Similar to Hoya aeschynanthoides Schltr. as both species have bilobed outer corona lobes, but differing in the size and colour of the flowers (3.5–4 mm in diameter, pink corolla, yellow and red corona lobes for Hoya bakoensis; ca. 5 mm in diameter and white for Hoya aeschynanthoides) and in the habit, that in Hoya aeschynanthoides is decumbent while Hoya bakoensis is a weak twining climber.

Malaysia, Borneo, Sarawak, Bako National Park, along Tajor Trail, 20 March 2015, Rodda M MR1042b (SING, holotype; SAR, KEP, isotypes)

Epiphytic climber with white exudate in all vegetative parts. Stems slender, weakly twining upward, internodes (1 mm–)2–7(–10) cm × 0.7–1.5(–2) mm, dull green, sparsely pubescent when young, scabrous; adventitious root sparsely produced along the stems and just under the nodes where they are paired. Leaves petiolate; petiole straight or recurved, 4–10 × 1–1.5 mm, dark green to maroon, sparsely pubescent; lamina ovate (lanceolate) (1.5–)2–4(–5) × 1–2.8 cm, base attenuate (acute), apex apiculate (cuspidate), dark green above, slightly pubescent on young leaves only, lighter green underneath with occasionally a slightly darker midrib and edge, glabrous, margin occasionally ciliate; penninerved, secondary veins obscure; colleters one at each lamina base, triangular to ovate 0.1–0.3 × 0.3–0.5 mm. Inflorescence pseudo-umbelliform, slightly convex, 10–15 flowered; peduncle 4–6 cm × 1–1.5 mm in diameter, dull green to brown, pubescent; rachis indeterminate (–7) × ca. 2 mm in diameter. Pedicel 3–7 × 0.7–0.9 mm in diameter, pale green with pink spots, papillose. Calyx lobes triangular, 0.7–1 × 0.5–0.8 mm, apex acute or round, light green with pink edge, sparsely ciliate; basal colleter one in each calyx lobe sinus, ovate, 150–200 × 80–100 µm. Corolla revolute, 3.5–4 mm in diameter, ca. 6 mm when flattened; corolla lobes basally fused, tube 1.2–1.5 mm long, lobes triangular, 2–3 × 2.4–2.7 mm, pink, pubescent inside, outside glabrous. Corona staminal, 2.8–3 mm in diameter, 0.9–1.1 mm high; corona lobes oblong, 1.2–1.4 × 0.7–0.8 mm, convex above, underneath sulcate with inrolled margins, outer process apex bifid, light yellow, inner process elongate, red, with a yellow round tip. Anthers broadly triangular, 400–450 × 550–650 µm, with apical triangular membranaceous appendage. Pollinia clavate, 200–250 × 90–110 µm, narrowing towards the base, apex obliquely truncate, with evident pellucid margin; corpusculum oblong, constricted in the middle, 80–100 × 50–60 µm; caudicle broad, spathulate, hyaline, 110–130 × 30–45 µm at the widest. Style-head 5 angled in cross section, with 5 lobes alternating with the stamens, style-head apex round, 200–300 µm long, ca. 0.5 mm broad at the base; ovary ovate with a narrow tip, 0.8–1 mm, each carpel ca. 0.3 mm wide at the base, light green. Fruit (unripe) a single linear follicle, 10 cm × 2 mm (widest), seed (unripe) narrowly lanceolate, 3–4 mm long, winged, long comose.

The species is named after the collection locality of the holotype, Bako National Park (Sarawak, Malaysia).

Hoya bakoensis is only known from Bako National Park, where it is common along Tajor Trail in moist, shady kerangas heath forest near a stream. The species is strictly epiphytic and it was found mostly germinating from the opening of small ant nests located inside hollow tree trunks (Fig. 4). The plants observed were forming small clumps of weakly climbing stems near the germinating point or more rarely were climbing towards the tree canopy potentially in response to low light.

Hoya bakoensis is locally common and well protected inside Bako National Park. Its conservation status is therefore Least Concern (LC) (IUCN 2014).

Hoya bakoensis is one of the 11 Bornean Hoya species in Hoya section Acanthostemma (Bl.) Kloppenb. Acanthostemma species are characterised by revolute corolla lobes, bilobed outer corona lobes and pollinaria with broad, spathulate caudicles. The other Bornean species in Acanthostemma are Hoya aeschynanthoides, Hoya acicularis, T. Green, Hoya beccarii Rodda & Simonsson, Hoya kloppenburgii T. Green, Hoya minutiflora Rodda and Simonsson, Hoya pubera Bl., Hoya revoluta Wight ex Hook.f., Hoya rundumensis (T. Green) Rodda and Simonsson, Hoya sigillatis T. Green and Hoya waymaniae Kloppenb.

Most of these species can be easily separated from Hoya bakoensis because their flowers are larger (>5 mm in diameter) or their inflorescence is markedly concave (in Hoya beccarii and Hoya revoluta). As mentioned in the diagnosis Hoya aeschynanthoides has white, only slightly larger flowers than Hoya bakoensis (ca. 5 vs. 3.5–4 mm in diameter). Additionally the bilobed outer lobes of Hoya aeschynanthoides appear to be less pronounced than those of Hoya bakoensis, but this observation is based solely on the examination of the drawing attached to the type of Hoya aeschynanthoides as only small buds are present on the specimen while no other specimens have been found at present. The flowers of Hoya pubera are also comparable in size with those of Hoya bakoensis, but they are yellow-orange, the corolla lobes are only apically revolute and the corona is more markedly raised in the centre. Lastly, Hoya minutiflora has the smallest flowers among all Bornean Acanthostemma (2.6–2.8 mm in diameter).

Malaysia, Borneo, Sarawak, Bako National Park, along Tajor Trail, 20 March 2015, Rodda M MR1042a (KEP, SAR, SING)