Schiedea waiahuluensis is an erect to spreading shrub, reaching 40 cm long. It has terete (cylindrical) stems that are green and glabrous in young plants but become conspicuously viscid and glandular-pubescent as they age. The stems are ascending and sprawling, becoming pendent as they elongate in the wild. It is many-branched, with side branches elongating to the length of the main axis and flowering simultaneously with the main axis. The leaves are opposite, ranging from 4.5–7 cm long (or up to 11.9 cm in cultivation) and 1.5–2.1 cm wide (up to 3.3 cm in cultivation). They are thin, green, and narrowly oblanceolate. The mid-rib is prominent, with 2–5 conspicuous veins branching from the mid-vein near the base of the leaf. Leaves on mature plants are glandular pubescent, with entire, slightly thickened margins. The apex is acute, and the base gradually tapers. The leaves are sessile (lacking petioles). The inflorescence is terminal and erect, with 9–27 (sometimes up to 45) flowers. The flowers are pseudo-axillary or borne on a central axis 20.5–36.5 cm long. Bracts are green and densely glandular-pubescent, with the lowermost bracts measuring 13–29 mm long and 7–13 mm wide. Distal bracts are smaller, measuring 5–12 mm long and 3–8 mm wide. Pedicels spread at anthesis, are densely glandular-pubescent, and measure 5–30 mm long (sometimes up to 40 mm at anthesis). The flowers are hermaphroditic and strongly protandrous. The sepals are usually 5–6 in number, subequal in size, measuring 4.5–11.9 mm in length and 2.5–5 mm in width. Sepals are green or whitish-green on the adaxial face, occasionally with whitish margins, and they are concave and ovate, positioned at a 90° angle to the pedicel. The adaxial side is glabrous, while the abaxial side is glandular-pubescent. Margins are entire and scarious, and the apex is apiculate. The nectary shaft is tubular and notched at the apex, measuring 2.0–5.0 mm in length, and it may be flap-like or tubular, with nectar released as stigmas become receptive. Stamens number 8–10 (sometimes up to 12) and are 5.3–10 mm long, with yellow anthers measuring about 0.9 mm long. The styles number 5–8 (sometimes up to 11) and are 3.6–10.0 mm long when receptive, with stigmatic papillae on the distal half. The ovary is 1.5–3.0 mm wide and 1.0–2.0 mm tall. The fruit is a capsule that is 2.6–3.2 mm long and subglobose, apparently dehiscing tardily. The seeds number between 13 and 82, viable following self-pollination, black, 0.4–0.6 mm long (sometimes up to 0.7 mm), orbicular-reniform, compressed, with transversely rugose faces and papillose margins. It grows in mesic to wet environments, often on steep cliffs and ridges in remote areas between 530–950 meters.