The large flower of Dietes grandiflora is characterised by prominent dark yellow nectar guides positioned on the lower halves of the three bigger (inner) tepals. The other (outer) three tepals are white with two thin parallel brown lines at the base. These plants flower well, justifying the grandiflora name for its big (10 cm in diameter) flowers on erect stems of 1 m tall, often blooming after summer rain.
The style is prominently branched into flattened blue or violet petal-like sections protruding above the inner tepals, and hiding the anthers.
The flowers last for only two or three days, rendering them just about unsuitable as cut flowers.
The large, oblong, green fruit capsules are borne erectly. They split open widely when ripe, releasing small dark seeds (Manning, 2009; Pooley, 1998; iNaturalist).