Schiedea spergulina is a dioecious, weakly erect to sprawling shrub. The stems are often swollen at the nodes, terete (cylindrical), 4-angled or ridged, and grow 3–6 dm long. It forms tangled clumps, with few stems emerging from the base. Primary stems are branched, weakly ascending to sprawling, with internodes measuring 0.8–2.5 cm long. The stems are green and glabrous proximally, while the distal stems and inflorescences may be glabrous or puberulent. The leaves are simple and opposite, with those of axillary stems often appearing fasciculate due to condensed internodes. Leaf blades are linear or linear–subulate, inconspicuously falcate, and weakly v-shaped in cross-section, measuring 3–7(–8.7) cm long and 0.08–0.12(–0.22) cm wide. The apex is attenuate and often slightly curved. The base is often connate around the stem and slightly tapering. Surfaces are glabrous, though distal leaves may be puberulent along the midvein and margins at the base, with erect, white–opaque hairs measuring 0.05–0.1 mm long. Blades are usually reflexed or recurved, becoming thin and membranous. The margins are slightly thickened, with only the midvein evident, which is slightly excentric. The leaves are sessile, and stipules are absent. The flowers are arranged in terminal inflorescences with 15–280 flowers measuring 6–10 cm long. The inflorescence is laterally contracted, with short lateral branches. The branches and pedicels are ascending to somewhat spreading and may be glabrous or puberulent, with hairs similar to those found on the leaves. The bracts are green, filiform to linear–subulate, with those on the central axis measuring 4.5–6 mm long and those on the branches and flowers measuring 1.3–2 mm long. The flowers are unisexual, with plants being dioecious. In staminate flowers, the calyx consists of sepals that are subequal, 2.5–2.65 mm long, narrowly ovate, yellowish-green with a yellow base, opaque, reflexed, and convex at the base, producing a small transverse bulge. The middle part is concave, curved upward, and broadly and shallowly navicular to nearly flat in the distal third, sometimes irregularly inrolled toward the apex. The sepals are oriented at an 80° to 100° angle to the pedicel, moderately puberulent with hairs 0.05–0.1 mm long. The margins are broadly and conspicuously scarious, sometimes ciliate in the distal half, and the apex is obtuse. Pistillate flowers have a similar calyx structure, with 2–2.25 mm long sepals. The corolla is absent. Staminate flowers contain 10 stamens with dimorphic filaments measuring 3–5 mm long. The antisepalous whorl is 2.8–3.9 mm long, while the alternate whorl is 2.3–3.5 mm long. Anthers are pale yellow and about 0.3 mm long. Pistillate flowers have 10(–11) vestigial stamens, with subequal filaments measuring 0.55–0.7 mm long. Their anthers are yellowish-white, ca. 0.3 mm long, and do not produce pollen. The pistillate ovary is superior, one-celled, with free-central placentation and 2–3 styles. Staminate flowers also have 2–3 weakly elongating but non-functional styles. The fruit is a narrowly ovoid capsule measuring 2–3.5 mm long. The seeds are reniform, compressed, and 0.7–0.8 mm long, with a transversely rugose surface. The perisperm is starchy and hard or rarely soft, while a true endosperm is absent. It is found on cliffs and in diverse dry shrubland and mesic forests between 60-800 meters.