Vachellia is a genus of flowering plants in the legume family, Fabaceae, commonly known as thorn trees or acacias. It belongs to the subfamily Mimosoideae. Its species were considered members of genus Acacia until 2009. Vachellia can be distinguished from other acacias by its capitate inflorescences and spinescent stipules. Before discovery of the New World, Europeans in the Mediterranean region were familiar with several species of Vachellia, which they knew as sources of medicine, and had names for them that they inherited from the Greeks and Romans. The wide-ranging genus occurs in a variety of open, tropical to subtropical habitats, and is locally dominant. In parts of Africa, Vachellia species are shaped progressively by grazing animals of increasing size and height, such as gazelle, gerenuk and giraffe. The genus in Africa has thus developed thorns in defence against such herbivory. The members of Vachellia are trees or shrubs, sometimes climbing, and are always armed. Of the 163 species currently assigned to Vachellia, 52 are native to the Americas, 83 to Africa, Madagascar and the Mascarene Islands, 32 to Asia and 9 to Australia and the Pacific Islands.

Vachellia rehmanniana commonly known as Silky Thorn in English and Sydoring in Afrikaans is a distinctive small to medium sized tree to 12 m tall, with a spreading crown. It has conspicuously reddish brown bark on young trees becoming darker with age, and dense golden yellow hairs on the youngest branches. It usually occurs solitary in wooded grassland and bushveld on a range of soils, often clay rich such as in vlei margins. It is a southern African species, quite resistant to frost. It can be found in Miombo and similar woodlands as well as Vachellia woodland with Vachellia karroo.