Senegalia is a genus of flowering plants in the family Fabaceae. It belongs to the Mimosoid clade. Until 2005, its species were considered members of Acacia. The genus was considered polyphyletic and required further division with the genera Parasenegalia and Pseudosenegalia accepted soon after. Senegalia can be distinguished from other acacias by its spicate inflorescences and non-spinescent stipules. Plants in the genus are native to the tropical and subtropical areas of the world, occurring on the Australian, Asian, African and South and North American continents, as well as in Wallacea.

Senegalia nigrescens, previously known as Acacia nigrescens and commonly known as Knob Thorn in English and Knopdoring or Knoppiesdoring in Afrikaans, is a South African indigenous species found in the Gauteng, KwaZulu-Natal, Limpopo, Mpumalanga and North West Provinces. In open soil it grows from 5 to 18 meters in height and as the common names in English and Afrikaans denote, this tree has attractive, very characteristic thorns, which are knobbed. Senegalia nigrescens is deciduous and the tree is usually leafless for the winter period and early spring. The leaf structure is distinctive of this species. There are only 4 to 6 leaflets per leaf and these are relatively large with tilted bases. Sometimes, when the new leaves start forming on the tree in spring, the new growth is bright red.