The flower of Dietes grandiflora is characterised by the prominent dark yellow nectar guides on the three bigger (inner) tepals. The other (outer) three are white with two thin parallel brown lines at the base. The style is prominently branched into flattened blue or violet petal-like sections protruding above the tepals and hiding the anthers. The flowers last for only two or three days, rendering them just about unsuitable as cut flowers. These plants flower well, however, justifying the grandiflora name with big (10 cm in diameter) flowers on erect stems of a metre tall, often appearing after summer rain.
The plant is a rhizomatous perennial with a tuft of long fibrous sword-like leaves arranged in a fan shape. The oblong green seed capsules eventually split open to release small dark seeds.
Dietes refers to the plant’s dual resemblance links to both the genera of Iris and Moraea. The plant is found in coastal parts of the Eastern Cape and Kwazulu-Natal among evergreen scrub (Manning, 2009).