Carissa are tough evergreen shrubs from dry woodland and beaches in eastern and southern Africa. They belong to the Apocynaceae family of plants, which includes Trachelospermum, Mandevilla, Catharanthus, Plumeria and Vinca. This species, sometimes called the Natal plum, has rounded, leathery leaves with sharp spines among them. It seems to be incredibly drought tolerant: mine survives in a hot, dry garden that is never watered by me. It is also said to be tolerant of salt-laden winds, as well as part-shade.
Starry white, fragrant flowers appear in late summer (sometimes earlier) and these may be followed by crimson fruit about 3 cm in length and resembling a small plum. My shrub has only reached about 1.5 m tall but 2 m wide in 15 years - the ultimate height of the species is said to be around 3-4 m, perhaps if it is looked after better than mine is; alternatively perhaps mine is one of the compact cultivars, such as 'Emerald Carpet'. The shrub can grow quite wide - mine is 2 m across. There is also a prostrate cultivar, 'Horizontalis'.
Generally speaking, this genus is frost sensitive, but copes with our Sydney winters well. In South Africa, the plant is often apparently used as a hedge to keep out predators with its spines. I have found that low-hanging branches may take root where they touch the ground; these can be dug up and potted to make a new plant. To keep it neat, prune wayward branches to shape.