Geniostoma pumilum is a sparingly branched shrub with stems that are sharply angled or slightly winged, and either glabrous or bearing a few scattered hairs. The leaves are simple and arranged oppositely. The blades are elliptic–obovate, measuring 4–10.5 cm in length and 2–4.5 cm in width, with an abruptly short-acuminate apex and a cuneate base. Surfaces are glabrous, with the lower surface pale and glaucous. Leaves are coriaceous at least when dry. Petioles are short, 0–1 cm long. Stipules are interpetiolar (and sometimes intrapetiolar), membranous, connate into a caducous cup 5–9 mm long, and distinct from the petioles. The flowers are bisexual (perfect) or functionally unisexual, actinomorphic (radially symmetric), and are arranged in terminal sessile cymes, with up to 15 flowers per inflorescence. Peduncles are 0–6 mm long, and pedicels are 8–11 mm long, elongating to 10–13 mm in fruit. Bracts are linear–filiform, 5–7 mm long, and bracteoles are about 4 mm long and glabrous. The calyx is 5-lobed, with lobes oblong to oblong–lanceolate, 6–11 mm long and 2–3 mm wide, united at the base and often bearing colleters on the inner surface. The corolla is also 5-lobed and orange in color, 15–16 mm long with a 9–10 mm tube, hirsute externally; lobes are narrowly deltate and 5–7 mm long. Stamens are five, alternipetalous, attached at the mouth of the corolla and exserted, often recurved post-anthesis, with anthers that are 2-celled, introrse, and sometimes have an apical appendage. The ovary is superior or partly inferior, 2- or 3-locular, glabrous or slightly pubescent near the apex, with numerous amphitropous ovules and axile placentation. The stigma is clavate to globular, approximately the same size as the ovary. The fruits are capsules, ovoid–trigonous, 9–14 mm long and 6–8 mm in diameter, 3-valved, and dehiscing septicidally or sub–septifragally. Valves have obscure to conspicuous keels near the apex, which are 0.5–1.5 mm wide, and terminate in a beak 1.5–2.5 mm long. Fruits mature green, often with a black–purple tinge, and become brown–black when over-mature. Seeds are numerous, ellipsoid to subglobular, often ± winged at both ends or all around, embedded in juicy orange to red pulp. The seed surface is minutely papillose to areolate, and seeds are brown to black. The endosperm is thick, fleshy, oily, and proteinaceous, with saccharose or hemicellulose, and the embryo is small and straight. It occurs on hummocks in bogs and along bog margins between 1,060-1,390 meters.