Terrestrial plant; stems 1 or more metres long, rooting at the nodes; internodes 1–8 cm long, 1.5–2.5 cm diam., grey-green, drying medium yellow-brown and longitudinally ridged; cataphylls to 14 cm long, reddish, sharply 2-ribbed, clasping the base of the petiole at first then deciduous. Leaves clustered at the top of the stem; petiole obtusely flattened adaxially, moderately soft, sulcate base, 20–32 cm long, 5–7 mm diam. in the lower half, 2–3 mm diam. near apex, drying dark brown; blade ovatecordate to subcordate, 17–22 cm long, 11–14.5 m wide, averaging 18.5 × 12.5 cm, 1.4–1.55 × longer than wide, 0.55–0.85 × as long as petiole, abruptly acuminate at apex, cordate at base, subcoriaceous, drying yellow-brown and matte on both surfaces, lighter below; midrib flat and drying dark brown above, raised and lighter coloured below; primary lateral veins 5–6 pairs, arising at 30–50º angle, flat above and slightly raised below, concolorous with midrib; basal veins 3–4 pairs, all free to the base; minor veins somewhat obscure and diffuse, those veins visible moderately widely spaced, the surfaces obscurely areolate when magnified. Inflorescences 2–4 per axil; peduncles terete, 5–6.5 cm long, c. 2 mm diam., drying dark brown; spathe 5–6 cm long, constricted in the middle between the tube and blade sections, faintly reddish outside, whitish on tube within at anthesis; spadix 4.5–4.8 cm long; staminate portion white, 2.2–2.5 cm long, 2.5–3 mm diam., the constricted part just above the sterile portion, 1.2–2 mm diam., sterile male portion indistinct, 4.5 mm long, 1.8 mm diam., the staminodia with pistilodes; pistillate portion green, 2–3 cm long, 4 mm diam.; pistils 1.5–1.6 mm long, stigma 1.6–1.8 mm wide, irregularly rounded; locules 3(–4) per ovary; style 0.3 mm diam., raised with a pale margin, funnel-shaped; ovary 1 per locule; 0.15 mm long; Infructescences remaining female portion of spadix c. 4.4 cm long, 7 mm diam.; fruits green; seeds 1.8 mm long, 1 mm diam.
The species is a member of Philodendron subg. Philodendron sect. Macrobelium subsect. Oligocarpidium and is characterised by its terrestrial habit, short thick internodes that dry medium yellow-brown and longitudinally ridged, the slender, soft obtusely flattened petioles, the ovate-cordate yellow-brown drying blades and especially by the clusters of tiny faintly reddish inflorescences in several upper leaf axils.
Philodendron merenbergense somewhat resembles P. strictum G. S. Bunting, but the latter has much larger leaves, which are more deeply lobed and have a prominent posterior rib (which is absent in P. merenbergense) and also has much larger spathes (13–18 cm versus 5–6 cm long in P. merenbergense). P. merenbergense also somewhat resembles P. devianum Croat, but that species generally has larger leaves, which are more deeply cordate or broadly sagittate, and especially has much larger inflorescences (14–15 cm versus 5–6 cm long in P. merenbergense).