteristic of this species. There are only 4 -6 leaflets per leaf and these are comparatively large with oblique (lopsided) bases. The rachis is recurved and bears small spines. New leaf growth is sometimes bright red.
Flowers and Fruit
Flowering is erratic and occurs between August and November. The scented flowers are borne in spikes about 100 mm long. In the bud stage the flowers are reddish-brown in colour, turning cream-white when fully open. The fruit is a pod about 100 mm long and 13-25 mm broad (the specific name 'nigrescens' means 'becoming black' and probably refers to the pods, which turn black as they ripen). They are eaten by Giraffe and other animals. Elephant eat the branches, leaves and shoots, Kudu browse the leaves and shoots and Giraffe, Monkey and Baboon eat the flowers.
The Knob Thorn trees are used by hole nesting bird species. The wood is hard and termite-resistant. It has been used to make posts and mine props. The species is drought-resistant but frost-tender. It also makes a good bonsai subject. It is a very hard timber which is very durable. It is not regularly used for furniture. It has exceptional impact toughness, thus good for Parquet flooring material.
The knobbed thorns of the Acacia nigrescens are thought to be for protection against Elephants destroying the bark, and in fact in some areas where Elephants no longer occur the thorns do seem to have receded.
Where Knob Thorn Are Found
Knob Thorn have a wide distribution, occurring in the drier parts of southern Africa as far north as Tanzania, often on deep, sandy soils and commonly in widely-spaced stands. It is a familiar sight to visitors to the Kruger National Park.