The flowers of Asclepias crassinervis grow on thick, dull-cream stalks in flat-topped, corymb-shaped clusters from stem-tips. The peduncles and pedicels are hairy.

The five corolla lobes are pointed, spreading to reflexed away from the flower centre. The lobes are brown near their margins in picture, white in the centres.

The erect, five-lobed corona in the flower centre arises around the staminal column base. The lobes are purple on the outside, partly white within. Their pointed tips curve in like hoods, slightly taller than the staminal column (Leistner, (Ed.), 2000; iNaturalist).