Stems of castilleja septentrionalis
generally grow vertically upwards, to a height of around one foot; they may branch a few times but are most often simple. Plants produce one stem or several. Stems are usually glabrous below, but have a covering of spreading hairs above. The narrow, undivided leaves grow at regular intervals all along the stems; they have a midvein and a pair of parallel, equally prominent side veins.
The upper stem bracts are greenish yellow to whitish yellow in color, glandular hairy, and rather shorter and wider than the leaves; more ovate than lanceolate. The bracts may have one pair (less often two) of small side lobes. The tubular corolla is enclosed by a calyx a little less than one inch long, divided into two main lobes, each subdivided into pointed segments. The corolla is longer, up to 1.2 inches, divided into a beaked upper lip and a shorter, narrower lower lip. The corolla contains four stamens.