The flowers of Karomia tettensis are axillary, emerging above the leaf connections on the branchlets, or terminal, at the branch tips. They grow from spring to autumn in sprays. The flowers may appear singly or in small clusters. The corolla is bright blue with a five-lobed lip, positioned on a saucer-shaped, shallowly five-lobed calyx that is mauve to pink.
The leaves are opposite, variable in size and shape, ranging from elliptic to obovate or oblanceolate and up to 11 cm long. Their margins may be entire, with shallow teeth or scallops. The South Africa species, Karomia speciosa, or southern Chinese hats, has more pronouncedly serrated or toothed leaves and the flowers have two-lipped corollas. The bark of Karomia tettensis is grey, or brown, rough and covered in lenticels. There may be spines on the branches (Coates Palgrave, 2002).