Perennial subshrub 1.2–2 m tall. Stems quadrangular, pubescent, more densely so at nodes with eglandular, spreading or retrorse hairs and reddish sessile glands; inflorescence axis also with shorter glandular hairs; young shoots tomentose. Leaves petiolate; blades paler beneath, ovate, 50–120 mm long; 25–40 mm broad, shallowly serrate, base rounded to cuneate, sometimes asymmetric, pubescent with red sessile glands, grey tomentose beneath when young; petioles 8–15 mm long. Inflorescence terminal, branched at base, lax; verticils 2–6-flowered, 5–10 mm apart; bracts inconspicuous, soon deciduous; pedicels 4–5 mm long, spreading, forming a right angle with the calyx. Calyx deflexed, 4 mm long at anthesis, pubescent with reddish sessile glands; fruiting calyx 7 mm long; posterior lip erect; lateral lobes of anterior lip deltoid with posterior margin extended towards posterior lip; median lobes of anterior lip lanceolate, subulate at apex; longer than the lateral lobes. Corolla usually deflexed, pinkish, 10 mm long; tube 7 mm long, straight, dilating slightly towards throat; anterior lip cucullate enclosing stamens. Stamens 4, declinate, enclosed within the anterior lip; posterior pair inappendiculate, glabrous, adnate to the corolla above the midpoint of the tube; anterior pair glabrous, adnating nearer throat, anthers dorsifixed, synthecous. Disk 4-lobed, with anterior lobe larger. Ovary glabrous, deeply 4-lobed; style gynobasic, capitate, branches rounded, equal, adpressed. Nutlets brown, obovate with a small basal scar, minutely tuberculed, producing a small amount of mucilage when wet. Fig. 1.

Differs from all other African species of Orthosiphon except Orthosiphon thymiflorus (Roth) Sleesen in being a shrub rather than a suffrutex sprouting annual stems, or an annual herb. Differs from Orthosiphon thymiflorus in being larger and more erect, 1.2–2 m tall, rather than straggling to 0.2–1.2 m tall; and with longer larger discolorous leaves, 50–120 mm long, and greyish tomentose beneath, rather than 10–40 mm long and sparsely pubescent. The flowers of Orthosiphon cinereus are also more strongly deflexed.

Named after the grey tomentose indumentum of the stems and abaxial leaf surfaces.

Endemic to Angola.

Damp area in forest; 1610–1700 m, alt.

No recent gatherings of this species have been collected and the specimens mostly lack precise localities. This species is best viewed as data deficient.

This is a distinctive, easily recognized species, differing from the most large-leaved specimens of Orthosiphon thymiflorus which lack the discolous, greyish leaf indumentum and have spreading rather than deflexed flowers. Orthosiphon thymiflorus is found throughout tropical Africa and the large leaf size seen in specimens similar to the type of Orthosiphon longipes Baker represent an extreme of the variation Fig. 2. Orthosiphon longipes was placed in synonymy of Orthosiphon thymiflorus in Paton et al. (2013). The shrubby habit is seen in Asia and Madagascar Orthosiphon, Orthosiphon aristatus (Blume) Miq. and Orthosiphon adenocaulis A.J.Paton & Hedge being respective examples and in Orthosiphon americanus Harley & A.J.Paton from Colombia, the only New World member of the genus. However, the species in Africa are mainly suffrutices with annual stems being produced from woody rootstocks. The little habitat information available from the specimens of Orthosiphon cinereus suggests that the species occupies a wetter forest than the seasonally dry forest habitat of Orthosiphon thymiflorus and other Angolan species.