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Knob Thorn

The knob thorn (Acacia nigrescens) brings a beautiful colour to the landscape at this time of the year. Knob thorns are plentiful in the Lowveld, their yellow flowers drawing a distinctive contrast to the dull grey appearance of the bush veld at the end of autumn.

The knob thorns seem to be particularly abundant in their flowering this season, perhaps a result of the past summer’s good rains? The giraffes, baboons and monkeys eat the scented flowers. Elephants enjoy eating the branches. Elephants and kudus browse the leaves and shoots. The pods are also eaten by game.

It is a tree that can grow as large as 18 metres in height. A key attribute to its successful survival in the savanna habitat is that is has evolved with fire and is therefore fire-resistant. However, it is frost sensitive. Being deciduous it sheds its leaves during the winter months. New leaves grow in late spring and can even be red in colour as the tree begins to leaf up. Its thorns are characteristic in that they are knobbed at the base, hence the name Knob thorn, the knobby thorns occur on the branches and stems of the trees.

Knob thorns are drought resistant and their hard wood makes them termite resistant too. The wood is used for fence posts, home building material and has even been used for mine props and railway sleepers. It is also excellent fire wood. A variety of birds use the tree for nesting, especially those that use holes in trees to nest. Leaves are double pinnately compound with 4-6 leaflets per leaf.

The knob thorn is listed in the 2009 Red Data Listing, with a “no threat” status. Basic research shows that knob thorns have high tolerance levels to intense browsing pressure, but that such targeted trees show a decline in pod production. A tree that the knob thorn may be confused with by a viewer is Zanthoxylum spp, a collection of knobwoods.

A dozen interesting facts about the knob thorn:

  • They occur as far north as Tanzania
  • They prefer deep sandy soils that are well drained
  • The knobs can sometimes be absent in individual trees
  • Knob thorns make good bonsai specimens
  • The Zulu name is umKhaya, and in Afrikaans, knoppiesdoring
  • Acacia in Latin means “thorny” and originates from the Greek word akakia
  • Nigrescens in Latin refers to the colour black.
  • The flowers contain a considerable amount of condensed tannins
  • They host the larvae of the dusky charaxes butterfly
  • It almost always has a single trunk
  • It is seldom used for making furniture as the wood is difficult to work and cut
    It is a very good wood for making “jukskeie”

Close up of Knob Thorn flower

Characteristic knobs on stems

Close up of flowers & leaf arrangement

Facts researched on the internet, words by John Llewellyn.