Costus convexus sp. nov. (Fig. 10) shares with C. glaucus Maas the presence of glaucous shoots and leaves especially in young plants, but differs from that species by a globose inflorescence enveloped by the upper leaves and by the presence of distinctly convex bracts.
3–5 m tall, young shoots and young leaves glaucous. Leaves sheaths ca. 20 mm diam; ligule truncate to obliquely truncate, 20–45 mm long; petiole 20–22 mm long, blushed in pink; sheaths, ligule and petiole glabrous; lamina narrowly elliptic, 31– 57 × 12–16 cm, adaxially pale green and glabrous, abaxially green or purple in very young plants, glabrous to sparsely puberulous, base acute to slightly cordate, apex acuminate (acumen ca. 15 mm long). Inflorescence globose to ovoid, 8–20 × 5–8 cm, wrapped tightly by the upper leaves, terminating a leafy shoot; bracts, bracteole, calyx, ovary, and capsule glabrous. Flowers adaxially oriented to erect; bracts distinctly convex, reddish on the outer margins, greenish at the centre, coriaceous, ovate, 55–65 × 40–45 mm, apex obtuse, callus green, indistinct; bracteole boat-shaped, 40–45 mm long; calyx red, 15–25 mm long, lobes shallowly triangularto deltate, 4–10 mm long; corolla deep pink, 80–85 mm long, glabrous, lobes 50–60 mm long; labellum yellow, distal edge horizontally spreading, broadly obovate, 60–70 × 60–70 mm, lateral lobes strongly striped with pink or red, middle lobe reflexed,with yellow honey mark, margin irregularly dentate to lobulate; stamen dark pink,30–40 × 12–15 mm, not exceeding the labellum, apex obtuse, anther 12–15 mmlong. Capsule ellipsoid, 12–30 mm long.