Senegalia nigrescens (Knob Thorn)
Plant image

Acacia nigrescens is a deciduous, small to medium-sized tree with a long cylindrical shape and rounded crown. It has knobs on the trunks and on branches with persistent thorns arising on the knobs.

Name & classification

Botanical name:
Senegalia nigrescens

Common names:
Knob Thorn (English); Knoppiesdoring (Afrikaans); muunga (Tshivenda); mooka (Sepedi); umKhaya (isiZulu)

Synonyms:
Acacia nigrescens, Acacia pallens

Plant family:
Fabaceae
- Pod-bearing family, legumes, peas & beans

Plant categories:
Trees

SA tree no:
178
Zimbabwe tree no:
195

Name derivation & history:
Nigrescens means 'becoming black' and is thought to refer to either the colour of the thorns or the pods.

Its common names in English and Afrikaans refer to the very characteristic thorns borne on knobs on the stem.

Pictures

Features

Leaf habit:
Deciduous

Height:
5 to 18 metres

Plant shape:
The knob thorn is a small to medium tree with a long cylindrical shape and rounded crown.

Leaf description:
The leaf consists of 2 or 3 pairs of pinnae (primary divisions of a compound leaf) with 1 or 2 pairs of leaflets per pinna.

Spine or thorn description:
Thorns are in pairs below the leaves

Flower description:
Flowers are yellowish-white and form in elongated spikes. They appear before or with the new leaves making the tree very conspicuous.

Flowering months:
August to November

Fruit description:
Fruit are dark brown, thinly textured pods borne in pendant (hanging downward) clusters.

Bark or stem description:
It has knobs on the trunks and on branches with persistent thorns arising on the knobs. The trunk is approximately 0.5 metres in diameter on mature specimens but can reach 0.75 metres.

Habitat

Natural distribution:
Knob thorn is a species with a wide distribution range, occurring from Tanzania southwards to KwaZulu-Natal. It is a familiar sight to visitors to the Kruger National Park.

Habitat:
Knob thorn is often on deep, sandy soils and commonly in widely-spaced stands.

Water requirements:
Waterwise (little water required)

Frost tolerance:
Semi-tender (tolerates some cold but not freezing)

Light conditions:
Sun

Other Characteristics

Drought resistant
Wind resistant
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:
Acacia nigrescens flowers are a dietary component for giraffes. Its flowers contain almost three times as much condensed tannin as leaves. Giraffes consume large quantities of flowers resulting in distinct browse lines on the trees.

Knob thorn leaves and pods are also included in the diet of elephant, kudu, duiker, impala and steenbok.

Knob thorn trees are the host of hole-nesting bird species and the larvae of the dusky charaxes butterfly.
Attracts birds
Attracts insects
Attracts mammals

Other information

Uses & Cultural aspects:
The wood is hard and drought and termite-resistant but frost-tender; it has been used to make fence posts and mine props.

The knob thorn yields good quality firewood producing lasting coals and severe heat.

It also makes a good bonsai subject.

It is not regularly used for furniture because it is difficult to cut.

Cultivation:
Acacia nigrescens is an attractive garden tree which grows into a lovely shade tree.

It can be cultivated from seed soaked in hot water and left overnight. Then it can be planted in separate plastic bags the following day. When a seedling develops a taproot, it must be planted in a larger bag. Seedlings are easily transplanted.

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