Costus dubius (Afzel.) K.Schum. — Plate, 1904

Maas-van de Kamer, H., Maas, P. J. M., Wieringa, J. J. & Specht, C. D., 2016, Monograph of African Costaceae, Blumea 61 (3), pp. 280-318 : 295-296

6. Costus dubius (Afzel.) K.Schum. — Plate 2a; Map 5

Costus dubius (Afzel.) K.Schum.(1904) 409. — Zingiber dubium Afzel.(1813) 9. — Type : A collection by Afzelius , Sierra Leone, without location. As no original material of Afzelius could be traced we here designate the following neotype from the same country: Pyne 88 (neo K; isoneo P), Sierra Leone, Moyamba District , Kori Chiefdom, Gbonjema, (‘Bonjema (Kori)’), 21 Dec. 1955.

Costus maculatus Roscoe (1825) 218,pl. 82;K.Schum.(1904) 408. — Costus afer var. maculatus (Roscoe) Baker (1898) 299, syn. nov. — Type: Roscoe’s plate 82 (1825).

Costus littoralis K.Schum. (1904) 395. — Type: Dinklage 1701 (holo B), Liberia, Grand Bassa, near Fishtown , 14 Aug. 1896.

(acumen 10–30 mm long). Inflorescence many-flowered, ovoid to ellipsoid or narrowly so, 5–20(–30) by 3–6 cm, terminating a separate leafless shoot 5–40(–100) cm long, or rarely terminating the leafy shoot; bracts, appendages of bracts, bracteoles, calyx, ovary and capsule glabrous, except for some erect hairs <1 mm long at the base of the calyx and apex of the ovary. Flowers 1 per bract, rarely 2; bracts green with reddish margins, sometimes becoming red in fruit, coriaceous, bulging, very broadly ovate, 1.5–3 by 2–3.5 cm, callus sometimes present and then to c. 2 mm long; appendages generally absent; bracteole boat-shaped, 20–25 mm long, callus 2–3 mm long; calyx 12–18 mm long, lobes broadly ovate-triangular to deltate, 3–5 mm long, callus very inconspicuous; corolla white to cream, 25–55 mm long, glabrous, tube 12–15 mm long, lobes narrowly elliptic, 25–40 mm long; labellum white, inner side with central yellow nectar guide, funnel-shaped, broadly obovate when spread out, 35–40 by 30 mm, margin crenate; stamen white, 25–30 by 8–9 mm, anther 6–8 mm long. Capsule ellipsoid to obovoid, 10–15 by 6–12 mm. Seeds 2–2.5 by 1.5 mm.

Distribution — North East Africa ( South Sudan); West Africa ( Burkina Faso, Ghana, Guinea, Ivory Coast, Liberia, Nigeria, Senegal, Sierra Leone, Togo); Central Africa ( Burundi, Cameroon, Central African Republic, Congo Brazzaville, Congo Kinshasa, Equatorial Guinea, Gabon); East Africa ( Kenya, Tanzania, Uganda); Southern Tropical Africa ( Angola, Malawi, Mozambique).

Habitat & Ecology — In primary rainforest, savanna forest, coastal forest, secondary forest, or swamp forest, in shady places, also along roads and rivers and in plantations, on clayish soil at elevations of 0–1400 m. Flowering and fruiting: all year through.

Field observations — Costus dubius plants are the only cultivated species of Costus often setting seed in greenhouses. Their seedlings germinate easily and tend to take over other pots with different species of Costus growing beside them.

IUCN Conservation Status — Least Concern.

Notes — Costus dubius can be recognized by its inflorescence generally near the base of the plant terminating a separate leafless shoot, inflorescence composed of 1-flowered green unappendaged bracts and flowers having a white labellum with a yellow nectar guide. The leaves of this species are glabrous and the shoot is often covered by reddish dots.

For the differences between C. dubius and C. afer see under C. afer .

For the differences between C. dubius and C. dinklagei , see under C. dinklagei .

Of the two syntypes of C. trachyphyllus (now in the synonymy of C. afer ) Schweinfurth 3268 (B destroyed, K), from South Sudan, Western Equatoria, ‘am Turu bei Uando’s Dorf’, 10 Mar. 1870, probably belongs to C. dubius , because of its white flowers mentioned on the label (‘fl. albo’).

As in many species of Costus , inflorescences are generally terminating a leafless shoot but can (rarely) terminate a leafy shoot. Plants with both types of inflorescences have both been included in the description.

Some specimens of C. dubius do have a very long ligule (1-lobed or obliquely truncate, acute, 25–30 mm long). These specimens are: Breteler 1209 from Cameroon, Ekwuno 12 from Nigeria and Harris 3211 from Congo.

The collection by Friedmann 3919 has been found East of the African continent on the Seychelles, on the island of Mahé. Because on the label is written: ‘subspontanée’, we suppose this collection might not represent a natural distribution.