The flowers of Holothrix grandiflora grow in an erect spike of up to 40 flowers with bracts present below the flowers. The flowers are slightly spaced and facing somewhat towards one side of the stem. The sepals are up to 9 mm long and hairless.
The two petals are divided in from 5 to 9 thread-like, fleshy lobes each. The lip is divided into 13 to 26 thread-like, fleshy lobes. The broadly conical spur is up to 4,5 mm long and slightly curved. The flower colour is creamy white with some green or lilac.
Flowering happens late in summer until after midautumn and is not fire dependent.
The specific name, grandiflora, is derived from the Latin words grandis meaning large and florus meaning flowered, referring to the large flowers of the species (Vlok and Schutte-Vlok, 2015; Liltved and Johnson, 2012; Bond and Goldblatt, 1984; iNaturalist).