The divergent, curving spurs of Diascia whiteheadii differentiate it from D. longicornis that also has long and conspicuous spurs, borne closer together below the flower.
D. whiteheadii flowers grow solitary on slender pedicels from leaf axils, the pedicels remaining erect in fruiting. The small calyx is five-lobed.
The two-lipped and five-lobed corolla is mauve with a darker centre. Small yellow spots are windows near the spurs visible inside the corolla and out, occurring among several shades of mauve and red-purple. The spurs are less floral here, more like cow-horns. The corolla diameter is up to about 18 mm, the spurs from 18 mm to 22 mm long.
The blooming season is from late winter to after midspring. The photo was taken in September (Manning, 2007; Manning and Goldblatt, 1997; iNaturalist).