Astelia argyrocoma is an epiphytic, rhizomatous perennial herb. Its leaves are simple and arranged in a 3-ranked pattern. The leaf blades are ensiform, measuring (25–)40–100 cm in length and 0.4–1.1 cm in width. The surfaces of the leaves are covered with scales; the upper surfaces are coherent and peel off in small pieces, while the lower surfaces are closely appressed and woolly pubescent beneath the scales. The base of the leaves is densely shaggy pubescent, and the upper surfaces exhibit a silvery-green color, with the lower surfaces being silver. The leaf margins are entire, with parallel veins and a sheathing base. Stipules are absent. The flowers are on scapes (leafless stalks from roots) that can reach up to 50 cm long, with a shaggy pubescent texture. The staminate (male) flowers form in racemes 6–10 cm long, pedicels 4–6 mm long, and bracts 4.5–7 mm long. The pistillate (female) flowers form in shorter racemes 2–7 cm long, with pedicels 2.5–5 mm long and bracts 3–5 mm long. The plant is dioecious, having unisexual flowers on separate plants. The tepals are green or sometimes reddish-brown, pubescent towards the base. The staminate flowers have tepals 4.2–5 mm long (outer) and 4–4.5 mm long (inner), while the pistillate flowers have tepals 3–4 mm long (outer) and 2.4–4 mm long (inner). The staminate flowers feature six stamens with anthers 0.6–0.7 mm long and filaments 1.3–2 mm long. The anthers are not fertile in pistillate flowers, measuring 0.2–0.6 mm long, with filaments 0.4–1 mm long. The ovary in pistillate flowers is superior, measuring 2.5–4 mm long, with a style approximately 1 mm long. Staminate flowers have a pistillode 2–3 mm long. The fruit is an orange to yellowish-orange berry, measuring 4–6.5 mm long and 3–4 mm in diameter. The seeds are black, fusiform (shiny), and 2–3 mm long. It is typically found as an epiphyte or sometimes on wet rotting logs in diverse mesic and wet forests at 600–1,280 meters.