Hemiepiphytic, rarely terrestrial; stem appressed-climbing or scandent; internodes semiglossy, sometimes 1-ribbed, 1–10 3 1–5 cm, longer than broad, medium green to brown, drying gray-brown, obscured by cataphyll fibers; roots few per node; cataphylls 8– 20 cm, sharply 2-ribbed, red or green, tinged red, persisting semi-intact at upper nodes, weathering to thick, matted, pale fibers with much of the epidermis intact, drying reddish brown. LEAVES erect-spreading; petioles firm, flexible, 13.5–70 cm 3 3–8 mm (dry), subterete, narrowly sulcate with thick, blunt, erect margins at base, becoming flattened with 3 ribs toward apex adaxially, rounded abaxially, dark green, faintly striate, semiglossy, drying dark reddish brown; geniculum 3-ribbed, to 1 cm, darker than petiole, drying darker than petiole, almost black, somewhat shrivelled; blades triangular-ovate, subcoriaceous, moderately bicolorous, abruptly acuminate at apex, deeply cordate at base, 18–59 3 10–31 cm, 1.4– 1.9(–2.3)3 longer than wide, 0.7–1.1(2.1)3 longer than petiole, broadest 6 at the middle, margins sometimes slightly concave at point of petiole attachment, otherwise rounded, adaxial surface semiglossy, dark green, drying dull, pale to dark redbrown, abaxial surface glossy, paler than adaxially, drying semiglossy, paler than adaxially; anterior lobe 16.5–41.5 3 10–31 cm, 2.3–4.3(–5.5)3 longer than posterior lobes; posterior lobes 3–17 3 3.5–15 cm, rounded to obtuse, often bluntly acute at apex, directed downward, rarely slightly outward; sinus parabolic to spatulate to hippocrepiform, 1.5– 16.5 cm deep; midrib broadly convex to flat to sunken, paler than blade, or matte and olive-green with short pale lineations adaxially, convex or round-raised and darker than blade abaxially, drying darker than blade on both surfaces; primary lateral veins 4 to 6 per side, 2–3.5 cm apart, spreading from midrib at 50º–60º angles, downturned at midrib, straight out to margin, drying nearly concolorous adaxially, darker or lighter than blade abaxially; interprimary veins sometimes nearly as prominent as primary lateral veins, more often obscure; basal veins 5 to 10 per side, with 1 to 4 free to base, 4 to 6 coalesced for 6.5 cm, posterior rib barely naked or naked to 2 cm, drying darker or lighter than blade abaxially; minor veins moderately obscure. INFLORESCENCE 2 per axil, erect or curved downward; peduncle 3–6.2 3 2– 5 cm (dry), shorter than petiole, white and coarsely streaked at apex; spathe long-tapered, apiculate, 16– 17.5 cm, slightly constricted above tube; spathe blade white outside, paler and glossy inside; spathe tube rosy burgundy, white-speckled toward blade outside, red-violet inside; spadix sessile, 12–15 cm; pistillate portion pale green, cylindrical, ca. 3.5 cm 3 8 mm (dry); staminate portion ellipsoid, ca. 10 cm 3 12 mm at widest point (dry), wider than pistillate and sterile staminate portions; sterile staminate portion cylindrical, to 4 cm 3 7–8 mm (dry); pistils ca. 2 mm; ovary 6-locular, ovules with axile placentation, 10 to 20 per locule. INFRUCTESCENCE unknown. JUVENILE PLANTS differ in having leaf blades with smaller, less well-developed lobes.
Philodendron tricostatum is a member of subgenus Philodendron, section Philodendron, subsection Philodendron, series Fibrosa and is characterized by the short internodes to 10 cm usually obscured by long, thick, matted, pale cataphyll fibers with large pieces of epidermis intact, and the triangular-ovate, deeply cordate leaf blades on slender petioles to 70 cm long. In addition, the inflorescences are stout, with a long apiculum, burgundy red on the spathe tube and white on the spathe blade. The light red-brown color when dry is also distinctive.