Crossandra greenstockii is a low-growing, erect herb up to 300 mm tall, with unbranched, annual stems from a woody rootstock. Leaves are large, broadly ovate, narrowing to a stalk-like base, hairy, and in a basal rosette.

The inflorescence is a terminal, unbranched spike, 50 to 80 mm long. Bracts are broad, overlapping, glandular hairy, margins toothed with a recurved, sharp tip. Flowers are 5-lobed, about 12 × 20 mm, bright orange to red with a yellow centre. Flowers are borne during summer (from October to March). The fruit is a green capsule that becomes brown when matured, containing 4 seeds.

Crossandra greenstockii is similar to C. zuluensis (Zulu crossandra), which can be distinguished from C. greenstockii because it has smaller, oval leaves, with slightly wavy margins; it has hairless bracts with long spines; and the flowers are large, about 30 mm wide, 6-lobed, dark orange with a yellow centre, borne in in a short elongate inflorescence in spring and summer (from September to March).